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AC 2000.2: The Astrographic Catalogue on
The Hipparcos System
2024-03-19
AC 2000.2: The Astrographic Catalogue on
The Hipparcos System
Sean E. Urban, Thomas E. Corbin and Gary L. Wycoff
United States Naval Observatory, Washington, DC
∧
Erik Høg, Claus Fabricius, and Valeri Makarov
Copenhagen University Observatory, Denmark
Catalogue of Positions Derived from the Astrographic Catalogue
Measures. Positions are on the Hipparcos System (HCRS, J2000.0) at the
Mean Epochs of Observation.
The AC 2000.2 is a revised version of the 1997 release of the
AC 2000 (Urban et al. 1998). It was decided that the availability of
an improved reference catalogue
and the inclusion of the Tycho-2 photometry would be sufficient to
warrant a complete re-reduction of the data and a new
distribution of the catalogue.
The AC 2000.2 is a
catalogue of positions and magnitudes of 4,621,751 stars covering
the entire sky around the epoch of 1900. The data
are derived from the images measured and published as part of the Astrographic
Catalogue (AC). The positions are on the Hipparcos reference frame
(ESA 1997), having originally been
reduced plate-by-plate using an updated version of the Astrographic Catalog
Reference Stars.
Each of the 22 zones of the AC has been reduced independently,
since telescopes, observing techniques and measurement methods varied.
This document describes the history behind the Astrographic Catalogue,
the reference catalogue used to transform the measurements to equatorial
coordinates, the methodology employed in this transformation,
the resulting catalogue and information about each participating observatory.
2 The Carte du Ciel
The Carte du Ciel was an international effort begun more than a century ago to
determine positions better than 0.5 arcsec for all stars 11th magnitude
and brighter using photographic plates and, using another set of plates, to
publish charts representing the relative positions of all stars of 14th
magnitude and brighter. The charts – generally called CdC –
proved to be very expensive to photograph
and reproduce, so many institutions did not complete this part of the work.
However, the astrographic program
designed to measure all stars to 11th magnitude was completed. Actually,
the original goal of 11th magnitude was generally surpassed. In fact, some
observatories routinely measured stars as faint as 13th magnitude.
These plate measures, as well as the formulae used to transform them to
equatorial
coordinates, have been published in what is known as the Astrographic Catalogue
(AC).
In total, 20 observatories from around the world participated in
exposing and measuring the AC plates.
Each was assigned a specific zone, between
two parallels of declination, to photograph. In order to compensate for
any plate defects, each area of the sky was to be photographed twice, using
a two-fold, corner-to-center overlap pattern. This pattern was continued
even at the zone boundaries; each observatory's plates would overlap with
those of the observatories responsible for the adjacent zones.
The participating observatories agreed to standardize the type of telescope,
so each plate photographed had a similar scale of approximately 60 arcsec/mm.
The measurable areas of the plates were 2 × 2 degrees, so the
overlap pattern consisted of plates that were centered on every
degree band in declination, but offset in right ascension by two degrees.
The first plates in the even degree bands were centered at right ascension
0 hours 0 minutes; the first plates in the odd degree bands were centered
with right ascension several minutes higher (corresponding to approximately
one degree).
In addition to the overlap pattern and type of telescope, the
observatories also agreed to expose a grid,
called a réseau, on each plate. It was originally used to monitor
emulsion shifts.
After the shifts were demonstrated to be quite small, the practice of exposing
a réseau on each plate was continued because it aided in the measuring
of the star positions by letting the measurer refer each image position to
the grid lines. Each réseau unit was approximately 5 mm.
The réseau orientation defined the plate's x,y coordinate system.
All participating observatories (with the exception of Vatican) used one of
two measuring methods, short-screw or eyepiece scale.
In both methods, a set of spider wires was centered over an image, then
the distance traveled by the slides carrying the spider wires was read.
With the short-screw method, this distance was read off of the screws
used to move the slides. Whereas for the eyepiece scale method,
this distance was inferred by a
scale in the focal plane of the microscope. In general, the eyepiece
scale method was faster, but less precise.
Although telescope type, plate size, and use of a réseau were standardized,
many other factors, such as reference catalogue used,
reduction technique and printing formats were left up to the individual
institutions.
3 Published Data
Most observatories published their results in several volumes, each of
which consists
of measures from plates centered on the same degree of declination.
Generally, each line in the printed volumes contains data for one star,
including the measured
x,y value, a measure of brightness (magnitude or diameter), the plate
number and a running number on the plate.
Other data concerning epoch of exposure, hour angle at mid-exposure,
air temperature, barometric pressure, réseau used, observer,
measurer and measuring machine are usually provided by plate
in separate tables. Additionally, provisional
plate constants used to transform the x,y measures to standard coordinates
are typically supplied.
The published data have been transferred to machine-readable
form via double-entry (that is, typing each record twice to remove most
keying mistakes) including errata found in the published volumes.
An additional literature search has been conducted on all
zones to increase the probability that all published errata not found with the
original volumes have been corrected. Each page of each volume was searched
for pertinent notes. If important notes were found, they were entered and
the measures to which they refer were flagged. A summary of the data entry
information can be found in Table tab:keypunch.
4 Compiling a New Reference Star Catalogue
The reference catalogue used throughout the individual plate reductions
of the 1997 version of AC 2000 (hereafter referred to as AC 2000.1) was the
Astrographic Catalog Reference Stars (ACRS; Corbin & Urban 1988,
Corbin & Urban 1990, Corbin & Urban 1991).
This was compiled on the system of the FK5 as realized by the
International Reference Stars (IRS, Corbin 1991), and utilized the
best data and reduction techniques available in the early 1990s.
Since its compilation, the FK5 has been superseded by
the Hipparcos Catalogue. It was recognized that with the increased
number of stars in the Hipparcos Catalogue over the FK5/IRS
(~ 118,000 vs. ~ 40,000), the conversion of
the catalogues making up the ACRS to the
system of Hipparcos could be done more rigorously than the earlier
conversions to the FK5 system. Since the systematic errors could be
determined and removed more thoroughly, the final combined catalogue would
be better. Additionally, the FK5/IRS data become
very sparse fainter than magnitude 9.0, whereas Hipparcos contains
over 35,000 stars listed as V=9.0 or fainter. This magnitude extension
allows the removal of systematic errors in the fainter stars – something
that was problematic for the original ACRS.
In addition to Hipparcos, several new catalogues unavailable at the time
of compiling the original ACRS have been released.
The most notable are the Tycho-1 Catalogue (ESA 1997),
the Twin Astrograph Catalogue
(TAC, Zacharias & Zacharias 1999), FOKAT (Polozhentsev et al. 1989),
and a recompiled Second Cape Photographic Catalogue
(CPC2, Zacharias et al. 1999).
Due to these improvements
in astrometry, it was decided to compile a new
reference catalogue. In this document, this new catalogue will be
referred to as ACRS_1999. The ACRS_1999 is not being released as
a separate catalogue because the data used in computing
the astrometry were subsequently used in computing the proper motions of
the Tycho-2 Catalogue (Høg et al. 2000). The ACRS_1999
can be thought of as a specialized subset of the data going into Tycho-2,
which completely supersedes it.
4.1 Conversion of input catalogues to HCRS
The Hipparcos Catalogue (ESA 1997) plays a special part in the ACRS_1999.
It is recognized as the optical realization of the International Celestial
Reference Frame, ICRF (IAU 1999). The frame defined by the Hipparcos
data is termed Hipparcos Celestial Reference Frame, HCRF (IAU 2001); the
HCRF contains all Hipparcos stars with the exception of those believed
to be multiple. These data – taken from the Hipparcos Catalogue – were
used to convert all input catalogues
to the HCRF by the method described below.
In total, 145 different observational catalogues (termed input catalogues
in this document) were included specifically
to strengthen the positions of the ACRS_1999 stars when brought to the
epochs fo the AC plates. A list of
these catalogues is found in Table tab:acrs.
Each of the catalogues was converted to a standard format. Next,
zero point corrections and elliptical
aberration terms were applied, when necessary, to convert
the positions to roughly coincide with the FK5 J2000.0 system. Following this,
the Hipparcos stars contained in each catalogue were identified.
Differences in right ascension and declination were computed for these
stars following the application of the Hipparcos proper motions to bring the
Hipparcos positions to the epochs of the input catalogue positions.
To remove the systematic differences between these input
catalogs and Hipparcos, ``local'' mean differences between each
catalogue and Hipparcos were made, and corrections to the input catalog
positions based on those differences were applied. To compute
these differences, for each star in
the catalogue, nearby stars in common with Hipparcos were
identified. In order to compute local systematic errors and not
follow the random errors, a minimum number of stars in common to
the input catalogue and Hipparcos had to be enforced.
This minimum number was based on each input catalogue's random errors.
For catalogues with large random errors, more Hipparcos stars were needed.
The range in required Hipparcos stars was from 3 for the most accurate
catalogues to 30 for the least accurate.
Additionally, since determining local systematics was the goal,
only those stars in common with Hipparcos and close on the celestial sphere
were used.
For astrographs, typically the Hipparcos stars within
two degrees of the star to be corrected were utilized. For transit circles,
typically the Hipparcos
stars within 30 minutes of time in right ascension and 5 degrees in
declination were used. If the minimum number of stars discussed in the
previous paragraph could not be met, the area could be expanded. How
far this expansion was carried out was dependent on how quickly, spatially
speaking, the systematic deviations from Hipparcos were changing.
Catalogues whose density of Hipparcos stars was too low to adequately
perform a local reduction were dropped from the input catalogue list.
Once nearby catalogue entries of Hipparcos stars were found for each
input catalogue star, their
positional differences with the Hipparcos data – that is, their residuals –
were used to
compute the local reduction to the Hipparcos system. Weights were assigned
to each residual based on the distance to the
star being corrected.
To more heavily favor nearby Hipparcos stars, an elliptical, parabolic
weighting method was used, as seen in the following equation.
The value of wi is the weight of the ith residual, Dαi
and Dδi are the distances in right ascension and declination
between the catalogue star whose correction is being computed and the
ith Hipparcos star, and maxDα and maxDδ
are the maximum distances whose weight is non-zero in right ascension
and declination.
Once weights to each nearby Hipparcos residual are computed, the weighted
mean residual vector can be computed in the standard way. That is, for
right ascension,
Δ(α) = Σwi Δi(α)/Σwi
(2)
where Δ(α) is the weighted mean vector in the right ascension
direction,
Δi(α) is the
residual of the ith Hipparcos star. Similar formulation is used for
the declination vector. A summation is used to get the mean residual
vector, which is the
systematic difference between the Hipparcos Catalogue and the input
catalogue at that specific location. Hence, subtracting this vector
from a star position will put it close to the HCRF.
The above technique describes correcting a catalogue for systematic
differences with Hipparcos based on right ascension and declination.
However, it is well known that other systematic deviations exist,
most notably those based on magnitude and color. These, in turn,
may have a positional dependency associated with them; so, corrections
based on right ascension, declination, magnitude, color and all
combinations of the four were computed and applied.
This was done similarly to the technique just describe, however
the definition of ``local'' was expanded to include those stars
nearby not only in right ascension or declination, but also similar
in magnitude or color. In order for a magnitude-dependent correction to
be computed, typically only Hipparcos data from stars that were
within 0.1 magnitudes of the input catalogue star were used.
The same 0.1 magnitude difference in B-V was usually utilized in
performing color corrections.
4.2 Computing catalogue weights
Once a catalogue was believed to be on the HCRS, individual star differences
from Hipparcos were computed. (Note that the conversion to
the HCRS utilized weighted mean differences using several stars,
not individual differences.) These were used to compute the catalog
standard error and weight utilizing the well-known formulae
σα,δ = sqrt( Σ(xi)2– (Σxi)2/N/N)
(3)
and
Wα,δ = 1/σα,δ2
(4)
where σα,δ is the standard error per catalogue entry in
right ascension and declination, xi are the individual differences
between the stars in common between the input catalogue and Hipparcos in
right ascension and declination, and N is the total number of stars
in common between the two. The value Wα,δ is simply the
computed weight of the catalogue in right ascension and declination.
For some input catalogues, the number of observations per catalog
entry given. In these cases, an error (hence weight) per observation
could be computed. These catalogues have a ``Yes'' under the
σ/sqrt(N) column of Table tab:acrs. For many catalogues, however,
only a standard error (hence weight) per catalogue entry could be computed.
These catalogues have a ``No'' under the
σ/sqrt(N) column of Table tab:acrs.
This procedure was used for most catalogs; a few, however, had error estimates
available for each star that closely coincided with the newly computed
estimates. In these cases, the additional information in the form
of published error estimates (hence weights) was used.
4.3 Computation of ACRS_1999 positions and motions
The reduced positions were grouped together by indivisual star. Mean positions,
proper motions, and error estimates were computed following the method
outlined by Corbin (1977), briefly described here.
4.3.1 ACRS_1999 mean positions
Computations are made using position unit vectors, defined by
xi = cosδi ×cosαi
(5)
yi = cosδi ×sinαi
(6)
zi = sinδi
(7)
Weighted mean (x,y,z) values, denoted as ({bar}x,{bar}y,{bar}z)
and their associated epochs can be easily computed using the computed weights.
Note that the weights wx, wy, and wz are equal to wα, wα,
and wδ, respectively. From ({bar}x,{bar}y,{bar}z), a
conversion back to mean right ascension and declination, ({bar}α,
{bar}δ), is made using the
formulae
{bar}α = \left( {bar}y/{bar}x \right)
(8)
and
{bar}δ = \left( {bar}z/sqrt( 1 – {bar}z2) \right)
(9)
4.3.2 ACRS_1999 proper motions
To compute proper motions, first
the time derivatives of (x,y,z), denoted ({dot}x,{dot}y,{dot}z),
are computed using
{dot}x = Σxi ⋅wi ⋅τi/Στi2⋅wi
(10)
with similar equations for {dot}y and {dot}z. The variable τi
is the epoch difference between the ith catalogue position and the
weighted mean epoch.
Finally, proper motions in right ascension and declination, µα,
µδ, are computed using
The U.S. Naval Observatory typically computes standard error estimates based on
the residuals of catalogue positions making up the mean position and
proper motion. This has been termed the scatter method, because it
is based on the scatter of the data around the computed value.
The formulae used are detailed in Corbin (1977). For standard errors of the
mean right ascension and declination, σ{bar}α,
σ{bar}δ, we have
where the values of Δxi, Δyi, Δzi are the
residuals of the ith catalogue position from the
computed (x,y,z) values at ith catalogue epoch. For the computation
of the standard errors of µα and µδ we have
where n is the number of catalogue positions used to compute {bar}α,
{bar}δ,µα, and µδ. Even a cursory look at the
above formulae will show that computation of the standard errors using this
method cannot be made if there are only two catalogue positions
used. In this case, the formulae
were used. Similar formulae were used to compute position and proper
motion errors in declination.
4.3.4 Refining the catalog
Since the purpose of compiling the ACRS_1999 was to provide a source
of reference stars used to reduce the AC data on to the system defined
by Hipparcos, it was decided to utilize the astrometry from the
Hipparcos Catalogue for the ~30% of the stars in common.
Hence the Hipparcos data were substituted for the compiled data.
Hipparcos stars flagged with a G, V, or X in the Double and Multiple
Flag field (H59) were removed from the ACRS_1999, as their astrometry
is suspect. Additionally, stars marked in that same field with an
O (orbit stars) were removed if the semi-major axis exceeded 100 mas.
All stars marked as orbit stars in the Washington Double Star Catalog
(WDS; Worley and Douglass 1996) were removed. Since measurements of
blended images on the AC plates are often suspect, when an ACRS_1999
stars was found within 5 arcsec of another ACRS_1999 star, both were removed.
Finally, all ACRS_1999 stars whose positional errors in either coordinate
at epoch 1900 were computed to be at 450 mas or higher were removed.
4.3.5 ACRS_1999 characteristics
The ACRS_1999 contains 391,838 stars distributed over the entire sky,
resulting in an average density of just under 10 stars per square degree.
The magnitude distribution, using the Tycho-1 visual magnitude, can
be seen in Fig fig:magdist.
The astrometric errors are magnitude dependent. Figures fig:poserr
and fig:muerr – indicating positional errors at the ACRS mean epoch
and proper motion at the same epoch, respectively –
show this dependency. The wide range in positional errors, from near
1 mas for the brighter stars to over 30 mas for stars between magnitude
10 and 11, is due to the Hipparcos star distribution in the ACRS_1999.
Positional errors
in the Hipparcos Catalogue are generally 1 to 3 mas; for the best non-Hipparcos
catalog, Tycho-1, the errors run from 10 to 50 mas. This is reflected in
Fig. fig:poserr. For stars brighter
than 8th magnitude, the primary source of astrometry is Hipparcos. Between
8 and 11, the ratio of Hipparcos to non-Hipparcos stars drops, and hence the
errors increase. Beyond about magnitude 11, this ratio begins to
increase, so the positional errors once again drop.
This same trend is reflected in the proper motion errors, seen in
Fig. fig:muerr. The span in errors is less, however, since the Hipparcos
values are generally between 1 and 3 mas/year whereas without Hipparcos,
values of 2 to 5 mas/year are normal. No downturn of errors in the
faintest stars is seen; this is where the Hipparcos proper motion errors
are their highest.
Having the ACRS_1999, with a density providing an adequate number of reference
stars on each AC plate (35 on average) and being on the Hipparcos system,
allows one to continue the plate reduction process.
5 Preparing the Data for the Plate Reduction Software
It was necessary to prepare the data for the plate
adjustment software. The preparation process consisted of three
discrete tasks: matching the images with reference stars;
matching images with those of the same star on another plate; and
converting the data to a standard format. The last two steps
were performed during the AC 2000.1 work. However,
since they are an integral part of the reduction process, the descriptions
are repeated here.
5.1 Matching images with the reference stars
Equatorial coordinates for all AC images were computed from the rectangular
coordinates via the published plate constants. The ACRS_1999 data were then
brought to the average epoch of each zone and transformed to the AC equinox,
B1900.0. A positional match was then made between the AC and ACRS_1999.
Checks were
made to ensure that illegitimate matches had not taken place. An example
of an illegitimate match is an ACRS_1999 star that matches with two separate
AC images that are on the same plate. Also at this stage, all plates were
checked to ensure that each contains an adequate number of
identified reference stars.
Fewer than expected matched reference stars on a plate may indicate problems
with the published constants or plate centers. These plates were investigated,
and the problems were corrected.
5.2 Matching images with those on other plates
The same equatorial coordinates computed in the previous step
were used to identify images of the same
stars that lie on different plates. Images within 2.0 arcsec
of each other were generally
identified as the same star at this stage.
Illegitimate matches were investigated and treated appropriately.
Each image was then assigned an internal star number that is unique
for each star in the zone, regardless of the number of plates on which it
appears. The data were verified to ensure that all images marked with
an ACRS_1999 number have the same
internal star number, and vice versa. No two images on one plate were allowed
to have the same internal star number.
5.3 Converting data to a standard format
The data were combined in one, standard format file. This file contained
all pertinent information about each plate and image. Plate information
such as plate centers, sidereal time of exposure, meteorological data,
measuring machine and measurer, réseau used,
emulsion type, and epoch of exposure were
included, if they were published.
Data pertaining to each star, that is the x,y values, image diameter
or magnitude,
internal star number, and ACRS_1999 identifier, were included.
A conversion from the published x,y units to units of millimeters, along
with a translation of the coordinates to ensure that the origin is in
the approximate plate center, was performed when necessary.
The persons responsible for the zone preparations can be found in
Table tab:people.
5.3.1 Conversion of the AC Magnitudes
The Astrographic Catalogue as originally published contains
magnitude measures – usually in the form of image diameters and
formula to convert them to stellar magnitudes. However these are
non-uniform between zones, in part
because of different techniques used by participating observatories. Many of
the published magnitude measures are unreliable, especially for the
faintest and
brightest stars. Thus, it is desirable to transform them to
a well-known – or often used – system, preferably to the
same system as the reference catalogue, thus facilitating the removal
of systematic errors that are a function of magnitude.
The plates used were most sensitive in the blue spectral region,
so a logical choice of systems was the Tycho-2 blue (BT). Tycho-2
contains about 2.5 million stars covering much of the AC magnitude range.
The Tycho-2 photometry was made available for
this in advance of publication.
Each zone was treated independently. Only stars identified in Tycho-2 as being
single and with negligible variability were used for the calibration.
Differences between AC and
Tycho-2 BT magnitudes as a function of AC magnitude were computed. Polynomial
expressions describing the results were computed via least-squares fitting.
Some extrapolation of these polynomials was required because for most
zones the magnitude limit is fainter in the AC than in Tycho-2.
(Usually an extrapolation
of less than 0.5 magnitude was used. Beyond that the corrections were held
constant). For stars not used to
compute the calibration – primarily non-Tycho-2 stars – the magnitudes found
in the AC 2000.2 are based on the published diameters and the
derived polynomial expressions. For stars in common with Tycho-2, the
Tycho-2 BT magnitudes are given.
6 Preliminary Reductions and Investigation of Plate Models
The core of the plate reduction software was the same as that used in the
reductions of the Cape Photographic Catalogue 2 data
(Zacharias et al. 1992). For all zones,
an eight-constant plate model, consisting of four
orthogonal terms (a,b,c, and d), two non-orthogonal terms (e and f) and
two tilt terms (p and q) was initially used, as shown in
Eqs. eq:xmodel1 and eq:ymodel1 .
ξ= ax + by + c + ex + fy + px2+ qxy
(19)
η= ay – bx + d – ey + fx + pxy + qy2
(20)
At this step, no corrections to the published
x,y values were applied. Investigations of radial distortions,
tangential distortions, magnitude equation, coma, periodic measuring errors
and réseau-dependent systematic errors were investigated for each zone
following the procedures outlined previously (Urban & Corbin 1996;
Urban et al. 1996). Since an uncompensated systematic error may
cause a star to be an outlier, only reference stars with residuals
over five times the standard deviation of unit weight of the solution
were removed while various plate models were investigated.
6.1 Corrections to the x,y values
Results of investigations of systematic errors may lead to applying a
correction to the published star measures prior to final plate constant
determinations. These are typically done on a zone-by-zone basis.
To investigate if radial distortion exists in the data, the reference star
radial residuals were plotted against distance from the plate center.
If a radial distortion was present, the corrections were described by a
non-zero function. Additionally, the radial residuals were examined for
magnitude, measurer and measuring machine dependence.
A similar investigation was conducted looking for
the existence of a tangential distortion, however none was found in any of
the zones.
The presence of a magnitude equation, which is a systematic offset of
stars based solely on
their brightness, was investigated by plotting the reference stars' residuals
with respect to the Tycho-2 BT magnitude. This was performed
separately for both the x and y coordinates. The existence of a
magnitude equation that is dependent on the x,y
measures was also investigated. Additionally, magnitude equation varying
with measurer and plate epoch was considered. Note that a magnitude
equation outside the range of the reference stars is extremely difficult
to find (Eichhorn 1974). It is possible to compensate for a magnitude equation
in the magnitude range of the reference stars, but it is unwise
to extrapolate those corrections to field stars that may be two or three
magnitudes fainter.
The presence of coma, a change in scale based on magnitude, is uncovered
in two ways. First, investigating the x,y-dependent magnitude equation will
reveal coma if it exists within the range of the reference stars.
Second, using fainter stars, a plot of the difference between the mean
position (computed from overlapping plates) and an individual image position
vs. plate coordinates of the individual position will, using data from hundreds
of stars and different magnitude ranges, reveal the presence of coma. If
coma was found, it was examined for variations with respect to plate epoch.
Errors caused by the measuring mechanism were investigated
by analyzing the residuals of reference stars as a function of
location on the x and y measuring apparatus (usually a screw or an eyepiece
scale, see Section 2). The presence of an error with a period
corresponding to the length of the measuring screw or eyepiece scale is quite
likely caused by the machining of that part, but may also be a function of the
measurer. In either event, these errors can be corrected.
Additionally, the presence of a remaining field distortion pattern
can be examined by using plots similar to those produced while investigating
coma. These patterns were investigated for dependence on magnitude, réseau,
measuring machine and measurer.
Using the information from these investigations, a plate model was
developed for each of the AC zones.
Equations (eq:xprime)
and (eq:yprime) show the corrected x,y values used in the final plate
adjustments.
x′= x+RD + TD + ME x + MC x
+ S x mx + MA x + FDP x
(21)
y′= y+RD + TD + ME y + MC y
+ S y my + MA y + FDP y
(22)
In Eqs. (eq:xprime) and (eq:yprime),
RD is the correction applied to compensate for radial
distortion; TD is the correction to compensate for tangential distortion;
ME and MC are corrections to compensate for magnitude equation and
x,y-dependent magnitude equation; S is the correction for coma; MA corrects for
measuring apparatus errors, and FDP compensates for any remaining field distortion
pattern. The variable m is the computed magnitude. By substituting
x′, y′ for x,y in Eqs. eq:xmodel1 and eq:ymodel1,
the 8 constants for each plate were computed.
The significant systematic errors found and corrected in each zone are listed in
Table tab:const.
7 Investigation of Discordant Data
Once a suitable plate model was determined, the computed positions were
used to investigate problems such as mismatched images, blended images
of multiple stars and typographical errors. Much of this work was
performed during the AC 2000.1 reductions; the description is
repeated here for completeness.
7.1 Incorrectly matched images from overlapping plates
Images may be incorrectly matched in a variety of ways. Images are originally
matched in the way described in the subsection ``Matching images with those on other
plates.'' Positions computed from the provisional plate constants can
change significantly due to the new reductions. So, images earlier believed to
be from two stars because their positions were dissimilar may now
be recognized as coming only from one if their new positions are closer.
To investigate this possibility, images closer together than a certain
distance were investigated. The distance chosen depended on the
range in plate epochs (since proper motion will then be a factor) as well as
accuracy of the x,y measures.
Images that fall outside the expected precision may be a result of
a double star, poor measurements of the same
star or good measurements of a star that has moved due to proper motion.
Deciding which was the case was often difficult because no
comparable catalogue with the epoch of the AC plates exists with which
to check the stars in question. The Preliminary Version of the Third
Catalogue of Nearby Stars (Gliese & Jahreiss 1991) aided in
detecting high proper motion stars, but this catalogue, as its name states,
contains only nearby stars. The Luyten's Two-Tenths Catalogue and its
supplement (Luyten 1979, 1980; Luyten & Hughes 1980) has also been used in
aiding in the identification of high proper motion stars, as was the
Hipparcos Catalogue.
The Washington Double Star
catalogue (WDS; Worley & Douglass 1996), the internationally recognized source
of visual double star data, was also used but is limited because of its
incompleteness over the AC magnitude range and its occasional use of AC data.
However, certain criteria were followed in deciding if two images were from
the same or different stars. These criteria were zone
specific, but for most zones images were identified as coming from the same
star if an image was within 3.5 arcsec of a mean position and there was no
indication of duplicity.
Images from different stars, but incorrectly identified as coming from
the same star, will result in a high standard deviation in right ascension
or declination, σα or σδ. Stars with the largest
σα and σδ were investigated for this possibility.
7.2 Duplicate entries
Virtually every zone has mistakenly printed some of its measures more than
once. These were easily found since the data in question was either exactly
the same as another record (in cases of true duplication) or the resulting
star positions of two records were ridiculously close for the telescope scale
and typical seeing. In general, images closer than 1.0 arcsec that appear
on the same plate were suspected of being duplicate entries. In cases of this
type, generally only one of the measures was kept.
7.3 Blended images
Images of double stars that are blended on one plate but discrete on another
require special treatment. Two possible problems exist under these
circumstances. First, if the blend is identified as one of the separate
images, then the computed separation of the double star will be smaller than
it is in actuality (since the blend will fall near the photocenter).
Second, if the blend is not identified with either
discrete image, then three sets of coordinates will be computed where only
two stars exist.
Under the first scenario (a blend is matched with a discrete image),
the data will consist
of a multiple star system with at least one star having a large
standard deviation of position, σα or σδ.
To investigate this possibility, the area around each star was searched for
the presence of another star. If a nearby star was
found, then the data were examined for a blend if σα or
σδ of either star exceeded a certain amount (usually
1.0 or 0.9 arcsec). Additionally, any two
stars closer than about 3.0 arcsec, or any triple or quadruple systems,
were examined.
Under the second scenario, (a blend is not identified with either of the
discrete images), then a star system
will appear to have one more member than it really has. To minimize this
occurrence, an area with a radius of about 10 arcsec
around each star was searched for the presence of two other stars.
Additionally, an area with a radius of about 15.0 arcsec was searched for three
other stars. If multiples were found, they were examined to ensure that
images were identified correctly. In the situation described above, blends
are discarded.
7.4 Large proper motion candidates
Due to the span of plate epochs, images from large proper
motion stars may be displaced from one another by several arcsec and hence
not be matched as coming from the same star. To minimize this occurrence,
a search in the Luyten's Two-Tenths Catalogue and its supplement
was made to identify high proper
motion stars. An additional search for large proper stars in the Third
Catalogue of Nearby Stars was also made. Stars in
the final catalogue having large standard deviations of position are known
high proper motion stars.
7.5 Typographical errors
A bright star may only appear to have one image if one of its records
contains a typographical error. To find and correct this, all bright,
single image
stars were investigated. (The exact magnitude limit depends on the zone.
Typically all stars down to 11.5 are investigated.)
The process involved generating positions for these images if one
altered one of the digits of the printed x or y value, then performing
a search within 2 arcsecs around these pseudo-positions. If another image was
found at one of the locations, a typographical error may be present.
For zones with x,y values published in millimeters, investigations
down to the tenths position were made (0.1 mm ~ 6 arcsec).
For those zones with x,y values published in réseau units,
investigations down to the hudreths position were made
(0.01 réseau unit ~ 3 arcsec). The
Twin Astrograph Catalogue (TAC; Zacharias et al. 1996),
was used to determine which of the two records, if either, should be changed
for all zones north of, and including, Tacubaya. For the more southerly zones
which are not covered by TAC, the Tycho Input Catalogue (Halbwachs et al. 1994)
has been used.
If a corresponding star was not found in the TAC or TIC, then a search in the
the Digitized Sky Survey using the Skyview interface was made to determine
which, if either, contains a typographical error.
To minimize the possibility of a printing error in the magnitude data,
a standard deviation of the magnitude, σ mag, was generated
from the computed magnitudes for every star appearing on more than one plate.
The stars with the
largest σ mag were investigated. Additionally, range checks were
made on the original data as well as the final computed magnitudes to ensure
that all values were reasonable.
7.6 Investigation of other potential problems
To investigate the possibility of a few plates being adjusted incorrectly
due to such oddities as several poor reference stars on a plate or
incorrectly removing accurate reference stars for poorer ones,
the positions of the 500 stars with the largest σα and
σδ were plotted for each zone.
Additionally, the percentage of stars on each plate whose σα
and σδ are in the highest 1% of that zone were computed. Plates
that have more of these than expected were investigated.
Positions of all stars were plotted to ensure that the data contain no missing
areas, such as a block of data having not been typed or accidentally
discarded. Positions of stars having only one image, and positions of stars
having multiple images, were plotted separately to investigate the
possibility of missing plates.
To avoid the presence of non-stellar objects in the final catalogue, a search
in the New General Catalogue of Nebulae
and Clusters of Stars (NGC), the Index Catalogue (IC), and the Second Index
Catalogue (Sinnot 1988) was made. The Digitized Sky Survey, using the
Skyview interface, was used to graphically show potential NGC and IC objects
present in the data. Those records found to be non-stellar NGC or IC objects
have been discarded.
8 Final Plate Model and Weights
It was necessary to ensure that the plate models developed in each
zone and the corrections applied to the data were still valid because
they were originally developed on the data that included erroneous
identifications
and typographical errors. Once any needed revisions were made, plate weights
were computed. For most of the AC zones, a plate's weight is
the inverse of the variance of the positions of the stars which it contains,
after removing the stars with the highest 1% σα and
σδ. The removal of these stars prior to computing the plate
weights reduced the possibility that a few, high proper motion stars
adversely affected the weighting of an entire plate.
No grating was used in the AC program, so bright stars are over-exposed on the
plates and should not be used in the final adjustment nor be included in the
final catalog. All stars with mean computed magnitudes brighter than
4.0 have been removed prior to final plate adjustments.
Also removed from determining the final plate parameters were reference stars
with residuals larger than 3.0 times the standard deviation of unit weight
of a plate solution. Once removed, the plate adjustment is performed again.
These stars are included in the
final catalog, but are treated as field stars.
9 Linking of Individual Zones
The Astrographic Catalogue was observed in discrete zones in the sky, and
the reductions, by necessity, were made on individual zones. However it
was desirable to link these together to make one, cohesive catalog.
To do this, stars in common to adjacent zones were identified. Blended
images, high proper motion stars, and typographical errors were investigated
as described above. Each star was assigned a new internal star number.
All images were combined to yield weighted mean right ascensions and
declinations on the system defined by Hipparcos (HCRS) at the weighted
mean epochs of observation.
9.1 AC 2000.2 plots
Figures fig:errra1 through fig:epoch1 clearly demonstrate
the zonal dependence
of the AC 2000.2. Figures fig:errra1 and fig:errdec1
show the average positional error for a single image with respect
to location on the sky. Of course individual errors will vary, and
many of the stars have more than one image. Figure fig:epoch1
provides information on the mean epochs of observation as function of
location of the sky. The combination of the positional errors and
epochs being a function of declination has consequences for any
proper motions derived using the AC 2000.2 data. That is, the proper
motion accuracies will also contain this dependence.
This was true for the now superseded ACT Reference and Tycho Reference Catalogues,
both of which utilized earlier reductions of the Astrographic Catalogue
data. It is also true for the Tycho-2 Catalogue, but
some of this zonal dependence was eliminated by the inclusion of additional
astrometric catalogues. Figure fig:zones1 shows, on the same
scale as Figs. fig:errra1 through fig:epoch1, the
observatories responsible for photographing and measuring each area of the sky.
The average density of the AC 2000.2 is 112 stars per square degree;
however, the sky is hardly uniform. The mean density as a function
of sky position is shown in Fig. fig:dens1. The galactic plane
is clearly visible, where some areas exceed 500 stars per square degree.
On the other extreme, for large areas surrounding the galactic poles,
the density drops by more than half of the average.
As a courtesy to the reader,
Figs. fig:errra1 through fig:dens1 are provided on this CD-ROM
in a larger format. They can be found on this directory in the files
errra1.ps, errdc1.ps, epoch1.ps, zones1.ps and dens1.ps; each is in postscript
format.
10 Cross-Referencing Information
The numbering between the AC 2000.1 and AC 2000.2 has been
maintained. It should be noted, however, that there is not a
strict one-to-one correspondence
between the two versions; the main reason is that some images now known
to be from high proper motions stars were not identified as such in AC 2000.1.
Note that AC 2000.1 contains 4,621,836 whereas AC 2000.2 contains
4,621,751 stars. However, the numbering between the two remains consistent.
In other words, the star numbered ``1'' in both versions refers to the same
star.
In order to aid users and to facilitate other work utilizing the
Astrographic Catalogue, most stars from the Hipparcos and Tycho-2
Catalogues have been identified. This cross-reference is not intended to be
100% complete; however, the vast majority of stars from these catalogues
are identified.
11 Description of AC 2000.2
11..1 Right ascension
The mean right ascension for each star as computed from its weighted images,
in units of hours, minutes and seconds of time, referred to the Hipparcos
system (HCRS, J2000.0) at the weighted mean epoch of observation.
11..2 Declination
The mean declination for each star as computed from its weighted images,
in units of degrees, minutes and seconds of arc, referred to the Hipparcos
system (HCRS, J2000.0) at the weighted mean epoch of observation.
11..3 B-Magnitude
The magnitude is either taken directly from the Tycho-2 Catalogue
or is an average of the computed magnitude based on the measured image
diameters from the AC plates. To determine which is the case,
one will need to check the
V-Magnitude field. All stars with the V-magnitude field non-blank
contain Tycho-2 photometry in both the B-Magnitude and
V-magnitude fields. Stars with the V-magnitude field blank
contain magnitudes based on the measured image diameters on the
AC plates, which should roughly correspond to the Tycho-2 BT system.
See the section ``Conversion of the AC Magnitudes'' for details.
11..4 Epoch
The mean epoch for each star as computed by its weighted images, in years.
11..5 Number of images used
The number of individual images used to compute position, magnitude (if
from image diameters), epoch and
standard deviation of position.
11..6 Standard deviation of weighted mean
The standard deviations of weighted means, σ\ and
σ\ are computed for every star with more than one
image. The formula used is:
σ2x = x – x2/N′– 1
(23)
where
x = αi, δi
(24)
and
N′≡N \left[ <w >2/<w2> \right]
(25)
where w is the weight of an individual observation (generally the same
for all stars on a plate).
11..7 AC 2000 number
This number is used in the reduction process to
identify all images of the same star that may appear on different
plates. This is generated at the U.S. Naval Observatory and
added to the original x,y data. When the x,y data are
released, these numbers can be used to link the data to the final catalog.
11..8 Hipparcos number
If a star has been identified as being in the Hipparcos Catalogue
then the Hipparcos number is provided. This cross-referencing
information is not 100% complete.
11..9 Tycho-2 number
If a star has been identified as being in the Tycho-2 Catalogue,
then the Tycho-2 identifier is provided. Zeros have been inserted
in blank fields. This cross-referencing information is not 100% complete.
11..10 V-Magnitude
The magnitude listed here is taken directly from the Tycho-2 Catalogue,
otherwise it is blank. There are 20 cases where the Tycho-2 BT
is given in the B-Magnitude column, but their is no corresponding
Tycho-2 VT magnitude. For these stars, the V-Magnitude is
set to .000 and a `3' is set in the Magnitude Flag field.
11..11 Magnitude Flag
This flag will give users caution regarding the photometry. A `1' is
given if the star has a B magnitude fainter than 13.5 or a V magnitude from
Tycho-2 VT fainter than 12.5.
A `2' is given if the star was identified
as a Tycho-2 star but the AC magnitude computed from the published diameters
is given. This is done when the Tycho-2 BT magnitude is either not
given or is listed as fainter than 14.00. No Tycho-2 VT
magnitudes are given
for these stars. In cases where both `1' and `2' are set, only
`2' is given. A `3' is given if the star was identified as a
Tycho-2 star, but there is no Tycho-2 VT magnitude. In these
cases, the Tycho-2 VT magnitude is set to .000. There are 20
such instances. Only 1 star (AC 408635, Tycho-2 107200084801) should
have a '3' flag in combination with another. For this star, the Tycho-2
BT is 13.51 and no Tycho-2 VT magnitude exists; the
flag is set to `3'.
11..12 Verification Flag
A `1' in this field indicates the star has a single image and is not
found in Hipparcos, Tycho-2, ACRS_1999 or the Hubble Guide Star Catalogue 1.2.
These ``stars'' may not exist but instead may be the result of typographical
errors, plate defects or other such blunders.
12 Participating Observatories
Work from the observatories that participated in the
photographing and measuring of the Astrographic Catalogue data are summarized
below. Some important characteristics of each zone can be found in
Table tab:obschar1 and in Appendix A. The telescope characteristics
agreed upon were a normal astrograph with an aperture of roughly 33 cm and
a this instrument having a focal length of 3.43 m. This created a scale
close to 60 arcsec/mm on the plates. All observatories, with the exception of
Nizamiah, used this type of instrument.
12.1 The Royal Observatory at Greenwich
The Royal Observatory at Greenwich was an original participating institution
in the Astrographic Catalogue project, sending representatives to the
International Congress on Astronomical Photography held in Paris in 1887. This
meeting outlined the plans for the Carte du Ciel project. Funding was provided
shortly following this meeting. A telescope built by Sir Howard Grubb
following the design agreed upon in Paris was delivered in May 1890.
Greenwich centered its plates between +65 and +90 degrees, with five plates
taken on the pole in different orientations.
In total, 1153 plates were exposed and measured in this zone. Three exposures
were made on each plate lasting six minutes, three minutes and 20 seconds.
The telescope was moved 20 arcsec between the exposures, so the six and
three minute exposures are offset from each other in declination, the six
minute and 20 second exposure are offset in right ascension.
The plate epochs span from 1892 to 1905.
After initially measuring some plates using micrometer screws, Professor H.H.
Turner realized that a different measuring technique was required if the job
were to be completed in a timely fashion. At his request, an eyepiece scale
type
measuring machine was built and greatly reduced the time to measure a plate.
Many other observatories adopted this measuring procedure.
Systematic measuring of the plates
began in October 1894. A duplex micrometer, which is a measuring machine capable
of measuring two plate simultaneously, was put into use in February 1895.
This aided the identification of images of the same star but on different
plates since it was possible to arrange the plates in the machine so that
the same field of sky was coincident under the measuring apparatus. (Remember
that each plate overlapped surrounding plates so that each area of sky appears
on at least two plates.) All plates were measured in two orientations, with
the plates being rotated 180 degrees between measurements. In all degree bands
except the +65, +66, and +67 degree bands, the same measurer was used for
both orientations. Both the six minute and three minute exposures were
measured for all stars that appeared on the 20 second exposure.
Details can be found in the introductions to the published volumes
(Christy & Dyson 1904-1932).
12.2 The Vatican Observatory
The Vatican Observatory, located in Vatican City, was founded
in 1888 while the Carte du Ciel program was in its infancy.
Vatican staff members realized that
participation in this program would immediately give their
young observatory international recognition.
Pope Leo XIII commissioned Father Francesco Denza and Father Giuseppe Lais
to attend the Astrographic Congress and enroll the Vatican as one of the
participating institutions.
After being accepted as a participant, the Vatican commissioned
the Henry brothers of France to build the telescope and P. Gautier to build
a machine to measure the stars on the plates.
Father Denza describes the finished telescope:
``The instrument consists of two parallel telescopes: The photographic
telescope
with an aperture of 33 cm and a focal length of 3.43 m; and the finding
telescope or collimator with 20 cm aperture and a focal length of 3.6 m. Both
are housed in a metal tube with a rectangular cross section of 37 by 68 cm.
Both objectives are fixed on the same block of bronze at one end of the
tube and at the other end is the photographic plate holder and the eyepiece
of the collimator. A thin metallic diaphragm separates the two telescopes.
...The photographic objective is a doublet of flint and crown and it is
both achromatic and aplanic for the most intense chemical rays of the
spectrum.'' (Denza 1891).
The telescope was installed in the Leonine Tower in 1891. This tower,
located on the highest point of Vatican Hill,
was originally constructed in 840 AD under Pope Leo IV as a
defense against the Saracen invasions. It is about 20 meters above
ground (about 100 meters above sea level) with walls about 4.5 meters in
thickness.
The Vatican was assigned the strip of sky between +55 and +64 degrees on
which to center the plates. In order to achieve the two-fold sky coverage
1040 plates would be needed. (Actually, 1046
plates were exposed and measured.) The job of photographing and developing
the plates was
carried out primarily by one person, Father Lais, who worked on this for
over 25 years until the time of his death in 1921. Lais's aid, Carlo
Diadori, completed the photographing in 1922.
For the first several years
that Father Lais was working at the telescope, no plates were being measured.
Father Johann Hagen,
appointed director of the Observatory
in 1906, was committed to seeing the project completed. He soon
realized that the machine built by Gautier to measure the
stars was too slow. After investigating different measuring
techniques employed by other institutions, he decided on the use of an eyepiece
grid. In the method used by Vatican, a 10 × 10 mm area of a plate,
corresponding to 2 × 2 réseau intervals, was magnified in a
microscope along with a grid which is segmented into 0.05 mm steps.
The edge of the grid was aligned with the edge of the
2 × 2 réseau interval
area. The location of a star within the magnified area was read from its
apparent position on the grid. This method was indeed
efficient, allowing the Vatican to be one of the first observatories to complete
its assigned zone despite utilizing minimal manpower.
However, the accuracy was not as high as can be obtained with the
measuring techniques used at other participating institutions.
Vatican was the only observatory in the Astrographic
Catalogue program to use the eyepiece grid technique.
Also during his trips to other observatories,
Father Hagen saw extensive use
of women in measuring the plates, freeing the full-time, male
astronomers from this time-consuming, repetitive task.
He brought in three nuns from the Instituto di
Maria Bambina to measure the plates. These nuns worked from 1910 to 1921 and
measured the vast majority of the Vatican data. The Oxford University
Observatory agreed to compute the plate constants used to convert the
rectangular measures to equatorial coordinates.
For additional information concerning the history of the Vatican Observatory
and its personnel, see the book In the
Service of Nine Popes (Maffeo 1991). Additional details regarding the
Vatican's participation in the Astrographic Catalogue project
can be found in the introductions to the data (Vatican 1914-1928), written
in Italian.
12.3 The Catania Observatory
The Catania University Observatory, located in Catania, Sicily, centered
its plates between +47 and +54 degrees.
In total, 1010 plates were exposed and measured in this zone.
The epochs span from 1894 to 1932, but over 95% were exposed prior to 1906.
All the plates were measured using the short-screw method. Additional
details can be found in the introductions to the published data
(Catania 1907-1963).
12.4 The Helsingfors Observatory
The Helsingfors Observatory, located in Helsinki, Finland, centered its plates
between +40 and +46 degrees.
In total, 1008 plates were exposed and measured in this zone.
The epochs span from 1892 to 1909, but over 94% were exposed prior to 1897.
All the plates were measured using the short-screw method.
One screw was used; the plates
were rotated 90 degrees to measure both x and y coordinates. The plates
measured early in the work had images from both the longest and middle exposures
measured in one orientation. Later (after 1896), only the images from the
longest exposure were measured, but the plates were rotated 180 degrees between
measurements so these images were measured twice (this also helped remove any
bias a measurer may have).
Various aspects of the work are detailed in the introduction to Volume 1
(Helsingfors 1903-1937).
12.5 The Potsdam Observatory
In 1887, at the meeting of the International Congress which established the
Astrographic Catalogue, the zone with plate centers from +39 to +32 degrees was
assigned to the Potsdam Observatory, Germany. Potsdam
began photographing the plates by 1893 and 1226 plates were
exposed by the end of 1900. Each plate had two exposures of 5 minutes duration.
Unfortunately, the measurements of the plates lagged
behind the exposures. By the start of World War I only 406 plates were
measured. Following the war, Potsdam announced it could no longer continue
with the project, and a re-photographing of its zone was made by Oxford,
Uccle, and Hyderabad. The remaining plates were never measured.
An allied bomb during World War II destroyed virtually the entire set of
plates (Dick 1988, 1990).
In total, 406 plates were measured and published as the Potsdam zone.
These plate are scattered throughout the zone, so many are not overlapped
by others. All were measured using one of two short-screw
type measuring machines. The plates were measured in one direction only;
the plates were not rotated.
Various aspects of the work are detailed in the introductions
of the printed volumes (Potsdam 1889-1915).
12.6 The Nizamiah Observatory, Hyderabad
In 1887, at the meeting of the International Congress which established the
Astrographic Catalogue, the
zone from –17 to –23 degrees was assigned to the Observatory of Santiago, Chile.
By 1900, the work was still not progressing, so a proposal to establish an
observatory in Montevideo, Uraguay was made. This, too, did not progress so
Santiago asked to re-undertake the project. At the same time, the
Nizamiah Observatory, located in Hyderabad, India, offered to work on this
zone as well. So, in 1909 there were two observatories offering to work on
the –17 to –23 zone. A resolution passed by the Congress in 1909 assigned the
–17 to –20 zone to Hyderabad. After completing the photographing and
measuring of these four bands in 1920, the International Astronomical Union
recommended that Hyderabad continue photographing down to –23 degrees
declination.
This work was completed in 1928. This zone between –17 and –23 degrees is
known as the Hyderabad South zone. Hyderabad also observed a section of
the sky in the Northern hemisphere that Potsdam was originally assigned. This
zone, between +36 and +39 degrees, is known as the Hyderabad North zone.
In total, 1260 plates were exposed and measured in the Hyderabad South zone.
The epochs span from 1914 to 1928, with only a handful taken after 1923.
For the Hyderabad North zone, 592 plates were exposed and measured.
The epochs span from 1928 to 1937, with just a very few taken after 1934.
The telescope used was not one of the Henrys' design, as all the other AC
participation observatories used. Instead of a 33 cm, the Hyderabad
instrument, built by Cooke and Sons of York, had an aperture of only 20 cm.
Its objective was described as a
``patent photo-visual lens''. The smaller aperture meant longer exposures
were required to achieve the desired magnitude limit set for the AC.
The telescope's focal length was 133 inches.
All the plates were measured using one of four eyepiece scale
type measuring machines, all built by Cooke and Sons.
Various aspects of the work are detailed in the introductions
of the printed volumes (Edinburgh 1918-1930, London 1934-1946).
12.7 The Uccle Observatory
The Royal Observatory of Belgium, located in Uccle, was assigned the zone
with plate centers running from +34 to +35 degrees. Although Uccle
was not an original participating observatory in the AC project, it became
one because the Potsdam Observatory, originally assigned to cover
this area, was unable to fulfill its commitment. The
telescope used was of similar design as the Henry brothers, but built by
Gautier. In total, 320 plates
were exposed between 1939 and 1950. The epochs of the plates are spread fairly
uniformly, except for a lack of plates exposed between mid-1943 and mid-1945.
The measurements took place at the Paris Observatory with the use of three
short-screw measuring machines.
The réseau used was one from the Toulouse Observatory.
An introduction can be found in Volume 1 of the printed catalog
(Paris 1960,1962).
12.8 The Oxford Observatory
The University Observatory at Oxford was originally assigned the zone +25 to
+31 degrees on which to center the plates. This is known as the Oxford I
zone. An additional
zone with plates centered on +32 and +33 degrees declination was photographed
at Oxford after Potsdam announced they would not be able to complete their
assigned area (+32 to +39). This two degree band is referred to as the
Oxford II zone.
The telescope used was the same
design as the Henry brothers' instrument located in Paris. The Oxford
lens was made by Sir Howard Grubb and attached to an existing Grubb 12
1/4 inch, which was utilized as a guiding instrument. All plates
of the Oxford 1 zone
were taken between mid-1892 and 1910, with over 80% exposed by the
end of 1903. These plates were measured mostly by boys from the New College
Choir School, and Mr. T. J. Moore, a gardener who was interested in astronomy.
The measuring apparatus was designed with an eyepiece scale, similar
to that employed
by the Greenwich Observatory in their AC work, with the exception that only one
plate was measured at a time whereas Greenwich measured two.
Mr. F.A. Bellamy of University Observatory at Oxford supervised much of the
work in photographing the 320 plates required to complete the Oxford 2
zone. He employed the same techniques used in the Oxford 1 zone
and used the same Grubb refractor operated 30 years earlier.
Mr. Bellamy would have photographed the entire zone himself, however he died
with 32 fields left unobserved. These 32 fields were exposed at the
Royal Observatory at Greenwich by Mr. H.G.S. Barrett with the telescope used
for the Greenwich Zone (+65 to +90). These last 32 plates were the only
plates in the Astrographic Catalogue that did not have a
réseau exposed on them, however they were measured with one clamped on
the glass.
All plates of the Oxford 2 zone were exposed between 1930
and 1936. (Actually there are two plates that appear to have typographical
errors in their epochs. One is dated in 1918; the other in 1930, but 8 months
prior to any other plate.) Mr. Barrett also supervised the
measurement and reduction of these plates at Oxford, using the
eyepiece scale technique.
However World War II intervened preventing
the determination of the plate constants for 92 of the fields. After the
war, the constants for these remaining plates were determined under Dr. H.
Kox at the Hamburg Observatory at Bergedorf.
A detailed introduction covering the participation of the
University Observatory at Oxford in the Astrographic Catalogue project
can be found in Volume 1 of the Oxford I zone catalogue (Turner 1906-1911),
as well as in the book The Great Star Map also by Turner. An
introduction to the
Oxford II zones can be found in Volume 1 of the Oxford II zone catalog
(Paris 1953-1954).
12.9 The Paris Observatory
The Paris Observatory agreed to photograph the zone with plate centers
between declinations
+18 and +24 degrees. The telescope used was the original Henry brothers'
instrument, after which all other AC telescopes were supposed to be patterned.
In total, 1261 plates were photographed and measured. The measuring technique
employed at Paris was the short-screw method, which was the
most accurate utilized with the AC plates.
The epoch range of the plates are
from October 1891 through November 1927, however only 7 plates were exposed
after 1907 and all but 100 were exposed prior to 1900.
An introduction to the Paris Observatory's participation in the Astrographic
Catalogue can be found in the introductions to the individual volumes
(Paris 1902-1932).
12.10 The Bordeaux Observatory
The Bordeaux University Observatory, located in Floirac, France,
was assigned the zone between +11 and +17 degrees
declination on which to center its plates. The telescope used was a similar
design to the Paris instrument and was built by the Henry brothers.
In total, 1260 plates were exposed between 1893 and 1925, all but five being
taken before 1913. The plates were measured at Bordeaux using the
short-screw method.
An introduction to the Bordeaux Observatory's participation in the
Astrographic Catalogue project can be found in Volume 1 of the published data
(Paris 1905-1934).
12.11 The Toulouse Observatory
The Toulouse University Observatory (France)
agreed to participate in the Astrographic
Catalogue project by taking plates centered between +5 and +11 degrees
declination. In total, 1260 were exposed and measured. The epochs vary
from 1893 to 1935, and were taken in three fairly distinct groupings. Most
were exposed before the end of 1910. Another set is taken between 1918 and
1922. The last few plates are scattered between 1930 and 1935. All the
plates were measured using a short-screw type measuring machine;
however, not all plates were measured at Toulouse. Ninety plates were measured
at the Bordeaux University Observatory and 36 were measured at the Paris
Observatory.
Various aspects of the work are detailed in the introductions
of the printed volumes (Paris 1903-1948).
12.12 The Algiers Observatory
The Algiers Observatory was assigned the zone between –2 and +4 degrees on
which to center its plates. All 1260 plates were exposed
between 1891 and 1911. The plates were measured using the
short-screw method.
Details about the Algiers Observatory's participation in the
Astrographic Catalogue project can be found in the introduction to the
catalogues (Trépied 1903,Paris 1903-1924).
12.13 The San Fernando Observatory
The Naval Observatory of San Fernando (Spain) was assigned the area between
–3 and –9 degrees declination on which to center its plates. The telescope
used was built by Gautier, with the objective made by the Henry brothers.
All of the 1260 plates were exposed between 1891 and 1917. Over 1000 were
taken before 1899, then only a few per year until 1917. The plates were
measured using a short screw micrometer.
An introduction to San Fernando's participation in the Astrographic Catalogue
project can be found in Volume 1 of the published data (San Fernando 1921-1929).
12.14 The Tacubaya Observatory
At the meeting of the International Congress which established the
Astrographic Catalogue, the
zone from –10 to –16 degrees was assigned to the National Astronomical
Observatory of Tacubaya, located near Mexico City, Mexico.
In total, 1260 plates were exposed and measured in the Tacubaya zone.
(Actually, one of the plates had an incorrect and unknown plate center and
has been discarded from the reductions, leaving a total of 1259 plates).
All but five plates were exposed between 1900 and 1912. The five later
plates were exposed between 1926 and 1938.
All the plates were measured using the eyepiece scale method.
An introduction to the history
of the work can be found in Volume 1 part 1 of the –15 degree zone
(Tacubaya, 1913-1962).
12.15 The Cordoba Observatory
At the 1887 Paris meeting, the
zone from –24 to –31 degrees was assigned to the La Plata Astronomical
Observatory. In 1900, the zone was re-assigned to Cordoba. The telescope
was installed at the end of 1901. In 1908, Dr. Thome, the Director of Cordoba,
died and Dr. Perrine was named new director. After some investigations,
Perrine decided to re-observe all areas. He discovered that the telescope
was out of focus and many of the plates were impaired, and that the plates
were centered on apparent place coordinates, not at those defined at
equinox 1900. Plates of this series were exposed starting in 1909 and
finished by the end of 1913. These plates were measured between 1909 and
1920. In total, 1360 plates were exposed as part of the Astrographic
Catalog.
The telescope used was one of the Henrys' design and build, following the
standard with a 33 cm
aperture and 3.47 m focal length. In the introduction written by
Perrine, he states that the guiding is difficult because the guide scope has
only a 19 cm aperture, as opposed to the more conventional 25 cm in use
by most of the participating observatories. The mount was built by Gautier.
Some plates showed a ``triangular distortion'' that was traced to a warp in the
ring which held the lens in place. This ring was replaced in 1911. The
lens, from August 9, 1910 until the end of the program, was stopped down to
11 inches. This, according to Perrine, greatly improved the image quality.
Virtually all plates were taken and developed by R. Winter or F.P. \linebreak Symonds.
Four exposures on each plate were made; two long exposures of the same
duration (both of 5 or 6 minutes) one medium exposure (of 60 to 90 seconds)
and one short exposure (of 5 to 8 seconds). The telescope was moved in
declination between exposures. In order to expedite the work, two measuring
machines of the short-screw type were retro-fitted with eyepiece
scales. As a result, 140 plates were measured using the short-screw
method, the remaining 1220 plates were measured using the eyepiece
scale method. In total, five different
measuring machines were used, allowing each measurer to have his or her
own machine.
The réseaux used were supplied by Gautier and Prin; four were used
throughout the work.
Investigations of two of the réseaux were made in
Paris and the deviations were found to be negligible; no corrections
for the réseaux were applied to the measures.
Only stars within one degree in right ascension and declination of the plate
center were measured. All stars having three images were measured
unless images ran together, which was the case of
the brightest stars. Four measures were made on each star; a measure was made
on both of the long exposures in both orientations of the plate.
(Following an initial measurement of all stars, the plate was reversed 180
degrees and all stars re-measured.)
In general, measures in the direct and reverse
orientations of the plate were made on the same day by the same measurer.
In total, 37 man-years went into measuring the plates.
Various aspects of the work are detailed in the introduction of
Volume 26 of the Observatory Results (Cordoba 1925-1934).
12.16 The Perth Observatory
At the International Congress which established
the Astrographic Catalogue, the Observatory
of Rio de Janeiro was assigned to photograph the area between –32 and –40
degrees declination. In 1900, the work at Rio had not progressed and so the
Perth Observatory undertook the task. The telescope used was from Sir
Howard Grubb, and was of similar design to other telescopes used for the
AC work.
Although observing was progressing, no resources were available to measure the
plates. At this time, the Perth Observatory was primarily a meteorological
station, and the meteorological work took precedence. This changed
in 1908 when the Australian Federal Government established the
Australian Weather Bureau. About this time, four women were hired as plate
measurers. Professor Dyson of the Edinburgh Observatory
offered assistance in the measuring. Perth accepted this offer and started
sending those plates of the –40 degree zone.
By 1915, the Edinburgh
Observatory completed its commitment by measuring all of the plates centered
on the –40, –39 and –38 degree
band. This area is known as the Perth-Edinburgh AC zone.
Observing continued at Perth until 1919, at which time all areas had been
photographed. Personnel at Perth measured all plates between –37 and –32
degrees; this is known as the Perth zone.
In total, 432 and 944 plates make up the Perth-Edinburgh and Perth zones,
respectively.
All plates had three exposures taken, one of 4 minutes, 2 minutes and
13 seconds, or that of 6 minutes, 3 minutes or 20 seconds. The change in
exposure times took place following a 1909 meeting of key personnel from
different observatories participating in the AC. At that meeting, many
people expressed concern about the uniformity of limiting magnitudes on
different plates. Many of the Perth plates were re-examined and found
to be unsatisfactory. The areas affected were re-observed and a method
of ensuring more uniformity was developed.
In general, no guiding of the instrument other than the sidereal drive
was made, as it was found unnecessary. All plates were the brand
Ilford ``special rapid''. All were measured using an eyepiece scale machine,
similar to the Oxford Observatory's. For all but 5 plates, the measurements
done at Perth were made in two orientations of the plate by the same
person; the plate being rotated 180 degrees between measurements. In total,
10 measurers were used at Perth.
For the plates measured at Edinburgh, images were measured in two orientations
of the plate, with the plate
being rotated 180 degrees between measurements. For the direct orientation,
the second exposure (either 2 or 3 minutes) was measured; the longest
exposure was measured with the plate in reverse orientation. Quite often
more than one measurer was used on each plate.
Various aspects of the work can be found in the introduction to Volume 23
of the Perth-Edinburgh data (Perth 1922, Paris 1949-1952) and in Volumes
1 and 17 of the Perth data (Perth 1911-1921).
12.17 The Royal Observatory at the Cape of Good Hope
The Royal Observatory at the Cape of Good Hope, South Africa, took the zone
between –41 and –51 on which to center its plates. In all, 1512 plates
were exposed and measured. The epochs range from 1897 to 1912, with 97%
of them being exposed prior to 1906. The telescope used was built by
Sir Howard Grubb. Two machines were
used to measure the plates, both were designed by David Gill and built by
Repsold of Hamburg (Gill 1898).
Both machines employed the short-screw measuring method and
were of similar design.
An introduction to the participation of the Royal Observatory at the
Cape of Good Hope in the Astrographic Catalogue
project can be found in Volume 1 of the published data (London 1913-1926) .
12.18 The Sydney Observatory
H.C. Russell, the Government
Astronomer at Sydney, was in attendance at the 1887
meeting of the International Congress. He committed the Sydney Observatory
to photograph the sky between declinations –52 and –64 degrees.
The lens used for the project was built by Howard
Grubb of Dublin, and it followed the general design established by the
International Congress. The lens was
delivered in December of 1890. Most of the telescope was built in Sydney.
The observing program began in earnest
in 1892. The telescope was moved twice in the course of the AC work; first
from inside Sydney to Redhill (located about 12 miles from Sydney) in 1899,
and then back to its original location in 1931. The observing was left
unchanged until 1912, when W.E. Cooke took over the project. He was unsatisfied
with the quality of many plates and eventually rejected and rephotographed many
of the areas. Until this time, plates were being sent to Melbourne for
measurement, but under Cooke the Sydney Observatory began to measure their own
photographs. Prior to Cooke's arrival, plates were positioned so the center
of the plate was in the sharpest focus (The astrograph used in photographing
the plates did not have a flat field of focus, so some areas of the plate
are in focus while others are not. This is true for all the telescopes used
in the Astrographic Catalogue work). Cooke altered this and made the ring
about 50 arcmin from the center the place with the sharpest focus. In 1926,
Cooke retired and James Nagle took over. He altered the place of best focus
in a ring about 40 arcmin from plate center. Nagle died in 1941, and
H.W. Woods took over. Some plates were found unsatisfactory or missing. The
remaining areas were photographed between 1944 and 1948.
In total, 1400 plates were taken as part of the Astrographic Catalogue.
All plates exposed between 1890 and 1930 were taken by James Short.
Plate measuring did not start for about six years after the first plates
were taken. Evidently there was a suggestion about having all plates from
all participating observatories sent to Paris for
measurement. This plan was not ever put in place, but the Australians liked
this idea so they decided that Melbourne would be used to measure both the
Sydney and Melbourne plates. As mentioned above, this changed with Cooke's
arrival and Sydney started measuring their own plates. There were four
short-screw measuring machines used in Melbourne, and two eyepiece
scale machines used at Sydney.
From this point on, all stars were measured twice with the plates being
rotated 180 degrees between measurements.
Various aspects of the work are detailed in the introduction of
Volume 53 of the data (Sydney 1925-1971).
12.19 The Melbourne Observatory
In 1887, following the meeting which established
the Astrographic Catalogue, the British government agreed to have the
Melbourne Observatory participate in the project. Melbourne was assigned
the zone from –65 to –90 degrees. The telescope used was built by Howard
Grubb of Dublin, and it followed the general design established by the
International Congress. The telescope was
delivered in December of 1890. The observing program started about one year
later in January 1892 and continued until 1927 (Actually, one plate was
exposed in 1940). Over 80% of the plates were exposed prior to 1898.
In total, 1149 plates were taken as part of the Astrographic Catalogue.
All plates had three exposures of 5 minutes,
2.5 minutes and 20 seconds duration, with the exception of the plates taken
prior to February 26, 1892, whose exposures were slightly longer. The
réseau was exposed on the plates shortly after the plates were removed
from the telescope.
In total, six different réseaux were used during the
program; three were supplied by Gautier and three were made at Melbourne.
Plate measuring did not commence in earnest until November 1898, when six
women were hired at Melbourne. Two measurers were used for each plate, one
taking the northern half and one the southern. The plates were rotated
180 degrees and each half was remeasured by the same person.
All reference stars on each plate were measured
by both measurers.
Four measuring machines were used throughout most of the work; all used
short-screws for the star measurements.
The Melbourne staff tried using
an eyepiece scale measuring machine for the plates but found the measuring
error too high.
Periodic and progressive screw errors were investigated.
These were not applied, as they were found to be negligible by the Melbourne
astronomers. (Investiations of the screw errors performed at the
U.S. Naval Observatory as part of the AC 2000 work show this not to be
true.) Investigations into errors of the réseaux were made and these
corrections were applied to the data prior to publishing.
Various aspects of the work are detailed in the introduction of
Volume 1 of the data (Melbourne 1926-1929; Paris 1955-1958; Sydney 1963).
13 References
Catania, 1907-1963, Catalogo Astrofotografico Internazionale 1900
Vol 1 through 8
Christy, W.H.M., and Dyson, F.W., 1904-1932, Astrographic Catalogue 1900.0,
Greenwich Section Vol 1 through 6
Corbin, T.E. and Urban, S.E., 1988, IAU Symposium 133 Mapping the Sky,
ed. S. Debarbat, J.A. Eddy, H. K. Eichhorn and A. R. Upgren,
Kluwer, Dortrecht, p. 287
Corbin, T.E., and Urban, S.E., 1990, IAU Symposium 141 Inertial Coordinate
System on the Sky, ed. J. Lieske and V.K. Abalakin
Kluwer, Dortrecht p. 433.
Luyten, W.J., 1979, 1980, New Luyten Catalogue of Stars with Proper Motions
Larger than Two Tenths of an Arcsecond (Minneapolis: University of Minnesota)
Luyten, W.J. and Hughes, H.S., 1980, Proper Motion Survey with the
Forty-Eight Inch Schmidt Telescope. LV. First Supplement to the NLTT Catalogue
(Minneapolis: University of Minnesota)
Maffeo, S. 1991, In the Service of Nine Popes: 100 years of the Vatican
Observatory, Vatican Observatory Foundation
Melbourne, 1926-1929, Melbourne Astrographic Catalogue 1900.0, Vol 1 through 3
Paris, 1902-1932, Catalogue Photographic du Ciel, Vol 1 through 7
Paris, 1903-1924, Observatoire D'Alger Catalogue Photographique du Ciel, Vol 1
through 7
Paris, 1903-1948, Observatoire du Toulouse, Catalogue Photographique du Ciel,
Vol 1 through 7
Paris, 1905-1934, Observatoire de Bordeaux, Catalogue Photographique du Ciel,
Vol 1-7
Polozhentsev, D., Potter, K., Sal'Es, R., Selaiia, K., & Bystrov, N.,
1989, Astronomicheskii Zhurnal 66 425
Potsdam, 1889-1915, Publicationen des Astrophysikalishen Observatoriums zu
Potsdam, Photographische Himmelskarte Catalog, Vol 1 through 7
San Fernando, 1921-1929, Catálogo Astrográfico para 1900.0,
sección del Observatorio de Marina de San Fernando, Vol 1 through 8
Sinnot, R.W., 1988, NGC 2000.0, The Complete New General Catalogue and Index
Catalogue of Nebulae and Star Clusters by J. L. E. Dreyer, ed. R. W. Sinnott,
Sky Publishing Corporation and Cambridge University Press
Sydney, 1925-1971, Astrographic Catalogue 1900.0, Sydney Section, Vol 1
through 53
Worley, C.E. and Douglass, G.G., 1996, The Washington Double Star Catalog
Zacharias, N., de Vegt, C. Nicholson, W. and Penston, M.J., 1992,
A&A 254 397
Zacharias, N., Zacharias, M.I., Douglass G.G., and Wycoff, G.L., 1996,
AJ 112 (5), 2336
Zacharias, N. & Zacharias M., 1999, AJ 118 2503
Zacharias, N., Zacharias M., & de Vegt, C., 1999, AJ 117 2895
A Notes on Participating Observatories Data Characteristics
Greenwich notes:
Five plates are centered on the pole.
A duplex micrometer was used; two plates were measured simultaneously.
Formats of the published volumes reflect this.
Vatican notes:
One plate has a published epoch of 1891.
A measuring grid was used instead of eyepiece scale or screw.
Most plates were measured by one of three women. Measurer ``E'', who measured
about 50% of the plates, was significantly worse than the other two.
Plate weights, computed at USNO, are based on measurer.
Catania notes:
Over 95% of the plates were exposed prior to 1906.
Errata are found throughout volumes, mostly at the end of each plate's
x,y data.
Measures of large images (bright stars) are often listed many times
(with slightly different values).
Multiple stars were often measured both as center of light AND individual
images.
One plate has been taken with the same plate center as another, but only
faint stars measured on the second. Hence only reference stars near the
edges were available on that plate.
Many typographical errors in the printed volumes were found by USNO. These
have been corrected.
Helsinki notes:
Over 95% of the plates were exposed before 1897.
Helsinki was under Russian rule during the period of exposure and
measurement of the plates.
Potsdam notes:
The number of plates exposed was 1226, of which only 406 were
measured and published. No funds after World War I were available to continue
the work.
All but 34 plates were destroyed during World War II.
In cases where stars were measured and published twice, USNO used the
mean of the measures in the reductions.
Personal equation tables (describing observer dependent magnitude equation)
are published. USNO developed its own corrections by observer that supersedes
the Potsdam tables.
Generally, only stars brighter than 11.0 were measured
Plate weights, computed at USNO, are based on measurer.
Hyderabad North notes:
Nizamiah Observatory had the only instrument not of standard design.
Objective diameter was 20 cm (not 33 cm) so longer exposures were required
to achieve the desired limiting magnitude. The plate scale was approximately
61 arcsec/mm.
This zone was photographed in response to the
Potsdam region not being completed.
India was under British rule during the period of exposure
and measurement of the plates.
Uccle notes:
This zone was photographed in response to the
Potsdam region not being completed.
One plate was broken prior to diameter measurements.
Measurements were made at Paris.
Hour angles of exposures are not given.
Oxford II notes:
This zone was photographed in response to the
Potsdam region not being completed.
Thirty two plates in this zone were exposed and measured at Greenwich. These
are the only plates of the AC without a reseau printed on them.
One plate has a published epoch of 1918, 12 years prior to the
supposed start of this region.
Kox (Hamburg Observatory) determined plate constants for 92 of the plates.
All others were computed at Oxford.
Oxford I notes:
Some reseaux had rulings of 5.04 mm (5.00 mm was standard).
Introductions indicate which ones, but the introduction appears to be wrong
for 3 plates.
Paris notes:
Only 7 plates were exposed after 1907.
Corrections indicated in the ARI version (1991) of the keypunched data
have been applied.
Additional typographic errors found in Bull. of CDS 12 (p32-40) by Dunham
and Herget were applied, where needed.
One plate was printed with the x,y measures of two plates (see end of
Volume 2).
Nine plates were measured using Baillaud method, not utilizing the
reseau. Since the field distortion pattern applied during
USNO's redudction is believed to arise from the reseaux, these
corrections were not applied to these nine plates.
Notes indicate that several stars were not measured to their full
precision (0.1 microns). Images were not kept if the x,y values were not
given to 1.0 micron or better.
Bordeaux notes:
Both Toulouse and Bordeaux have plates centered on +11 degrees.
Only five plates were taken after 1913.
Corrections indicated in the ARI version (1991) of the keypunched data
have been applied.
Many more typographical errors were found by USNO in this zone than
most other zones.
Toulouse notes:
Both Toulouse and Bordeaux have plates centered on +11 degrees.
Ninety plates were measured at Bordeaux and 36 plates were measured at
Paris.
Corrections indicated in the ARI version (1991) of the keypunched data
have been applied.
Notes indicate that several stars were not measured to their full
precision (0.1 microns). Images were not kept if the x,y values were not
given to 1.0 micron or better.
The odd zones(+5,7,9 and 11) are split. The
plates of the first part (usually 0 to about 6h)
do not have reseaux corrections applied, but information
was given as to which
measuring machine (1 or 2) and which reseau was used. After the split,
there is no information regarding micrometer or reseaux.
For the even zones, there is information regarding where the plates were
measured and often what types of plate emulsions were used. However, there
are no data regarding which reseau was used.
Algiers notes:
Algeria was under French rule during the AC data acquisition.
San Fernando notes:
Y-coordinate increases to South
80% of the plates were taken before mid-1898.
One plate was exposed with Saturn and 3 of its satellites.
Four records have footnotes saying ``body moved during exposure of the
plate''.
Tacubaya notes:
All but five plates were taken before 1913.
One of the plates was not used by USNO because it has an unknown plate
center.
Many typographical errors in the printed volumes were found by USNO.
These have been corrected.
The type is very poor in some of the volumes.
Hyderabad South notes:
The Nizamiah Observatory had the only instrument not of standard design.
The objective's diameter was 20 cm (not 33 cm) so longer exposures were
required to achieve the desired limiting magnitude. The plate scale
is approximately 61 arcsec/mm.
India was under British rule during the program.
Five reseaux were used, some with non-standard rulings.
Rulings of 5.00, 5.04 and 4.985 mm used.
Y increases to South.
Cordoba notes:
1220 plates were measured using an eyepiece scale, 140 were measured with
a short-screw.
Errata were found in Information Bulletin of CDS 22 79-86 (1982).
Lens was stopped down to 28 cm in August 1910 to give better images.
Reseaux with rulings of 5.00 mm were used for all plates except those
numbered 2001 through 2290. These have 5.05 mm between rulings. Note that
this is slightly different than what is given on page xviii of Volume 26.
Many typographical errors in the printed volumes were found at USNO.
These have been corrected.
Perth notes:
This zone covers only those plates that were both photographed AND
measured at Perth (see Perth-Edinburgh notes).
A change of scale for plate 2063 through 2209 is present. This was
the result of someone altering the telescope focus.
A Scale value, usually given as a letter, is used as an estimate of
magnitude for all but the brightest stars.
Very few typographical errors were found at USNO.
Perth-Edinburgh notes:
This zone covers plates taken at Perth, but measured at Edinburgh,
Scotland.
Very few typographical errors were found at USNO.
Y increases to South.
Cape notes:
Images measured with micrometer one have significantly lower mean
errors than those measured with micrometer two.
Actual diameters were measured for many stars, but the faintest stars
were given a ``density'' measurement, dependent on how dark each image
appears.
South Africa was under British control during the AC work.
Sydney notes:
The telescope was relocated twice during the AC work.
The plate location of sharpest focus altered during the work, depending
on observatory director (see Section 12).
Plates measured prior to 1912 were sent to Melbourne for measurement,
using screw method. Later, the plates were measured at Sydney with
an eyepiece scale.
The plates were exposed in three discrete epoch spans, 1892 to 1901,
1918 to 1929, and 1944 to 1948.
Many typographical errors in the printed volumes were found at USNO.
These have been corrected.
Authors suspect a magnitude equation by measurer, but no identification
of the measurers of individual plates is given.
Scale value as estimate of magnitude is given for the majority of images.
X increases to the West
Melbourne notes:
Over 80% of the plates were taken before 1898.
One plate has an epoch of 1940.
Additional Melbourne errata were found in Sydney Volume 53, pg 64.
Byte-by-byte description of AC 2000.2 using format
requested by the CDS.
Bytes
Format
Units
Label
Explanations
1- 2
I2
h
RAh
Right Ascension (hours)
4- 5
I2
min
RAm
Right Ascension (minutes)
7-12
F6.3
s
RAs
Right Ascension (seconds)
14
A1
—
DE-
Declination (sign)
15-16
I2
deg
DEd
Declination (degrees)
18-19
I2
arcmin
DEm
Declination (minutes)
21-25
F5.2
arcsec
DEs
Declination (seconds)
27-31
F5.2
mag
B(mag)
Magnitude - Tycho-2 BT if col 86-91 are
non-blank; else from image diameters
33-40
F8.3
yr
Ep
Mean epoch of position
42-43
I2
—
Num
Number of images used
44-49
F6.3
arcsec
e_RAs
?Standard deviation of mean, RA
50-55
F6.3
arcsec
e_DEs
?Standard deviation of mean, Dec
57-64
I8
—
AC2000
AC 2000 Number
66-71
I6
—
HIPP
?Hipparcos Number
73-84
A12
—
TYCHO-2
?Tycho-2 ID
86-91
F6.3
mag
V(mag)
?Magnitude - Tycho-2 VT else blank
92-92
I1
—
MGFL
?Magnitude flag
93-93
I1
—
VER
?Verification Flag
Characteristics of each observatory's plates
Zone
Country
declination
mearuring
num of
epoch
or region
of centers
technique
plates
Greenwich
England
+90 +65
scale
1153
1892 1905
Vatican
Italy
+64 +55
grid
1046
1895 1922
Catania
Sicily
+54 +47
screw
1010
1894 1932
Helsinki
Finland
+46 +40
screw
1008
1892 1910
Potsdam
Germany
+39 +32
screw
406
1893 1900
Hyderabad N
India
+39 +36
scale
592
1928 1938
Uccle
Belgium
+35 +34
screw
320
1939 1950
Oxford II
England
+33 +32
scale
320
1930 1936
Oxford I
England
+31 +25
scale
1188
1892 1910
Paris
France
+24 +18
screw
1261
1891 1927
Bordeaux
France
+17 +11
screw
1260
1893 1925
Toulouse
France
+11 +5
screw
1260
1893 1936
Algiers
Algeria
+4 –2
screw
1260
1891 1912
San Fernando
Spain
–3 –9
screw
1260
1891 1918
Tacubaya
Mexico
–10 –16
scale
1259
1900 1938
Hyderabad S
India
–17 –23
scale
1260
1914 1929
Cordoba
Argentina
–24 –31
both
1360
1909 1913
Perth
Australia
–32 –37
scale
944
1902 1919
Perth-Edinb.
Australia
–38 –40
scale
432
1903 1914
Cape
S. Africa
–41 –51
screw
1512
1897 1912
Sydney
Australia
–52 –64
both
1400
1892 1948
Melbourne
Australia
–65 –90
screw
1149
1892 1928
Numbers of images in each zone and institutions aiding in keypunching
the data, by zone
Zone
stars
images
keyed and verified
(thousand)
(thousand)
Greenwich
179
322
USNO
Vatican
256
480
CIDA, Venezuela (verified USNO)
Catania
163
320
USNO
Helsinki
159
284
USNO
Potsdam
108
143
CIDA, Venezuela (verified USNO)
Hyderabad N
149
242
USNO
Uccle
117
159
USNO
Oxford II
118
161
USNO
Oxford I
277
471
Strasbourg
Paris
254
436
Strasbourg
Bordeaux
224
355
Strasbourg
Toulouse
270
433
Strasbourg
Algiers
200
330
Strasbourg
San Fernando
226
346
USNO
Tacubaya
312
518
USNO
Hyderabad S
293
521
USNO
Cordoba
309
467
Sternberg, Russia
Perth
228
402
USNO
Perth-Edinb.
139
202
USNO
Cape
545
901
USNO
Sydney
431
743
USNO
Melbourne
218
392
Univ of Fla., USNO
Personnel involved in USNO plate reductions
Zone
prepared
reduced
Greenwich
Gary Wycoff, John Martin
Sean Urban
Vatican
John Martin
Sean Urban
Catania
John Martin, Gary Wycoff
Sean Urban
Helsinki
Harry Crull, Edward Jackson, David Hall
Sean Urban
Potsdam
Marion Zacharias
Marion Zacharias
Hyderabad N
Edward Jackson
Sean Urban
Uccle
Edward Jackson
Sean Urban
Oxford II
John Martin
Sean Urban
Oxford I
John Martin
Sean Urban
Paris
Edward Jackson
Sean Urban
Bordeaux
Edward Jackson
Sean Urban
Toulouse
David Hall
Sean Urban
Algiers
Edward Jackson
Sean Urban
San Fernando
Gary Wycoff
Sean Urban
Tacubaya
John Martin, Gary Wycoff
Sean Urban
Hyderabad S
Edward Jackson, Sean Urban
Sean Urban
Cordoba
Gary Wycoff
Sean Urban
Perth
David Hall, Gary Wycoff, John Martin
Marion Zacharias
Perth-Edinb.
David Hall, Gary Wycoff
Sean Urban
Cape
Sean Urban
Sean Urban
Sydney
David Hall, Gary Wycoff
Sean Urban
Melbourne
Gary Wycoff
Sean Urban
Corrections applied to x,y data prior to final least squares
adjustment. Also includes single image precisions by zone.]Corrections applied
to x,y data prior to final least squares adjustment. Also includes single
image precisions by zone.
In the table, a ``Y'' indicates a correction was applied,
an ``n'' indicates it was not.
Values in parentheses indicates an additional dependence on those
characteristics.
mg=magnitude, mr=measurer, mp=microscope, rs=reseau, ep=epoch,
x=x-coordinate, and y=y-coordinate.
Note that Ox II/Grn ``zone'' refers
to the 32 plates observed at Greenwich for the Oxford II zone.
Zone
radial dist
Mag Eq.
Coma
Screw
FDP
\ ra
\ dec
Greenwich
Y(mg)
Y
n
n
n
.29
.30
Vatican
Y(mg)
Y
n
Y(mr)
Y(mp)
.42
.44
Catania
Y(mg)
Y(x,y)
n
n
Y(mg)
.33
.30
Helsinki
Y(mg)
Y(x,y)
Y
Y(mr)
Y
.23
.22
Potsdam
n
Y(mr)
Y
n
Y
.26
.25
Hyder. N
Y
Y(x,y)
Y
n
Y(mg)
.33
.30
Uccle
Y(mg,x,y)
Y
Y
Y
Y(mg)
.33
.38
Oxford II
n
Y(x,y)
n
n
Y
.33
.32
Ox II/Grn
Y
Y
n
n
n
Oxford I
n
Y(x)
n
n
Y
.32
.31
Paris
Y(mg)
Y
Y
Y(mp)
Y(rs)
.21
.21
Bordeaux
n
Y(ep)
n
Y
Y(rs)
.22
.21
Toulouse
Y(mg,mp)
Y(y)
n
Y(mp)
Y(rs)
.29
.28
Algiers
Y(mg)
Y(y)
n
n
n
.19
.18
San Fer.
Y(mg)
Y
Y
n
Y(mg)
.32
.33
Tacubaya
n
Y(x)
n
n
Y(mg)
.25
.24
Hyder. S
Y(mg)
Y(x,y)
Y
n
Y(mg,rs)
.31
.32
Cordoba
Y(mg)
Y(x,y)
n
Y(mp)
Y(mg)
.32
.29
Perth
Y
Y
Y
n
Y
.29
.28
Per-Edin.
Y(mg)
Y(x,y)
Y
Y
Y(mg)
.29
.28
Cape
n
Y(x,y)
Y(ep)
Y(mp)
Y(mg,rs)
.28
.26
Sydney
Y(mg)
Y(mp,rs,x,y)
n
Y(mp)
Y(mg,rs)
.44
.40
Melbourne
Y
Y
n
Y(mp)
Y(mg)
.34
.32
Catalogues used in the ACRS_1999 (units of mas)
Catalogues used in the ACRS_1999 (units of mas)
Name
type
σα
σδ
σ/ sqrt(N)
1ST CAPE FUND 1900
Transit Circle
589
564
Yes
2ND CAPE FUND 1900
Transit Circle
529
462
Yes
2ND MELBRN GC 1880
Transit Circle
1380
859
Yes
3RD MELB GC 1890
Transit Circle
1507
1117
Yes
ABBADIA 00 –3/–9
Transit Circle
975
849
Yes
ABBADIA 00 +16/+24
Transit Circle
988
810
Yes
ABBADIA 00 –2/+4
Transit Circle
951
929
Yes
AGK2 (BERGEDORF)
Astrograph
205
198
No
AGK2 (BONN)
Astrograph
261
259
No
AGK2a
Transit Circle
165
217
No
AGK3
Astrograph
204
191
No
AGK3R
Transit Circle
75
107
No
ALBANY –2/+1 1900
Transit Circle
840
844
Yes
ALBANY –20/–40 00
Transit Circle
836
1156
Yes
ALBANY 10
Transit Circle
294
282
No
ALGER AG 1900
Transit Circle
1156
958
Yes
BERGEDORF I - 25
Transit Circle
432
540
Yes
BERLIN 10 79/90
Transit Circle
504
617
Yes
BERLIN 1920
Transit Circle
559
660
Yes
BONN 00 +40/+50
Transit Circle
506
461
Yes
BONN 20
Transit Circle
533
504
Yes
BORD 00-II RE-OBN
Transit Circle
397
377
Yes
BORDEAUX II 1900
Transit Circle
1038
734
Yes
BUCHAREST –11/+11
Transit Circle
381
493
Yes
CAMC Series
Transit Circle
176
237
Yes
CAPE I - 50
Transit Circle
368
373
Yes
CAPE II - 25
Transit Circle
468
453
Yes
CAPE III - 25
Transit Circle
474
420
Yes
CAPE –40/–52 1900
Transit Circle
743
671
Yes
Catalogues used in the ACRS_1999 (units of mas), continued
Catalogues used in the ACRS_1999 (units of mas), continued
Name
type
σα
σδ
σ/ sqrt(N)
CAPE 17 –30/–35
Astrograph
391
407
No
CAPE 18 –35/–40
Astrograph
319
350
No
CAPE 19 –52/–56
Astrograph
298
319
No
CAPE 20 –56/–60
Astrograph
210
193
No
CAPE 20 –60/–64
Astrograph
196
200
No
CAPE 21 –64/–68
Astrograph
182
187
No
CAPE 21 –68/–72
Astrograph
190
195
No
CAPE 21 –72/–76
Astrograph
211
211
No
CAPE 21 –76/–80
Astrograph
186
202
No
CAPE 22 –80/–89
Astrograph
176
227
No
CAPE G. C. (1900)
Transit Circle
674
600
Yes
CAPE I - 25
Transit Circle
469
459
Yes
CAPE II - 50
Transit Circle
467
588
Yes
CAPE ST 50 –30/–35
Transit Circle
742
668
Yes
CAPE ST 50 –35/–40
Transit Circle
534
510
Yes
CAPE ST 50 –52/–56
Transit Circle
722
608
Yes
CAPE ST 50 –56/–60
Transit Circle
330
344
Yes
CAPE ST 50 –60/–64
Transit Circle
302
336
Yes
CAPE ST 50 –64/–68
Transit Circle
374
398
Yes
CAPE ST 50 –68/–72
Transit Circle
376
380
Yes
CAPE ST 50 –72/–76
Transit Circle
372
416
Yes
CAPE ST 50 –76/–82
Transit Circle
328
370
Yes
CAPE ST 50 –82/–90
Transit Circle
226
326
Yes
CORDOBA 6429 ST 00
Transit Circle
722
715
Yes
CORDOBA D 1950
Transit Circle
589
628
Yes
CORDOBA E 1950
Transit Circle
618
952
Yes
CPC2
Astrograph
Ind.
Ind.
NA
FAYET +5 TO +15
Transit Circle
559
607
Yes
FAYET –5 TO +5
Transit Circle
165
240
No
FOKAT
Astrograph
220
238
Yes
Catalogues used in the ACRS_1999 (units of mas), continued
Catalogues used in the ACRS_1999 (units of mas), continued
Name
type
σα
σδ
σ/ sqrt(N)
GREENWICH 1910
Transit Circle
426
324
No
GREENWICH 9Y2
Transit Circle
950
854
Yes
GREENWICH I-50
Transit Circle
504
487
Yes
GREENWICH II-25
Transit Circle
450
466
Yes
HEIDELBERG ZOD 50
Transit Circle
375
453
Yes
HIPPARCOS
Satellite
Ind.
Ind.
NA
KONIGSBERG 25
Transit Circle
183
199
Yes
KONIGSBERG 25-II
Transit Circle
482
426
Yes
KUSTNER - BONN 00
Transit Circle
368
332
No
LA PLATA 3710 S 50
Transit Circle
712
658
Yes
LA PLATA A 1925
Transit Circle
808
790
Yes
LA PLATA B 1925
Transit Circle
811
679
Yes
LA PLATA C 1925
Transit Circle
801
698
Yes
LA PLATA D 1925
Transit Circle
1000
867
Yes
LA PLATA E 1925
Transit Circle
911
857
Yes
LA PLATA EROS 1930
Transit Circle
500
387
No
LA PLATA F 1935
Transit Circle
485
518
Yes
LA PLATA GC 1950
Transit Circle
564
585
Yes
LEIDEN 25 XV/1
Transit Circle
460
576
Yes
LEIDEN 25 XV/2
Transit Circle
458
664
Yes
LEIDEN 25 XV/4
Transit Circle
2308
2069
Yes
LICK 17 HARVD AG
Transit Circle
246
345
No
LICK 28 20/30
Transit Circle
222
202
No
LICK ZODIACAL 1900
Transit Circle
424
335
Yes
LUND 50 FAINT AG
Transit Circle
710
749
Yes
LUND AG 25 35/40
Transit Circle
661
696
Yes
MADISON 1910
Transit Circle
444
468
Yes
MUNICH A 1900
Transit Circle
681
749
Yes
MUNICH B 1900
Transit Circle
730
680
Yes
NICOLAEV –5/–20 50
Transit Circle
306
367
Yes
NICE
Transit Circle
111
148
No
PARIS 1900
Transit Circle
1299
848
Yes
Catalogues used in the ACRS_1999 (units of mas), continued
Catalogues used in the ACRS_1999 (units of mas), continued
Name
type
σα
σδ
σ/ sqrt(N)
PARIS ASTR +17/+25
Astrograph
290
323
No
PERTH 83
Transit Circle
211
405
Yes
PERTH VOL 2 1900
Transit Circle
717
713
Yes
PERTH VOL 3 1900
Transit Circle
899
974
Yes
PERTH VOL 4 1900
Transit Circle
844
803
Yes
PERTH VOL 5 1900
Transit Circle
909
798
Yes
PERTH VOL 6 1900
Transit Circle
763
779
Yes
PFKSZ 50
Transit Circle
59
62
No
PULKOVA 1900
Transit Circle
1272
914
Yes
PULKOVO 10 39/44
Transit Circle
300
288
No
SAN LUIS 1910
Transit Circle
716
661
Yes
SCHWACHER STERNE
Transit Circle
108
148
No
SRS
Transit Circle
268
367
Yes
STRASBOURG AG 1900
Transit Circle
1005
908
Yes
SYDNEY 1499 IMD 25
Transit Circle
782
771
Yes
SYDNEY –51/–63.5
Astrograph
84
80
No
SYDNEY –48 TO –54
Astrograph
156
144
No
TOULOUSE 00 III-B
Transit Circle
462
380
No
TOULOUSE III
Transit Circle
684
572
Yes
TOKYO PMC 86-89
Transit Circle
463
543
Yes
TYCHO-1
Satellite
Ind.
Ind.
NA
USNO TAC
Astrograph
Ind.
Ind.
NA
W1J00
Transit Circle
204
236
Yes
W2J00
Transit Circle
274
319
Yes
WASH 25 ZOD 6-IN
Transit Circle
349
405
Yes
WASH 33 –10/–20 33
Transit Circle
564
438
Yes
WASHINGTON 00 9-IN
Transit Circle
590
535
Yes
WASHINGTON 20
Transit Circle
642
540
Yes
WASHINGTON 250
Transit Circle
262
381
Yes
WASHINGTON 350
Transit Circle
290
383
Yes
WASHINGTON 40 9-IN
Transit Circle
370
447
Yes
WASHINGTON AG 1900
Transit Circle
1217
976
Yes
WIEN AG 1900
Transit Circle
806
807
Yes
Catalogues used in the ACRS_1999 (units of mas), continued
Catalogues used in the ACRS_1999 (units of mas), continued