Next: Relation between and
Up: Position switched
Previous: Position switched
Contents
When the stability of the system is long enough, we can share the same off
for several independent on-positions measured in a row (e.g.
ON-ON-ON-OFF-ON-ON-ON-OFF...). The first key point here is the fact that
the on-positions must be independent. The OTF is an observing mode where
the sharing of the off can be used because the goal is to map a given
region of the sky made of independent positions or resolution elements.
When sharing the off-position between several on, Ball (1976) showed
that the optimal off integration time is
|
(23) |
where
is the number of on measurements per off. Replacing
by its optimal value in eq. , we obtain
|
(24) |
We thus see that the rms noise decreases when the number of independent on
per off increases. It seems tempting to have only one off for all the on
positions of the OTF map. However, the second key point of the method is
that the system must be stable between the first and last on
measurement. To take this point into account we must introduce
- The concept of submap, which is a part of a map observed between two
successive off measurements.
-
, which is the area covered by the
telescope in each submap.
-
the number of such submaps needed to cover the whole map
area.
-
, the typical time where the system is stable. This time
will be the maximum time between two off measurements, which is noted
.
-
, the number of coverages needed either to reach a given
sensitivity or to exhaust the acquisition time.
-
and
are the times spent respectively on and
off per independent measurement and per coverage.
We note that the number of on per off (
) is a purely
geometrical quantity. This implies that the time spent off is linked to the
time spent on by Eq. both in each individual coverage
and when averaging all the coverages.
Next: Relation between and
Up: Position switched
Previous: Position switched
Contents
Gildas manager
2014-07-01