As with the other ISO instruments, the SWS will need time for
switch-on, warm-up, stabilisation, internal calibration, etc., following
changes in the instrument configuration. Most of these overheads are of no
importance for the general observer, since they occur only once for
every period of SWS use.
There are however several sources of overhead that are directly related to
a specific observation. The time budget of a proposal will be charged for
these overheads, the main sources being:
- internal photometric calibration (about 5% of total integration time)
- internal reference scans and dark current checks (about 15% of total
integration time).
Overhead for satellite slewing (3 minutes) adds to these
internal overheads. Satellite overheads are described in the ISO Observer's
Manual.
Internal SWS observation overheads can be roughly defined as follows:
AOT SWS01:
198 sec for option 1 and 2
318 sec for option 3 and 4
AOT SWS02:
120 sec fixed
+ 26 sec per line observation
+ 10% of the pure integration time
(when merging, the overhead of the longest of the two observations applies)
AOT SWS06:
120 sec fixed
+ 92 sec per measurement
+ 15% of the pure integration time
(when merging, the overhead of the longest of the two observations applies)
AOT SWS07:
120 sec fixed
+ 92 sec per SW measurement
+ 15% of the SW pure integration time
+ 20% of the FP pure integration time
The number of grating observations can increase considerably when the
F-P-required time at a certain aperture is much longer than the SW time.
A single SW observation is repeated as often as possible.