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Overheads

For a specific observation, the observing time is composed of:

exposure time - the time during which the target, and for chopped observations also the sky background, is observed
overheads - the time to prepare the spacecraft and the instrument for the observation

The exposure time is the sum of individual integration times, e.g. per filter or per aperture, calculated on the basis of the specified flux levels. To calculate the overheads the detailed command sequences for the instrument modes must be known. Approximate overheads are given below. These overheads must be added to the exposure time derived for an observation.

target acquisition (per target): 180 sec
initial instrument set-up: 15 sec
wheel positioning (for each filter, aperture and polariser requested) : 10 sec

For all AOTs, except PHT05, PHT25, and PHT40:

calibration with FCS (per detector): 16 sec

For the spectrophotometric mode (AOT PHT40) only:

dark exposure: 32 sec

For the detectors P2, P3 and C100 only:

stabilisation of heated detectors (per detector): 90 sec

If the P1 detector is used in connection with either P2 or P3, the observation with the non-heated P1 should be performed first. This will lead to a reduced heating overhead since the stabilization of the heating of P2 or P3 can at least partially occur during the observation with P1 (see Sect. 6.4.6 for an example). Note that this does not apply to C200 which is also a non-heated detector because C200 is sensitive to the thermal radiation of the other heated detectors.

For scans and maps the time needed for slewing between the individual spacecraft raster positions must be added. It depends on the total number and the separation of the pointings, and the user is referred to the ISO OBSERVER'S MANUAL for details.

For concatenated observations, the overhead of 180sec for target acquisition has to be added only for the first observation in the sequence. All the following need only 20sec per observation (for concatenation see the ISO OBSERVER'S MANUAL).

Next: Photometry in Single Up: THE COOKBOOK Previous: Astronomical Considerations when

Tue Dec 6 17:24:45 MET 1994