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For a specific observation, the observing time is composed of:
- exposure time - the time during which the target is observed;
for chopped observations each chopper plateau time interval adds to
the total exposure time,
- 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.
To calculate the overheads the detailed command sequences for the instrument
modes must be known. These overheads must be added to the exposure time
derived for an observation. In the remainder of this section we itemize
the main elements in an observation causing overheads.
In the next section we describe how to estimate the target dedicated time
taking all possible overheads into consideration.
- target acquisition (per target): 180 sec
for tracked solar system targets: 300 sec - for each command that is sent to the instrument (to move a wheel, to
activate a detector, to switch on a heater, to activate the FCS, etc.)
an overhead is required of 1-3 sec
- initial instrument set-up: 25 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): 32 sec
For the spectrophotometric mode (AOT PHT40) only:
For the absolute photometry mode (AOTs PHT05 and PHT25) only:
- the heated FCS measurement is as long as the sky measurement
- (optional) dark measurement: 256 sec
- (optional) cold FCS measurement: 128 sec
For the detectors P2, P3 and C100 only:
- stabilisation of heated detectors (per detector): 90 sec
For PHT03:
- If the P1 detector is used in combination with P2, the observation
with the non-heated P1 is performed first. This will lead to a reduced
heating overhead since the stabilization of the heating of P2 can at
least partially occur during the observation with P1.
The PHT03 logic does not apply in PHT22 to C200 which is also a non-heated
detector because C200 is sensitive to the thermal radiation of the C100 heater.
Next: Estimation of the target
Up: Overheads
Previous: Overheads
R.J. Laureijs, U. Klaas, D. Lemke, U. Herbstmeier, P. Abraham
Wed Aug 7 12:14:12 MET DST 1996