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In the following we list the advantages and disadvantages of the 4 possible
observing modes for use to observe faint point sources. The modes are
presented in ``Detection limits and observing strategy for very faint
point sources'', SAI/96-188/Dc (30 Aug 1996).
All observations refer to 256 sec on-source integration time and 1 filter.
In this section we indicate an advantage by a plus (+) sign and a
disadvantage by a minus (-) sign in the lists. A zero (0) indicates that
the item is neutral and should be regarded as a comment or caveat.
Case: 4 positions on-source, 3 positions on background position 1,
2 positions on background position 2, 64 sec per position:
on-target time (OTT) =1727 sec
- + long detector stabilisation times
- + photometric calibration well understood: centre-field-of-view
observations, majority of photometric calibration observations
are taken in staring mode
- + gives the best detection limit
- + 2 FCS measurements, one at begin and one at end
- + sequence sky-source can be arranged to cancel out long term drifts
- - first FCS measurement after first sky measurement
- - expensive in performance time
- - in multi-filter mode FCS measurements for only one filter
Case: at least 2 on-source plateaux, 2 off-source plateaux:
OTT=1018 sec.
- + efficient in time
- + allows to cancel out long term detector drifts
- - photometric calibration not yet completed
- - considerable signal losses: depend on chopper frequency
(more losses for fast chopping), flux jump, ...
- - flux calibration needs either transient correction or
``dynamic calibration'' which is a calibration established for
each chopper frequency/(flux jump) combination.
- - chopping offsets, i.e. the presence of beam asymmetries,
vignetting, image degradation...
- - in multi filter mode only 1 FCS measurement per detector
- - application of chopper avoidance cone gives scheduling
restrictions
Case: C100, 
raster, 46'' step size, 42 sec integration
time per point: OTT=979 sec.
- + very efficient in time
- + long detector stabilisation times
- + continuous exposure
- + photometric calibration well understood: similar to staring mode
- + gives good detection limit
- + 2 FCS measurements per filter
- + internal flat field consistency, if raster step size = pixel size
- - no direct long term detector drift correction, needs baseline fit
Case: C100, raster, oversampling=1, 107 sec effective
integration time (off-centre, coverage factor=29): OTT=1033 sec.
- + efficient in time
- + gives good detection limit
- + high spatial resolution possible
- + high data redundancy
- + differential scans which can be used to assess baseline drifts
- + 2 FCS measurements per filter
- 0 photometric calibration still needs additional investigations:
the analysis of the effect of a relatively fast chopper sweep
frequency in combination with small step sizes is ongoing.
- - includes all (disturbing) chopper effects. However, since the
same sky position is viewed through several chopper angles'
chopper effects appear in practice to be less disturbing.
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Up: General ISOPHOT observing strategies
Previous: 2 Overview on sensitivity
U. Klaas, P. Ábrahám, M. Haas, U. Herbstmeier and R.J. Laureijs