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7.1 Overview
Conceptually, for a given AOT the derivation of Standard Processed
Data (SPD) can be divided into three main stages:
- ramp processing:
Readouts of an integration ramp in V are reduced to a signal
in V/s.
- signal processing: Signals are corrected for instrumental
effects and averaged over a given period in time.
- in-band power calibration:
- For PHT-P and PHT-C, one or more FCS measurements that were
collected according to the AOT logic are processed to determine
photometric calibration factors. These factors are used to convert
the signals from
the sky measurements to an in-band power (in W) on the detector for
the selected filter band. In case FCS measurements cannot be applied,
the conversion is based on a default detector responsivity.
- In the case of PHT-S the signals per pixel are converted to
Jy/pixel by comparison with similar signals obtained from
standard stars.
SPD processing derives a record for each consecutive period in the sky
measurement for which the power on the detector is expected to be constant:
- a staring measurement yields one SPD record;
- a raster yields one SPD record per raster position;
- a chopped measurement yields two SPD records: one
and one mean record;
- a PHT32 sky measurement contains one SPD record for each
chopper plateau.
Apart from the mean in-band power, each record contains additional
information such as the power uncertainty, median value,
etc. (see Section 13.3).
The above description of SPD records applies to all AOTs except AOT PHT40
(PHT-S spectro-photometry).
SPD for PHT40 contains flux densities (in Jy)
instead of in-band powers (in W). In the processing level following SPD
(see Chapter 8) the PHT-S flux densities are converted
into flux densities per wavelength interval (in
)
and surface brightness units (in
).
Besides the SPD products that yield the processed data for the sky
measurements, a number of other products are derived based on non-sky
internal measurements. The products refer to the following type of
measurements (see Section 2.5):
- FCS measurement: The resulting product is
also referred to as a Calibration-A (`Cal-A') file.
- Dark instrument configuration measurement: This measurement type is
obtained with AOTs PHT05, PHT25, and
PHT40 and is used to subtract
dark signals (in case of PHT05 and PHT25) or to identify
possible detector memory artifacts from a preceding measurement
of a bright source in case of PHT40.
- Cold FCS measurement: This type of measurement occurs in the
absolute photometry AOTs PHT05 and PHT25 to correct for
straylight affecting the heated FCS measurement.
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ISO Handbook Volume IV (PHT), Version 2.0.1, SAI/1999-069/Dc