3.3.9) The PIA dynamic calibration method (for PHT-S chopped measurements)

PIA offers from V.8.0 the user a new way (alternative to the old one) of calibrating data from chopped PHT-S observations. This method is taken as the default (from PIA V8.1 on) and also applied in the pipeline OLP 8. For simplicity we will call it also "dynamic calibration". The "dynamics" in this case is not based on the evolution of the signal affected by transients compared to the calibration measurements (as in the staring case) but by the use of a signal dependant correction as a first order correction to a special derived spectral response function (SPRF). The SPRF has been obtained by calibration observations performed exclusively in chopped mode (thus containing the signal losses caused by chopping).

Details on this method can be found in Calibration of ISOPHOT-S Chopped Measurements.

Basics of the calibration method

Relatively faint calibration stars from mK = 1.65 to mK = 7.54, measured in triangular and rectangular chopper modes were used for deriving the spectral response function SPRF. The measurements were reduced using PIA. An individual SPRF for each measurement was determined as usually,  taking the ratio between the signal difference (son - soff) and the model flux for each calibration star, as delivered by M. Cohen and P. Hammersley. A mean <SPRF> was derived by averaging the individual SPRFs. Clear differences (up to 30-40%) were seen between the so derived <SPRF> and the SPRF derived by staring observations, not homogeneously distributed over the full spectral range, showing special signal losses by certain pixels due to chopping.

The individual SPRFs vary systematically with the brightness of the calibration stars, defining a first order correction to the mean <SPRF>, which only depends on the signal difference (son - soff). This simplified approach is based on the fact that the signal difference corresponds basically to the brightness of the source. The background at the ISOPHOT-S wavelength range is dominated by Zodiacal Light which varies whithin a small range.
 

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Chapter history:

Date	Author	Description
05/12/1998	Carlos GABRIEL (ESAVILSPA-SAI)	First Version (PIA V8.0) - (Based on J.Acosta's internal PIDT document 'Calibration of ISOPHOT-S Chopped Measurements')
02/11/1999	Carlos GABRIEL (ESAVILSPA-SAI)	Update  (PIA V8.1)