IRAS Explanatory Supplement
VI. Flux Reconstruction and Calibration
B. Determination of Relative Flux
B.1 Overall Procedure to Determine Relative Photometry
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Each survey scan was initiated and terminated with a pair of flashes of the internal reference source to monitor the responsivity of the system. Because it was assumed that responsivity changes were slow, responsivity values between internal reference source flashes were determined by linear interpolation between these flashes. In principle, therefore, the responsivity was determined separately for each scan. If, as discussed below, both of the flashes at one end of a scan were unusable, the extrapolation was continued to the closest accepted flashes on adjoining scans. There was, however, never an interpolation across a bias boost (see Sections II.C.5, III.B.6, III.C.4 and IV.A.7 ) since the bias boost largely erased the detector's memory. In this case, the detector's responsivity was assumed to equal that calculated from the most recent flash accepted.
The responsivity relative to the flashes of the internal reference source, as determined from the flashes at the ends of the scans, was used to scale both point sources and extended emission. As seen in Fig. IV.A.2, the actual variations in the responsivity as measured by the flashes of the internal reference source are typically < 10% in the 12, 25 and 60 µm bands and < 15% in the 100 µm band.
As discussed in Section IV.A.4, the responsivity is a function of the spatial frequencies present in observing a source. Multiplicative factors appropriate for either the point source responsivity or the extended emission ("DC") were applied in the source detection processing (Sections V.C, V.G). No attempt has been made to adjust the calibration of the small extended sources to account for the appropriate spatial frequencies encountered (Section V.E). Additionally, the observations were reduced assuming a responsivity independent of the total flux falling on the detector; as indicated by the observations of Fig. IV.A.4, this assumption was clearly wrong in the 60 and 100 µm bands.
The design of the entire detector and electronics system used in IRAS was DC coupled, thus allowing in principle, a measurement of the absolute brightness of the sky from a measurement of the total photo-current. There was, however, no absolute radiometric reference included in the experiment in order to evaluate Vtia[off] directly, an essential quantity in determining the true sky brightness. The reconstruction of the offset Vtia[off] for each of the detector channels was the most difficult part of the effort to produce properly calibrated absolute sky brightness data for all parts of the sky. For example, in order to determine the total incident flux on each detector to an accuracy of one percent, it would be necessary to determine the absolute value of the DC voltage level at the input of each amplifier to about 1 microvolt. Despite this handicap, the extraordinary DC stability of the system, when coupled with the calibration procedures described below, provided an accurate and sensitive measurement of the sky brightness in all four wavelength bands.
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