IRAS Explanatory Supplement
VI. Flux Reconstruction and Calibration
B. Determination of Relative Flux
B.2 Photometry of Point Sources and Small Extended Sources
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The point source responsivity obtained from the measurements of the internal reference source as described above were applied directly to the output of the point source recognizer. No attempt was made in the reconstruction to correct for the cross-scan response profile other than in a statistical sense. However, as part of the calibration procedure using the flashes of the internal reference source and the celestial source NGC 6453, the shape of the cross-scan response of each individual detector (Section IV.A.3) was combined with the knowledge of the cross-scan position of the standard calibration track to produce a single correction factor for the standard calibration response. This correction factor would result in zero average error for point sources passing over uniformly distributed cross-scan positions. In the 25 µm band the difference between the measured amplitude and the average amplitude never exceeded 10%, while in the other bands the difference did not exceed 5%. The variations in the amplitudes caused by the cross scan response implies that a minimum uncertainty of 5% must be assigned to all fluxes determined in the survey.
The small extended sources were treated like the point sources except that a 10% and 8% increase in responsivity was assumed at 12 and 25 µm to approximate sources typically 4 times the nominal point source template in spatial extent. Except for this small correction, no attempt was made to take the variation of the responsivity with size of the source into account; see Section IV.A.4. Although this procedure incurred large uncertainties, the uncertainties introduced because of a lack of knowledge of the true spatial distribution of the sources dominated the basic photometric uncertainties.