ISSA Explanatory Supplement

Gain Errors

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As discussed in Appendix D, the BasketWeave DeStriper (BWDS) makes use of intensity difference histories. These histories were generated by comparing the intensities from a given detector with the intensities of all other detectors of the same band which cross the path of the first detector at any time during the mission.

These intensity difference histories revealed a correlation between the magnitude of differences and intensity, suggesting gain errors. The average percentage gain error was computed from intensity difference data for each detector at 100 µm. This was done by using the intensity-weighted differences which, at 100 µm, gave more importance to the detector differences near the Galactic plane. The intensity differences were converted to percentage gain errors using the intensities from the crossing detectors. Only intensities with the zodiacal foreground removed were readily available. This was considered viable for 60 and 100 µm since the emission due to the zodiacal dust is a small percentage of total emission near the Galactic plane at these wavelengths.

The average gain errors and associated population sigmas at 100 µm, broken down by detector, were computed as shown in Table E.1. A positive sign suggests that the intensities for that detector are too large relative to other detectors in the band and should be reduced by the indicated percentage. A negative sign suggests that the intensities for that detector are too small relative to other detectors in the band and should be increased by the given percentage. These adjustments were not applied to the IRAS data.

Table E.1 100 µm Gain Errors
Detector 01 02 03 04 05 06 07 56 57 58 59 60 61
Gain Err (%) +11 +01 +06 -05 +11 +04 +15 -01 +02 -01 -20 -05 00
Pop. Sig (%) 8.0 5.1 5.5 6.2 7.8 5.2 7.1 6.2 5.5 6.1 5.0 7.1 6.9

The analysis was repeated for 60 µm using a subset of the IRAS survey data known as the mini-survey (IRAS Explanatory Supplement 1988 §III.C.11). Results are tabulated in Table E.2.

Table E.2 60 µm Gain Errors
Detector08 09 10 12 13 14 15 32 33 34 35 37 38
Gain Err (%) +10 -02 -09 +09 +02 -06 +13 +08 +03 -05 +09 -03 -23
Pop. Sig (%) 3.8 3.2 3.3 9.2 2.9 5.0 5.9 3.0 4.0 3.4 4.2 3.9 5.2

These values do not fluctuate with Galactic longitude, which suggests that they are not a function of photon exposure and therefore are not due to hysteresis. The values presented above are expected to approximate DC gain errors. To a lesser extent, they may also reflect errors in determination of the detector solid angles.

Residual hysteresis effects were also noted in all bands. These were relatively short-lived with time constants roughly of the order of 10 to 20 seconds.


IRAS Catalogs and Atlases: Explanatory Supplement 1988, ed. C. A. Beichman, G. Neugebauer, H. J. Habing, P. E. Clegg, and T. J. Chester (Washington, D.C.:GPO).

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