IRAS Explanatory Supplement
XI. Known Processing Anomalies
F. Artifacts in the Digital Image Data Base
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The final calibration corrections for baseline and scale factor in the 16.5° image digital data base delivered in November 1984 (third HCON coverage only) were based on the Zodiacal History file. There was insufficient time before the delivery date to perform the corrections properly on the time ordered data and reassemble the high resolution maps. As a result, the 2' digital data show a patchwork structure of 30' squares which varies in intensity from one part of the sky to another. The maximum discontinuity across the edge of one of the squares amounts to about 5 digitization levels of the satellite's electronics in all bands (Section II.C) regardless of the instantaneous sky brightness. This translates to maximum jumps of about 0.9, 2.0, 0.5 and 1.0 MJy sr-1 at 12, 25, 60 and 100 µm, respectively. The problem should be of little concern in high brightness regions such as the Galactic plane but may compromise the data for some purposes in low brightness regions near the Ecliptic and Galactic poles. In most cases the patchwork amplitude is smaller than the residual striping left by the flat fielding procedure.
No attempt was made to smooth out the discontinuities. A number of fields where the problem was most severe were deleted and are listed in Table XI.F.1. The calibration of the third coverage will be corrected properly and a complete data set reissued. The photographic data products released in November, 1984 do not suffer from the problem because no corrections were applied to these data as discussed in Section X.D.3. The first and second HCON coverages as well as the reprocessed third HCON will be properly corrected.
Table XI.F.1
7 18 19 20 21 22
23 24 36 43 44 56
58 82 92 116 127 140
141 170 173 185 186 187
188 190 191 192 201 203
205 209 210 - - -