4.3.7 Astrometry - Including Distortion and Pointing
Absolute pointing knowledge of the focal plane array pixel centers based on the Pointing Control System is better than:
· 24 µm : about 1.4´´, 1 sigma radial
· 70 µm : about 1.7´´, 1 sigma radial
· 160 µm : about 3.9´´, 1 sigma radial
From the science data frames themselves, refined pointing knowledge of sources must be based on centroiding of objects visible in a MIPS image. The uncertainties will depend on the signal-to-noise ratio of those source detections and the related uncertainty in the centroids, and on the accuracy of prior knowledge of the positions of some or all of the sources. Uncertainty in relative positions between sources (source offsets) can be much lower than those associated with absolute pointing knowledge, for detections that can support this type of analysis.
The pointing offsets used in mapping of individual frames into the post-BCD products are derived from an optimal offset estimation method, e.g., point source extraction. If insufficient point sources are available, then the data are aligned using spacecraft and CSMM pointing knowledge only. Individual frames are adjusted such that the overlap regions are statistically similar, within the limitations of the large-scale background and slow response characteristics of the Ge:Ga detectors. This implies that some edges might be visible for certain types of varying backgrounds. This will especially be true for adjacent legs of a scan map over a region with significant and varying background. The absolute level to which edge effects and scan leg baseline offsets will be removed in the post-BCD products will not be known with certainty until more flight data are obtained. For observers requiring very uniform maps of extended source regions, there should be some expectation of additional required data processing to correct for edge effects and scan leg baseline offsets in the Ge:Ga data of a scan map. Such effects are minimized in the other MIPS observing modes. The zodiacal backgrounds are maintained only within an individual AOR.