The distortion at 24 microns is consistent with what was expected pre-launch. The 24 µm photometry can vary by about 2-10% across the array for data prior to the distortion correction. The BCD pipeline puts the distortion keywords (see Appendix C for a list of the keywords) into the header, but does not correct for the distortions. The Post-BCD pipeline does in fact correct for the distortions such that the mosaics are resampled to have a default pixel size on the sky of 2.45x2.45 arcsec. The photometry part of MOPEX can do single-frame photometry and takes into account distortion on the individual frames.
7.1.10 Large- and small-scale gradients
For large scan maps, we have noticed a slow large-scale gradient across the entire map, which is along the direction of a scan leg. Since we scan along directions perpendicular to the ecliptic plane, this may very well actually be real, physical variations in the zodiacal light. However, sometimes the gradients do not match in adjacent scan legs. Some fraction of this effect may be due to residual read-2 effects.
Gradients are also seen within individual 5x5 arcmin BCDs, but despite being in the scan direction, these are not likely to be astronomical in nature. Some may be due to poor flat-fielding, and can be corrected with self-calibration (see section 8.1.2). However, systematic gradients are seen as a function of scan mirror position, especially in photometry mode, which uses the largest range in scan mirror angle. These may be due to scattered light changes as a function of scan mirror position. These are likely to be additive.
Finally, as another common ''gotcha'' to list here, there are many, many asteroids to be found, even in observations obtained some considerable distance from the ecliptic. Beware of 24 micron sources without short-wavelength counterparts, especially near the ecliptic plane! Most of the point sources in Figure 7.18 are asteroids.
Figure 7.18: Beware of asteroids! Most of the point sources in this frame are asteroids.