Background Variations During Scans

Summary

Time dependent background variations during scans are removed from the images by matching frame backgrounds to previous frames during coaddition. This process causes the dc level of image backgrounds to diverge from the measured frame backgrounds, which will cause errors in the Poisson noise component computed from these backgrounds using standard photometry packages. The plots below show the difference between the min & max frame background divided by the minimum background during the scan for the nothern (69302 scans) and southern (69451 scans) telescopes. This is an upper bound on the fractional error in the background for each scan. The Poisson component of the noise is proportional to the square root of the background. Note that these plots exaggerate the problem more in J band, as the noise is not background limited.

2MAPPS V3 Fix

This problem was fixed in the 2MAPPS V3 pipeline by setting the background for each coadd to reflect the average frame background. This introduces dc offsets between sequential coadds that will have to be removed when the images are mosaiced. This problem is only significant for a small fraction of scans, but was corrected to avoid a continuing source of confusion.

Northern Telescope Background Variations

Southern Telescope Background Variations