| | 1 | == The Galactic Plane and w-band Astrometry == |
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| | 3 | (Up to the [wiki:IPP_for_PS1 IPP for PS1]) |
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| | 5 | MOPS has been reporting astrometry problems in the plane for some time. Richard and Larry pointed me as some problem exposures which exemplified the situation: o6783g0438o and o6783g0456o. I found that several chips failed astrometry and were reported as failing. A number of other chips had poor solutions but were not detected as poor. Looking at the astrometry analysis in detail, I discovered that the reference star selection was poorly matched to the w-band filter in the plane. Specifically, there is a density limitation to avoid excess cross correlation effort. Since the cross correlation is a problem of order N^3^, this is a reasonable concern. However, in the plane, the greater depth of w-band meant that the selected stars would frequently not overlap the non-saturated gpc1 detections (in magnitude range). Thus, chips could easily fail because there were not enough good stars for a solution, and poor astrometry could result from the limited numbers as well. |
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| | 7 | I modified the density limits to allow good overlap between reference catalog stars and w-band gpc2 stars. I was able to choose values which did not increase the processing time too much. I ran a large number of tests for 1100 3pi exposures scattered across the survey to prove that the new parameters did not degrade astrometry generally. In fact, the new results were nearly universally better, so I am happy to install them. |
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| | 9 | This improvement will also help diff operations in the plane. For the bad pair of exposures from Larry, the old parameters resulted in many skycells having 30-60k diff detections (due to dipoles). the new parameters generally keeps the numbers close to 10k. |
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| | 11 | The following figures illustrate the astrometry failure and the diff detections. |
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| | 13 | [[Image(dense.20120524.g.png,700px)]] |
