Explanation of Overlap Issues

Chas Beichman's analysis of the overlap regions from the 25,545 scan pairs in the Winter-00 Release (click here) lists a set of scans in Table 3 where the photometric offsets in two or more bands are greater than 0.04 mag. I have examined each of these scans to determine the cause of the offsets. By looking at the delta J mags vs. RA for the overlapping sources in the offending scans it appears that the offsets are due to night-to-night calibration differences. In all cases the offending scan pairs are those that fall in different nights. Scan pairs from the same night do not have a marked offset suggesting that the major discrepancy is between nights. To examine these plots just click on the night in question below:
970608n51 (tile=10794)
970614n19 (tile=10602)
980705s45 (tile=313717)
971119n118 (tile=12238)
980319s142 (tile=313716)

Data found in the overlap statistics for these scans suggests the offending scan is one from a different night as above, however, the number of data points are fewer and therefore can not be conclusive.
990331n35 (tile=7111)
980115n93 (tile=7486)
980302n80 (tile=12926)

Last update: 24 January 2000 slw@ipac.caltech.edu