This page discusses stacks with an excess in false detections in LAP.ThreePi.20120706

Bill Sweeney

psphotStack has found a large number of false detections in some of the stacks. This issue was discussed in December in the context of y band and changes were made to detect cases where the number of detected sources in y band was much higher than r, i, and z and turn off the feature where all forced photometry is performed in all bands for sources only detected in y.

Now that we have completed stack photometry for the most of the sky we can see how widespread the problem is.

The following plot shows the number of detections per skycell as a function of RA and DEC in g and in y band. The region near the galactic plane is clearly visible as is M31. Note the various regions where the detection count is much higher than the surrounding regions. These are the skycells under discussion in this wiki page

Example y band stack 1715750 skycell.1927.074 23.1369h, 27.004d

Stack_id 1715750 is an example that shows this problem. 55284 sources were detected in y band while < 6400 were detected in the stacks for the other 4 filters.

The following image shows the stack with the individual detections displayed as yellow or green circles. Note that there are areas which fall in chip and cell gaps from one or more of the inputs do not show the enhanced detections. This indicates that the phenomena is likely due to a subset of the input warps

This image shows the input warp images. Here we can see that the area with lower detection count matches the outline of the fourth and fifth images. Since the fourth image was rejected it looks like the excess detections come from areas where the fifth image contributes. This is warp_id 612302. Examining the image we find obviously unusual.

Here is a copy of the log file for the stack. stk_1715750.log