This wiki describes the zoology investigations of the false positives when looking at the images (not necessarily the flags).

First observations

Many of the stack skycells show a bias (more false detections on the top or bottom half of a cell). These biases tend to run horizontally on a chip and often do not continue onto neighboring chips.

Plotting pixel/sqrt(variance) for cells and subset of cells

The stack that I investigated was stack 336346 (skycell.1316.089). I chose a square that had a lot of false detections (green), and one that did not (red):

I then chopped the cell that had a lot of false detections into two (top half = red, bottom half blue), and divided the pixel values by the sqrt of the variance. A gaussian was fit to each of these parts, and it is in log space (y):

• black = the cell with few false detections,
• blue = the bottom half of the cell with the false detections
• red = the top half of the cell with the false detections

plotting psf_mag vs chisq

These are plots of inst_psf_mag vs chisq for g band (if one of psf_mag or ap_mag is NULL, it is ignored). The other bands look very similar. The first plot is for the true detections, the second for the false detections, and the third is for both (true = black, false = red). The only pattern I see is that faint = fake.

plotting psf_mag vs psf_mag - ap_mag

These are plots of inst_psf_mag vs inst_psf_mag - ap_mag for g band. Similar to the other plots, the entries with one of psf_mag or ap_mag = NULL are ignored, and it looks very similar for all bands.