2013-05-07

In an attempt to determine if the v2 factor in the psphotSignificanceImage calculation was necessary, I first looked at single frame detections. For a given section of an image (with many false positives due to row-to-row issues), there do not appear to be any issues with secondary peaks around bright objects. (key: red: matched detections between v2 and no-v2; blue: only found in v2; green: only found in no-v2)

However, looking at the deep stack i/055 histograms from 2013-05-01, it is clear that with v2 disabled, a new population of bright false positives arises. Overplotting matched (green) and unmatched objects (blue; although note that many are objects that were not found in SDSS, as they appear to fall on faint real sources), bright stars do seem to have adjacent false positives:

Taking a profile of the smoothed image for one of these stars shows the cupped S/N profile that leads to this issue (and that v2 was designed to fix). However, it seems that the v2 factor could be changed from 100 to 1000 to ensure that the expected S/N peak is not overly amplified.

2013-05-01

False positive histograms

I've been attempting to address the false positive rate, and believe part of the issue is with the v2 calculation in psphot. Disabling this appears to correct the worst issues in the y/027 stack. There is also a minimal change in the distribution of matched objects. The i/055 stack does not improve when v2 is disabled, so this is not likely to be a generic fix for all false positive issues.

v2 calculation

The significance image in psphot is calculated as:

(S/N)² = (I + (I / 100)²)² / V

where I is the smoothed image value, and V is the smoothed variance image value. Based on a standard S/N calculation in terms of I² / V, this is equal to

(S/N)² = (I² / V) * (1 + 2 * (I/100) + (I / 100)²)