After thorough investigation, we concluded that most of the spurious DIASources originate from a single night (visit). A handful are from a 1-2 nights within a few days of the most problematic night, when there was extremely high sky brightness. The HiTS survey was during dark time so we can't blame the moon.
We tried a few different things to alleviate the problem:
- Is the background being overfit, and would the problem still appear with a constant background level? (no substantive difference when this was tried)
- Is the problem due to proximity to bright sources, and do we just need to do a better job of masking them? (no, it is a function of time - visit - and location on the CCD and is independent of where other sources are, as it appears happens on the same CCD in multiple fields)
- Can we fix the problem by adding illumination correction to the ISR steps? (see
DM-19874) ... tl;dr it changes things but doesn't fix them
I suspect we should just throw out the most problematic visit, but we need a way to quantitatively determine a "bad visit for difference imaging" a priori, and we need to ensure this problem isn't somehow caused only in the presence of decam-community-pipeline-enabled ISR.
Would you review this Yusra AlSayyad? It is directly relevant to the work we'll be diving into next week, and looking at the notebook should give you a feeling for the kind of analysis I have been doing and one type of false positive we are encountering.
There is essentially no code review (save a small bug fix for a custom plotting script). In the end we haven't fully solved the problem, but we have characterized it somewhat.