Yes please. While adding the calexp background back in should improve difference imaging results, in practice it doesn't right now. Changing these ip_diffim defaults to something that yields good results seems very sensible.

It's good that these config improve the performance but do you understand why? E.g. I could guess that the higher spatial order provides a better fit, but am not sure why one has freedom to add or not add background. Do you have reasons to expect that these improvements should be general and independent of dataset? 

As I understand it, the higher spatial order fit for the difference image background is instead of using the calexp background directly. The default of 0 would be a constant background which is not an accurate representation of reality. We have shown that turning doAddCalexpBackground off significantly reduces the number of false positive sources in both HSC and DECam, but aren't sure why adding in the calexp background to the difference images doesn't work as originally intended.

Zeljko Ivezic doAddCalexpBackground "undoes" the background subtraction performed by single-frame processing, so all of the background fitting is done by ip_diffim. I am told that historically SFP background fitting was worse and it made sense to redo the subtraction in ip_diffim, but that appears to no longer be true. Going forward I would continue to expect SFP to provide our best estimate of the background. I'm also not aware of significant other datasets using diffim at present except for DESC DC2, whom I will inform.

Thanks, that's very helpful! 

Implementation ticket is https://jira.lsstcorp.org/browse/DM-21398