Nick, many thanks for the information. With PSF photometry the sky annuli are not used at all, so implementing this approach directly is not possible, unless we introduce a sky annulus setting. But then, if we do this, wouldn’t such uncertainty estimate be fundamentally detached from the photometry algorithm used?
Fortunately, I have found that the iterative PSF photometry algorithm implementation in photutils provides flux uncertainties as output. Here is the link to version 0.14 of the program that reports total error = sqrt(flux_unc^2 + fit_unc^2) instead of relying on an external SNR estimate:
http://trafyx.com/wp-content/uploads/2021/04/metropsf-0.14.zip
I don’t think I have the necessary expertise to develop a new method to estimate uncertainties with PSF fitting photometry. It should be safe to use the flux fit uncertainty provided by the underlying algorithm. I wonder if you agree with this approach and if you could give this version a try.
Maxim