Nick, many thanks for the

Maxim Usatov

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:

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.