In this thread the user CavasRNC says:

Anyone who is interested in reading more on how to estimate the signal-to-noise ratios (SNRs) and the uncertainties in digital photometry can take a look at :

http://www.phys.vt.edu/~jhs/phys3154/snr20040108.pdf

http://adsabs.harvard.edu/abs/1991PASP..103..122N

The IRAF equation for error estimation is an approximation for SNR>>1. If someone doesn't trust that this is valid for all their data, it is better to manually calculate the errors using the SNR (see the first link for a brief explanation and the equations).

Can I quote the article (second link) as a valid reference for how IRAF calculates the final errors in a CCD photometry process*?

Thanks!

* Such as the one described in A User's Guide to Stellar CCD Photometry with IRAF by Massey & Davis (1992)

This isn't rocket science. In calculating the error on aperture photometry, you add the relative sources of error in quadrature: the read-noise, the photon noise front he object, and the photon noise from the sky. With PSF fitting it's a little bit trickier. This is all well documented in the various papers Peter Stetson has written on daophot. See, for example,
http://adsabs.harvard.edu/abs/1987PASP...99..191S
http://adsabs.harvard.edu/abs/1989HiA.....8..635S
http://adsabs.harvard.edu/abs/1990ccd2.proc...71S

Sadly, I don't have access to the last two of the three articles you posted Dr Massey.

I am actually performing PSF fitting, is there a reference for that or do the same articles by Stetson apply?

Thank you very much for your reply.

Yes, these are about PSF fitting. Yes, two of the articles can't be found on-line, but you would have to try an astronomy library, where they have conference proceedings and observatory publications.