Further to my problem regarding the use of ensemble reference stars. I found another possible problem with MAXIMDL. I did the analysis with another program called ASTAP. The results were similar but there were some differences. One significant one was the measurement of SNR. For the same star MAXIMDL gave me an SNR of about 1500. ASTAP gave me an SNR almost exactly half at about 750. So I checked with two other programs. AIP4WIN gave me 801 and MUNIWIN gave me 788. Can anyone suggest why a program would give an SNR double the right value?
I doubt MaxIm DL has a gross intrinsic error of this sort, which would have been noticed before, given that the code has been around for a long time and who wrote it. Again (just guessing) there must be some problem with the configuration or the set-up, such as the detector gain, what the sky background is, how things are being measured etc. Someone familiar with the program would have to look over your shoulder (maybe a Zoom call?) to figure out what's going on.