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Hi!

I am a Phd student at Montpellier in France and I work on a globular cluster NGC 4833. I want to get magnitudes of stars which composed this globular cluster. For that I use the protocol for crowded field which I found in the "Reference Guide to the IRAF/DAOPHOT Package" of Lindsey E.Davis. I have done all steps that are present in the protocol and even repeat some of them but when I fit the HR diagram with the magnitudes in two filters (Bb and V) there is a lot of scatter. There is more scatter when I fit the HR diagram with magnitudes given by the last *.als file than when I fit the diagram just with the magnitudes given by the first *.mag file (so just with DAOFIND and PHOT tasks). So the diagram is better with a simple aperture photometry than with a PSF, that is not normal...
Here is the google link drive where I put pictures of the two diagrams:
https://drive.google.com/folderview?id=0BwWlQpEaq7FYMmhRVnpKYTdpSDA&usp=sharing

Do someone have solutions to reduce this scatter?

I thank you very much.

Johanna.

I don't have a specific answer to your question, but wanted to suggest that you also have a look at the "User's Guide to Stellar CCD Photometry with IRAF" by Phil Massey and Lindsey Davis. This is available from

https://iraf.net/irafdocs/daophot2.pdf

You might also check other documentation in the https://iraf.net/irafdocs page related to photometry. A google search of "IRAF crowded field photometry" turns up other links to tutorials and manuals as well. Hopefully somebody more familiar with this type of analysis will be able to provide a direct answer.