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Hi,I am trying to perform surface photometry on an optical image of a galaxy to create a surface brightness profile as a function of galactocentric radius. I have been trying to use the ellipse task in the Isophote package to do this. The problem I keep running into is that whenever I fix the central position I get errors for all of the values that are very high. If I do not fix the center position then the center moves to a bright star and sometimes moves around more after that. Since I am wanting to create a brightness profile as a function of radius I want the center to stay in one place and stay were I originally set it, while getting a good fit for the rest of the parameters. I have tried several different Ellipticities and Position angles but if the central position is set to be fixed then I get large errors. I was wondering if there are any parameters that need to be set a certain way in order to obtain a good fit for a galaxy with this task or if there is any other advice to get this task to create a good fit. Any help would be greatly appreciated.Thanks

I see you have not received any replies. This application was developed at Space Telescope and, as the only IRAF science analyst at NOAO I have no experience with the software or surface photometry. So I suggest you write directly to the help desk at the institute; help@stsci.edu.Yours,
Frank Valdes