Diapl - Difference Image Analysis Package

[yba]

Diapl is the program based on DIA by Dr. P. Wazniak to do image subtraction. I use diapl2 to reduce my CCD data. When my CCD images' fwhm is about 5 pixels, the results are good, but when the fwhm is 15 pixels (my scale is about 0."28/pixel), the subtracted images are bad. I know that I must adjust the parameters in diapl2, but I do not know whose parameters should be adjused , especially the 'getpsf.par' which has so many parameters. I hope to get help from experienced users.
Thanks in advance.

 

[Drakkith]

What exactly are you trying to do with your images? What are you subtracting from them?

 

[yba]

I use diapl2 to reduce the globular clusters, subtract the constant stars' images and get the variables on the subtracted CCD frames, just as others do. I exactly follow the manual of diapl, step by step, do not change the parameters of diapl2 (except the necessary changes, for example, my CCD gain, min fwhm, maxfwhm etc.).
My fwhm varied from about 5 pixels to 15 pixels within one night. The subtracted images are good for small fwhm CCD frames, but bad for large ones. The 'bad' means:
1. On the subtracted frames, the constant stars do not disappear, e.g., they have remained rings around the core of the stars. Obviously, at least I should enlarge the value of the psf radius. But I do not know, when I change the value of the psf radius, which other parameters should I also change together, because I do not know the meaning of these parameters?
2. In order to check my results, I plot the light curve so obtained for very well known variable stars. There are large scatter at the points of the light curve obtained from large fwhm CCD frames, but small scatter at other points.

Is it possible please tell me which parameters should be changed together?
Thank you very much.

 

[Drakkith]

Hmmm, I think this is a little beyond my knowledge. I'm just an amateur astrophotographer and I don't use the same program as you do.