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Hi everybody,I'm working with frames with an unusually large FWHM: around 10-11 pixels. I'm following the 'A user's Guide to Stellar CCD Photometry with IRAF' by Massey & Davis (1992). I'm using a script I wrote, but I can't seem to substract the PSF stars and their neighbours cleanly, no matter what I try to do. Bellow I transcript the steps I follow and the parameter values I use (I've tried several small variations of this parameters values, with no satisfying results):1) Input of the following parameters:-[b:2af3e38604]smean[/b:2af3e38604] = 'Sky mean value' (obtained by: imexa + 'm' key over a clear sky patch. The value I've used is around 16)
-[b:2af3e38604]delta[/b:2af3e38604] = sqrt(smean*gain + rdnoise*rdnoise)/gain (Gain = 2.1, Rdnoise = 8)
-findpars.[b:2af3e38604]threshold[/b:2af3e38604] = 3.5*delta
-datapars.[b:2af3e38604]fwhmpsf[/b:2af3e38604] = 'typical FWHM value' (around 10-11 pixels)
-datapars.[b:2af3e38604]sigma[/b:2af3e38604] = 'SSTDEV value' (around 2)
-datapars.[b:2af3e38604]datamin[/b:2af3e38604] = smean -3*delta
-datapars.[b:2af3e38604]datamax[/b:2af3e38604] = 64000
-datapars.[b:2af3e38604]noise[/b:2af3e38604] = "poisson"
-daopars.[b:2af3e38604]psfrad[/b:2af3e38604] = 'radius of the larger unsaturated PSF star' (I've used mostly something around 30 pixels)
-daopars.[b:2af3e38604]fitrad[/b:2af3e38604] = 'typical FWHM value'2) Daofind run3) Input of the following parameters:-unlearn [b:2af3e38604]centerpars[/b:2af3e38604]
-unlearn [b:2af3e38604]fitskypars[/b:2af3e38604]
-unlearn [b:2af3e38604]photpars[/b:2af3e38604]
-unlearn [b:2af3e38604]psf[/b:2af3e38604]
-photpars.[b:2af3e38604]apertures [/b:2af3e38604]= 'typical FWHM value'4) Phot run5) PSF stars selection6) PSF run7) Input of the following parameters:-daopars.[b:2af3e38604]psfrad [/b:2af3e38604]= psfrad - fitrad ('radius of the larger unsaturated PSF star' - 'typical FWHM value'. I've been useing this values: 30 - 10 = 20)8) Nstar run9) Substar runAt this point I should have a 'name.sub.1.fits' frame with no PSF stars or their neighbours (according to Massey's guide), but I don't. The closest I get is a horribly substracted frame. I'm thinking there must be something wrong with the parameter values I use, but I can't figure out which ones.Any help will be very much appreciated!Gabriel

Hello,I'm afraid I don't have anything to add. Massey is the expert here. I am responding mostly to for us to know that at least you got an acknowledgement. I hope you have made progress. In many cases you just have to experiment with the parameters to get the best result you can.Yours,
Frank Valdes

Thanks anyway for your answer Frank.Cheers!Gabriel