[Moderator's Note: Forwarded on behalf of user]
After weeks of intensive research I finally managed to obtain a CMD of the globular cluster M53 using B,V filters. (See Attachment)
The magnitudes here are of course the instrumental ones.
The issues I'm facing are as follows;
(1). I used daophot/ daofind,pstselect,psf,allstar to do photometry for all the stars (both standard and non-standard) on the globular cluster defining an aperture radius of 3px in 'photopars'. (I read that this had to be equal to the FWHM of the radial profile of a typical star)
From what I've read, I understand that we have to do Aperture Photometry (apphot/ phot) on the globular cluster to extract the magnitudes of some standard stars. If we use an aperture size here different to what we used for psf photometry (allstar)-(3px), we have to do an aperture correction using a Curve of Growth, correct? Say we used a 5px aperture here as opposed to the 3px one in psf allstar photometry; we have to find the difference in magnitudes corresponding to 5px and 3px using the COG plotted using data obtained by Aperture Photometry? Then add this magnitude difference to all the stellar magnitudes obtained from psf allstar?
However, is it possible to refrain from doing an Aperture Correction if we used daophot psf allstar photometry (as opposed to doing aperture photometry-phot) to find standard star magnitudes as well, using the same aperture size we used on photometry on all the stars using psf allstar. (3px in this case)
So we do psf allstar photometry for both the whole star field and the standard stars without the need to touch aperture photometry.
Also, is the aperture correction required if we do aperture photometry on standard stars anyway, but with the same aperture size used for daophot/ allstar photometry? (3px aperture for both) The magnitude difference does become zero here but thought of asking just in case there is more to it than expected.
(2). I'm using the following transformation equations for calibration.
b=B + colorterm_b*(B-V) + kb &
v=V + colorterm_v*(B-V) + kv where k=(extincion*airmass + const)
(Ref: A user's guide to stellar ccd photometry with iraf; Massey.P, Davis. L, page 26)
I first tried to arrange it in the y=mx+c form; (b-
=colorterm_b*(B-V)+kb then find the colorterm and k with a plot (2 plots for each filter) between b-B and B-V obtained from standard stars. Even if I could determine colorterm and k this way there was no way I could convert my instrumental magnitudes to standard magnitudes using these equations. The method the iraf manual describes to solve this proved to be an arduous task. Is there any simpler way of coming up with a way to transform instrumental magnitudes into standard ones?