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fitz
 11/25/2014 07:29AM (Read 3035 times)  
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[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-Cool=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?


 
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fitz
 11/25/2014 07:30AM  
AAAAA
Admin

Status: offline


Registered: 09/30/2005
Posts: 4040
[Moderator's Note: Followup]


This is related to the previous email I sent you.

I found this in an IRAF tutorial by W.E. Harris (published in 2008);

"The *.als file gives RELATIVE magnitudes; it has no zeropoint that means anything much. Now you have to convert them to instrumental magnitudes like those in the aperture photometry file. To do this, you essentially need to know the mean difference ("offset") between m(allstar) and m(phot). That is, correlate the output from ALLSTAR and from PHOT star-by-star (write your own routine for cross-identifying them) and plot up [m(PH)-m(ALS)] versus m(PH). Find the mean difference, by concentrating most on the bright stars. Reject outliers and ignore the big scatter at the faint end. Add this mean difference to all the .als magnitudes,and you have now converted them to the aperture-photometry scale.
But now notice that the aperture photometry was done with a SMALL aperture, not the large "fiducial" one that contains all of the light and that you decided on from the standard stars (see Part I step 4 above). So the last thing you need to know is the magnitude difference [m(fiducial ap) - m(small ap)]. Use the curve-of-growth technique to determine this. That is, for a few bright isolated stars, do multiple-aperture photometry and find the magnitude difference directly on that image. Then, add that mean difference to all the aperture photometry. At last, you now have a full list of instrumental magnitudes of all the stars on the image, on the right large-aperture magnitude scale."

Regarding the 1st paragraph, since we do an initial "phot" in psf fitting photometry (using the minimum size apperture), doesn't it automatically assign a zeropoint value to the magnitudes obtained from "allstar"? Is it necessary to correct the differences between the magnitudes from the phot file ( say test.mag.1 ) and the allstar file (say test.als.1) ?

I do comprehend the requirement for the 2nd process (aperture correction), but I don't understand why we need to correct the magnitudes obtained from psf fitting due to the no zeropoint/ relative magnitudes issue.

 
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