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Helplog #36 -- Measuring Ellipticities and Position Angles

  • Monday, November 14 2005 @ 02:51 PM GMT
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Help Logs/Tips Question: I have a (long) list of x and y positions of stars in an image. I would like to measure the ellipticities and position angles of the images non-interactively. How should I do this?
NUMBER: 36
KEYWORDS: psfmeasure, ellipticities, position angles, imexamine
DATE: Mon Nov 14 12:53:53 MST 2005
FROM: valdes

Q: I have a (long) list of x and y positions of stars in an image. I
would like to measure the ellipticities and position angles of
the images non-interactively. How should I do this?

A: IMEXAM I suggest you look at the task PSFMEASURE. In the current
version of IRAF it is part of the OBSUTIL package. To do the
non-interactive job you want you would make a text file of x/y
for all your stars.

ob> psfmeasure starfield imagecur=mystars.dat >G dev$null
** Select stars to measure with 'm' and finish with 'q'.
** Additional options are '?', 'g', and :show.
NOAO/IRAF V2.12.2a-EXPORT valdes Mon 12:47:40 14-Nov-2005

Image Column Line Mag FWHM Ellip PA SAT
starfield 164.79 184.59 0.00 2.206 0.06 -23

Full width at half maximum (FWHM) of 2.2061

This is with the default parameters. Note the redirection of
the graphics to null file. You could redirect the graphics to a
metacode file viewable with (e.g.) GKIMOSAIC. However for 30,000
stars I don't think you want that.

The algorithms in PSFMEASURE are largely the same as in IMEXAM.

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