[AstroPy] Tr : Re : Aperture photometry

Jerome Caron jerome_caron_astro at ymail.com
Fri Apr 15 01:17:57 EDT 2011


Dear all
I realise that I forgot to put the list in copy, see my message below...
It looks that there are many interesting options about aperture photometry !! It 
would be interesting to compare performance of each one, in terms of quality and 
rapidity. Is anyone willing to investigate ?
Regards

Jerome Caron


----- Message transféré ----
De : Jerome Caron <jerome_caron_astro at ymail.com>
À : Sergio Pascual <sergiopr at fis.ucm.es>
Envoyé le : Jeu 14 avril 2011, 21h 11min 49s
Objet : Re : [AstroPy] Aperture photometry


Dear Sergio

You may want to have a look at the astro library I am developping. It's based on 
Pyfits:
http://astrosurf.com/caron/python_scripts.html

I am precisely interested in aperture photometry. I make light curves of 
asteroids; some of my data is processed by observatory of Geneve, Switzerland:
http://obswww.unige.ch/~behrend/page1cou.html#000121http://obswww.unige.ch/~behrend/page1cou.html#000129

http://obswww.unige.ch/~behrend/page1cou.html#000202

My main script for aperture photometry is "photometry_call.py", it calls two 
functions
- first one finds the (x,y) positions of the stars on each frame in the serie 
(max pixels and/or centroid of gaussian psf)
- second one makes aperture photometry
I also use the two other scripts in the folder photometry to check uniformity 
and detect bad images.

I found that my scripts give better results than common softwares as e.g. Iris 
or Au-dela (http://www.astrosurf.com/buil/us/iris/iris.htm or 
http://www.audela.org/). Magnitude noise is typically smaller by 10%; the reason 
is that I do shift every object (star, asteroid) so that its centroid, 
calculated by gaussian fit, is exactly centered on one pixel, before making 
the photometry. The translation is made by bilinear interpolation, which exactly 
conserves photometric information. Then the variability of PSF from image to 
image does affect all the objects in the same way (asteroid and ref stars) which 
is not true if they are not centered, as in the standard approaches. I plan to 
make a web page on this before summer. This feature can be deactivated with a 
flag, if you don't like it.
Another nice trick is that you can make photometry with as many different 
apertures you want in one run. At the end you can pick up the best results.

The documentation is a bit short, but all functions are described. If you have 
any question, I'll be happy to give some help.
Cheers

Jerome Caron



________________________________
De : Sergio Pascual <sergiopr at fis.ucm.es>
À : astropy at scipy.org
Envoyé le : Jeu 14 avril 2011, 19h 18min 19s
Objet : [AstroPy] Aperture photometry

Hi

Let's say I have an image as a  numpy array and I want to obtain the
summed values of the array inside an ellipse inside the image. In
other words, I wanto to make aperture photometry. Do you know of any
python astronomy package that implements this. It's a fairly basic
think, but I haven't found anything.

Regards

-- 
Sergio Pascual                        http://guaix.fis.ucm.es/~spr
gpg fingerprint: 5203 B42D 86A0 5649 410A F4AC A35F D465 F263 BCCC
Departamento de Astrofísica -- Universidad Complutense de Madrid (Spain)
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