[Issues] Sextractor config for SourceList creation

Gijs Verdoes Kleijn g.verdoes at astro.rug.nl
Wed Dec 13 08:18:54 CET 2006

Dear Hans,

Thanks for the report. I would like to add information on how photometry 
is *handled* in Astro-WISE to the HOW-TOs which currently tell how 
photometry is derived. For this reason I am happy to help out in 
resolving the remaining issue below. Currently the system is down here, 
so I cannot check the following statement: aren't the FLXSCALE of the 
RegriddedFrame and of the corresponding CoaddedRegriddedFrame identical 
in your test below? If so, I would presume that you should be able to 
use the FLXSCALE of any CoaddedRegriddedFrame (e.g., one composed of 
many RegriddedFrames) to set GAIN.  But we should carefully check how 
this FLXSCALE is computed from all the different FLXSCALEs of all the 
RegriddedFrames that went into the CoaddedRegriddedFrame.

Best wishes, Gijs

Hans Vaith wrote

>This is a followup to the problem that we (MPE) reported at the AstroWise 
>workshop concerning some parameters in SourceLists created from 
>ReducedScienceFrames, RegriddedFrames and CoaddedRegriddedFrames:
>1) When creating SourceLists from Reduced or Regridded Frames the zeropoint is 
>not applied. This can be fixed easily by setting the sex.conf parameter 
>MAG_ZEROPOINT to the zeropoint as returned by 
>2) When creating SourceLists from a CoaddedRegriddedFrame, the errors of the 
>fluxes and magnitudes are way too high. In an email conversation E. Bertin 
>pointed out that this is caused by not setting the GAIN parameter in 
>I have experimented with a CoaddedRegriddedFrame that was made from a single 
>RegriddedFrame. Creating the source catalog by running Sextractor manually, I 
>set the GAIN in sex.conf to the inverse of the FLXSCALE attribute of the 
>RegriddedFrame. This way one gets the same error for the magnitude as in the 
>case of creating the catalog from the RegriddedFrame directly. The flux error 
>is now also consistent with the result from the RegriddedFrame (consistent 
>meaning that the S/N ratio flux/flux_error is the same). How to handle this 
>in the case of more than one RegriddedFrame being coadded I don't know at the 
>moment (how to compute an overall fluxscale from the individual numbers)
>For reference, I appended the response that I got from Emmanuel Bertin after 
>sending him some data after the workshop:
>>I had a look at your data, and I
>>noticed that the co-added image are scaled with a very small number,
>>something like 10e-11!! That should not cause much trouble except that you
>>forgot to scale the GAIN (aka conversion factor) accordingly, that is by
>>something like several billions e-/ADU. If you set the proper GAIN, you
>>will get correct results, although I suggest you set the image zero-point
>>to a more convenient value (say, 30 or so), and use a non-zero the
>>MAG_ZEROPOINT SExtractor configuration parameter. Best regards,
>>                      Emmanuel.
>Dear Emmanuel,
>I contacted you last Monday during the AstroWise workshop in Leiden concerning 
>a problem that we have with understanding the output of Sextractor when 
>running it to create a catalog from a CoaddedRegridded image. The problem is 
>that the errors in the fluxes and magnitudes appear to be way too high. This 
>happens only with the coadded image, whereas the catalog created from the 
>associated regridded image looks fine.
>There is a zip file containing the images, weights (regridded and coadded 
>frames) and configuration files that we have been using when we ran 
>Sextractor. This file is stored on "ftp.mpe.mpg.de" and can be accessed via 
>ftp. The filename is "for_bertin.zip". To retrieve the file please login as 
>user "anonymous" and use your email address for the password. Then change to 
>directory "/pub/edi_esoc_dds/".
>Our Sextractor version is 2.4.4
>To give you an idea about the numbers, I listed magnitudes, fluxes and their 
>associated errors for a single source that is contained in the output 
>                regridded      coadded
>MAG_ISO         19.165         19.165
>MAGERR_ISO      0.009          7338.7
>FLUX_ISO        20819          2.16e-8
>FLUXERR_ISO     164.3          1.46e-4
>Your help is much appreciated.
>Best Regards
>Hans Vaith
>Issues mailing list
>Issues at astro-wise.org

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