[Issues] Sextractor config for SourceList creation

Hans Vaith hav at mpe.mpg.de
Wed Dec 13 09:46:37 CET 2006

Dear Gijs,

thanks for your response. As far as I can see the FLXSCALE is not computed for 
CoaddedRegriddedFrames at the moment. The attribute does not even exist for 
this class. I agree that using this number, once it is available and 
correctly computed for the CoaddedRegriddedFrames, would solve the problem.

Best wishes,

On Wednesday 13 December 2006 08:18, Gijs Verdoes Kleijn wrote:
> 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
> >frame.photom_params.get_specific_zeropoint(...)
> >
> >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 sex.conf.
> >
> >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 catalogs:
> >
> >                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
> >http://listman.astro-wise.org/mailman/listinfo/issues

Hans Vaith   Max-Planck-Institut fuer extraterrestrische Physik
             Postfach 1312, D-85741 Garching, Germany
             Phone :(Country Code 49) 89 30000-3831
             e-mail: hav at mpe.mpg.de (Fax: 89/30000-3569)

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