[Issues] Sextractor config for SourceList creation

[verdoes at astro.rug.nl]

Dear Hans,

As one step in the process to solve the current issue I have determined
how the coaddition is done algorithmically. I have added this information
in the how to on coadd.

Ewout and I are working further on solving the issue.

best wishes, Gijs
> 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,
> Hans
>
> 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)
>