[Issues] Sextractor config for SourceList creation
Hans Vaith
hav at mpe.mpg.de
Fri Mar 2 17:39:39 CET 2007
Dear Gijs,
thanks for the update on this and sorry for the long delay in responding. I
was on a long vacation and finished a Header Translator this week.
The changes that you and Ewout propose look good to me. After reading the
swarp manual I came to the same conclusion as you, that a map of gains would
ideally be required.
Best wishes
Hans
On Monday January 22 2007 17:38:30 Gijs Verdoes Kleijn wrote:
> Dear Hans,
>
> An update on the photometry issues:
> -regriddedframes: Ewout will change soon the implementation of
> SourceList such that it will use the appropriate zeropoint for these
> frames which will lead to correct magnitudes/fluxes and their associated
> errors. We will let you know when this has happened.
> -coaddedregriddedframe: as you know SourceList no determines properly
> the magnitudes/fluxes by default. Bow for errors in them: thinking more
> about it myself and talking to Emmanuel further on this I now understand
> there is no good way now to obtain good error estimates from a
> coaddedregriddedframe which is the product of >1 regriddedframes which
> each have their own zeropoints (hence FLXSCALE values). The information
> needed to do it is simply missing. A map detailing the resulting GAIN
> for each pixel would be a solution, but Emmanuel has no time to
> implement this in the near future. For the time being I therefore reach
> the conclusion that one has to decide oneself how to obtain approximate
> errors best for coaddedregriddedframes made out of >1 regriddedframes.
> Obviously, I can image that one uses some average from the GAIN values
> of the RegriddedFrames.
>
> If I come up with a better solution I'll let you know.
> Best wishes, Gijs
>
> verdoes at astro.rug.nl wrote:
> > 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)
> >
> > _______________________________________________
> > Issues mailing list
> > Issues at astro-wise.org
> > http://listman.astro-wise.org/mailman/listinfo/issues
>
> _______________________________________________
> Issues mailing list
> Issues at astro-wise.org
> http://listman.astro-wise.org/mailman/listinfo/issues
More information about the Issues
mailing list