[Issues] Sextractor config for SourceList creation
Gijs Verdoes Kleijn
verdoes at astro.rug.nl
Mon Jan 22 17:38:30 CET 2007
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)
>>
>>
>
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