[Issues] Sextractor config for SourceList creation
Gijs Verdoes Kleijn
verdoes at astro.rug.nl
Mon Mar 12 10:33:39 CET 2007
Dear Hans,
Thanks for your email.
-regriddedframes: Ewout has changed the implementation of SourceList so
that it computes the correct magnitudes/fluxes for regriddedframes.
-coaddedregriddedframe: solution is still that it is up to the user to
decide how to approximate best the errors for the photometry of a
coaddedregriddedframe of >1 regrids. As far as I know Emmanuel has not
made any changes to swarp for this problem. I propose that we try to
make a generic solution once it turns out this is indeed needed by users
in general. At the moment I think the current status is sufficient. Let
me know if you think otherwise.
Best wishes, Gijs
Hans Vaith wrote:
> 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)
>>>>
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