[Issues] Sextractor config for SourceList creation

[Hans Vaith]

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