[Issues] SourceList on a CoaddedRegriddedFrame has wrong error values

[Gijs Verdoes Kleijn]

Dear Johannes,

An approximation to the magnitude error for coadds can be computed by 
setting the following GAIN value in the sextractor parameters:

pars.SourceList.sexconf.GAIN=pylab.median(pylab.array([reg.gain.gain for reg in coad.regridded_frames])) / pylab.median(pylab.array([reg.FLXSCALE for reg in coad.regridded_frames]))


This approximation is reasonable across the full FoV if the gain and 
FLXSCALES of the regrids are not varying significantly which is true 
under many circumstances in the optical (i.e., dithered observations 
over a night or nights with similar observing conditions).

Some background on the origin of this issue:
Errors on photometry are computed using the formula in sextractor manual 
page 36. This is equal 
to:dmag=1.0857*math.sqrt(sl.sources[0]['FLUX_APER']/sl.sexconf.GAIN+sl.sources[0]['NPIX']*sl.sources[0]['BackGr'])/sl.sources[0]['FLUX_APER']. 
The formula assumes fluxes are in ADU which is not the case for the 
coadds…..

Note that the coadd weight frames contain the flux errors: the 
weightframe is inverse variance=1/sigma^2.

Best wishes, Gijs


Johannes Koppenhoefer wrote:
> Dear AWE users,
>
> I found irregular MagERR valuse creating a SourceLists on CoaddededRegriddedFrames and I was wondering if anyone has seen the same and has an explanation:
>
> The following example should be easy to reproduce:
>
> If I create a SourceList on a RegriddedFrame, I get reasonable MAGERR values.
> However if I create a CoaddedRegriddedFrame with only this single RegriddedFrame as an input and make a SourceList on this CoaddedRegriddedFrame, the MAGERR values are extremely large (~10^5). 
>
> We are having this problem now for months and I checked yesterday that it is still present in the most actual AWBASE checkout.
>
> Cheers,
> Johannes
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>   

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|dr Gijs Verdoes Kleijn         |   astronomer                     |
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