[Issues] why the illumination correction is underestimated in aw

[Fedor I. Getman]

Dear Philippe and Gijs,

first of all, my apologise for not commited to cvs my modification of 
illumination correction done several mounth before.
Here the modified method "get_inputs_for_fit_routine()" wich must be 
invocked instead original one in "make_fit()" of IlluminationCorrection 
class.
The idea is to adjust before fitting the residuals into the same 
surface, using common value over FOV - sky background (as determined by 
sExtractor during source detection).

In our tests with 3c273 mosaic, that modified method to derive 
IlluminationCorrection give reasonable results in comparision to SDSS mags.

-------------------------------------------------------------------------------------------------
    def get_inputs_for_fit_routine_new(self):
        '''This routine accumulates source information from the input
        PhotSrcCatalog objects in preparation for the overall fit.
        '''
        sigclipped_rawzeros_list = []
        pixelpos_list            = []
        error_list               = []

        gridpositions_dict = {}
        sigclipped_rawzeros_dict = {}
       
        for photcat in self.photcats:
            for gridposition in self.gridpositions[photcat.chip.name]:
                
gridpositions_dict[photcat.chip.name+"_"+str(gridposition.index)] = 
gridposition

            for src in photcat.photsourcelist:
                corrected_dmag = src.mag - src.instmag - 
(self.photcats[0].skybackground-photcat.skybackground)
                dmag_error     = math.sqrt(src.mag_err**2 + 
src.instmag_err**2)
                
sigclipped_rawzeros_dict[photcat.chip.name+"_"+str(src.index)] = 
(corrected_dmag, dmag_error)
           
        # do sigma-clipping
        mean, stdev   = 
get_mean_and_stdev([sigclipped_rawzeros_dict[key][0] for key in 
sigclipped_rawzeros_dict.keys()])
        median        = do_median([sigclipped_rawzeros_dict[key][0] for 
key in sigclipped_rawzeros_dict.keys()])
        sigma_clip    = abs(self.process_params.SIGCLIP_LEVEL * stdev)
       
        for key in sigclipped_rawzeros_dict.keys():
            if abs( sigclipped_rawzeros_dict[key][0] - median ) > 
sigma_clip:
                sigclipped_rawzeros_dict.pop(key)

        # Add the gridpositions and the corrected raw zeropoints to the 
lists defined
        # above
        for star in sigclipped_rawzeros_dict.keys():
                
sigclipped_rawzeros_list.append(sigclipped_rawzeros_dict[star][0] - median)
                pixelpos_list.append((gridpositions_dict[star].x, 
gridpositions_dict[star].y))
                error_list.append(1.)

        return pixelpos_list, sigclipped_rawzeros_list, error_list
-------------------------------------------------------------------------------------------------


Philippe Heraudeau wrote:
> Dear Gijs,
>
> As we discussed yesterday the illumination correction is underestimated
> in awe because of the rescaling of each individual chip to the median of
> the zeropoints (see ./astro/main/IlluminationCorrection.py)
> I simulated this in the 2 ps files you can download with:
>
> wget http://www.astro.uni-bonn.de/~heraud/illum_4_4.ps.gz
> wget http://www.astro.uni-bonn.de/~heraud/illum_8_8.ps.gz
>
> which illustrate this effect for 4x4 chip and 8*8 chip cameras.
> The higher number of chips the more "smoothing" and that's why you
> notice more the underestimation with CFHTLS data than with WFI.
>
> The figures show:
> a) the input illumination correction which is about 25% maximum from
> peak to valley.
> b) the 2D 2nd order polynomial fit of the input
> c) the input illumination after dividing the field by the number of
> chips and rescaling each chip to the median to simulate what is done in
> aw for 4x4 chips in illum_4_4.ps and 8x8 chips in illum_8_8.ps
> d) the 2D 2nd order polynomial fit of c) which clearly underestimate the
> input: only a few percent (6-7%) are seen instead of 25%
> Note: The minimum of the plotted surfaces is always set to 0 and the
> median of c is 0.
>
> We should solve this issue to get the proper illumination correction...
>
> Cheers,
> Philippe
>
>