[Issues] why the illumination correction is underestimated in aw

Philippe Heraudeau heraud at astro.uni-bonn.de
Tue Nov 17 13:06:36 CET 2009

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...


Philippe Heraudeau
Argelander-Institut für Astronomie Email: heraudeau at astro.uni-bonn.de
Auf dem Hügel 71                   Tel.: +49 (0)228 736729
D-53121 Bonn                       Fax.: +49 (0)228 731775

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