Applying an Illumination Correction
Applying an Illumination Correction The twilight flats at u', g', and r' are somewhat contaminated by OH skyglow emission that can sneak in to varying degrees depending on distance from the center of the megacam FOV. I don't yet know if this problem is present also at i'. It is not present at z'.
To correct for the OH contribution to the twilight flat simply divide all frames by an appropriate illumination correction frame. My convention has been to name the output files with .zpcor.fits suffixes to indicate that the zero-point is correct at all radii from the FOV center. The following assumes that the megamerged, sky-subtracted input frames are named with a skysubm.fits suffix:
me> vi fix_illum.macro # vi commands follow
:1,$ s/skysubm/zpcor/g
:1,$ s/zpcor/skysubm/
:wq
me> cl < fix_illum.macro
In other words, the macro just created ought to have this format:
mscarith SSA22.2885.skysub.fits / ../2007.1102/Illum_z.fits SSA22.2885.zpcor.fits
mscarith SSA22.2886.skysub.fits / ../2007.1102/Illum_z.fits SSA22.2886.zpcor.fits
mscarith SSA22.2907.skysub.fits / ../2007.1102/Illum_z.fits SSA22.2907.zpcor.fits
mscarith SSA22.2908.skysub.fits / ../2007.1102/Illum_z.fits SSA22.2908.zpcor.fits
mscarith SSA22.2909.skysub.fits / ../2007.1102/Illum_z.fits SSA22.2909.zpcor.fits
mscarith SSA22.2910.skysub.fits / ../2007.1102/Illum_z.fits SSA22.2910.zpcor.fits
mscarith SSA22.2911.skysub.fits / ../2007.1102/Illum_z.fits SSA22.2911.zpcor.fits
mscarith SSA22.2912.skysub.fits / ../2007.1102/Illum_z.fits SSA22.2912.zpcor.fits
mscarith SSA22.2913.skysub.fits / ../2007.1102/Illum_z.fits SSA22.2913.zpcor.fits
When this process has completed, inspection of the *zpcor.fits files should indicate that superficially at least they are very very flat.
Last updated November 26, 2007.