jitter and photometry
Until version 4.5.0, the sky estimation method used by the jitter (combine method) for ISAAC was slightly different from the one used for example by the dimsum IRAF command: The running filter always rejects the central value in the dimsum method, not in the jitter combine method.
A new sky estimation method is now available in the jitter which exactly implements the method used by dimsum. This method is named combine_mc (for combine minus central) and is the default sky estimation method used by the jitter since version 4.5.1.
Tests have been done by Marina Rejkuba to compare the photometry difference between dimsum and the jitter combine method (see dimsum-combine.ps.gz) and the photometry difference between dimsum and the jitter combine_mc method (see dimsum-combine_mc.ps.gz).
These tests show that the photometry difference with dimsum is drastically reduced with the use of the combine_mc method. The remaining difference can be attributed to the median filter applied at the end by jitter on the combined image to remove row saturation effects, and that can be switched off in the jitter INI file with the following keyword:
RowSubtractMedian = no ;
Angela Iovino (firstname.lastname@example.org) had written in the past a complete report about the defects of the sky-subtraction combine method as implemented in the jitter command. You can retrieve a gzipped postscript version of her analysis here.
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