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Software Packages AIRY 5.1 known issues:
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1- The background estimation done by PRE module (and FSM module and ANB module
for a star detection limit) can be contaminated by the object if the extension
of the object is a large fraction of the image frame. The estimated mean value
of the sky background and especially its rms value are then biased. 

SOLUTION: A simple solution is to compute directly the mean background using
the routine n_phot (found in .../caos/lib/, help at the IDL/CAOS prompt with
the command 'doc_library, "n_phot"'). For example, with a 0.01 arcsec pixel
size, a telescope collecting surface of 10 m^2, an integration time of 100 s
and an overall efficiency of 0.3, the K band mean background (per pixel) is: 

   0.3*(n_phot(0.,BAND="K",DELTA_T=100.,SURF=10.))[1]*0.01^2

Then it is necessary to enter by hand the value found within the PRE module GUI
(using the option "consider a user-defined background").


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revised: February 2012 
* Andrea La Camera
* Marcel Carbillet
