[Issues] overscan correction methods

[G.A. Verdoes Kleijn]

Dear Astro-Wisers,

Data reduction processes in the Astro-Wise system have the option to use
subtractions of overscan regions, i.e., 'overscan corrections'. 
Bias frames: if no overscan correction is used the masterbias frame will
contain both a typical overall bias level and the smaller scale bias
variations. The average of the overall bias level is a positive number
(mostly >100) while the average of the small-scale bias variations is
very close to zero (mostly <<1). Thus there are two main types of
masterbias frames those with the overall bias level (type MB0) and those
without (type MB1). Eight different ways are defined for performing
overscan corrections. In other words, MB1 consists of 8 subtypes. 

Thus for reduction processes which involve subtraction of bias frames it
must be assured that the appropriate masterbias is subtracted.
Currently, the Astro-Wise system does not enforce selection of an
appropriate masterbias (a warning is issued in some situations).
Espcially for automated processes involving bias subtraction (e..g, with
the recipes and TargetProcessing tools) it would be highly convenient to
have a check on overscan correction. With this in place it will be
possible as well to improve the default calibration files (i.e., the
"forevers").

Best wishes, Gijs