Observation pre-processing consists of standard bias, dark and flat-field
corrections. The package provides different commands to average images,
extract sub-images or normalize flat-fields. General information
related to preprocessing commands is available in the
section Dark, Bias and Flat-Field of SHOW/LONG and can be
obtained by:
Midas...> SHOW/LONG d
The command PREPARE/LONG extracts sub-images. The pixel positions
and 
of the lower-left and
upper-right corners of the extracted sub-image can be specified with the
keyword 
. The command processes a catalog by default or
an image if the name is provided with the extension .bdf.
The sequence:
Midas...> CREATE/ICAT flatcat ff*.bdf
Midas...> SET/LONG TRIM=10,20,500,550
Midas...> PREPARE/LONG flatcat exf
generates a catalog flatcat.cat referencing the files ff*.bdf
and extracts the sub-window [@10,@20:@500,@550] (see help file
of EXTRACT/IMAGE). The output files will
be called exf nnnn.bdf where nnnn is a four-digit number
starting at 0001.
Images can be combined using an average or median method with the
command COMBINE/LONG. This command processes a catalog of images
and generates a combined image. The two commands:
Midas...> CREATE/ICAT flatcat exf*.bdf
Midas...> COMBINE/LONG flatcat avflat AVERAGE
generate a catalog flatcat.cat referencing the files exf*.bdf
and average them to produce the image avflat.bdf. The option
MEDIAN can be used to eliminate particle hits when combining
dark exposures. Flat-field normalisation can be achieved by the command:
Midas...> SET/LONG BIAS=190. FDEG=2
Midas...> NORMALIZE/FLAT avflat normff
which performs a bias correction of value 
on the image
avflat.bdf and interpolates the flat-field continuum by a polynome
of degree 
to produce the image normff.bdf. It can be
noted that flat-field normalisation can require a different approach than
the one implemented in the command NORMALIZE/FLAT. The Section 1.2.4
(Flat-Fielding) of Chapter 6 in Vol. B presents alternative methods.