Pre-processing the spectra

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.