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This task is used for the extraction of the multiple spectra from a 2D image. The spectra are output into 1D list files, not into slots. In order to define the positions of the spectra on the image there must exist a previously created BOXES list file. The user should declare the box numbers they want by:
LEXT - IMAGE mode > EXTRACT 2-5 6-10 15or, if all spectra should be processed:
LEXT - IMAGE mode > EXTRACT ALL
EXTRACT has a large number of options allowing most of the sophisticated reduction processing to be preformed with the one command, provided the necessary lists are present.
The default is to extract a normal sky subtracted object spectrum for each box. The algorithm used weights the contribution of the object rows according to a gaussian model of the profile. This helps to optimise the signal to noise ratio. However the following qualifiers provide alternatives which may be useful for arc, night-sky or quick-look spectra:
If a normal weighted extraction is to be performed then sky will be subtracted unless the /NOSUB qualifier is specified. The subtraction process can be either automatic (the default) or interactive. In the interactive case the user fits the slit profile via the image display cursor. In the automatic case the program will provide a graphical display of its progress by specifying /PLOT.
It is also possible to scrunch the spectra to a linear wavelength scale with /SCRUNCH (requires a COEFS list), and to flux them with /FLUX or /DFLUX dependent on whether the fluxing standard is multiplicative or dividing (requires a FSPEC list).
The /TILT option can be used to account for tilt of spectral lines if it is large, but this is a slow process and will not usually show any significant difference over the default method.