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Spectrum extraction and calibration

Once the data in the image frame has been preprocessed the multiple spectra can be flat-fielded, extracted and calibrated. It is only required that there exist a BOXES list containing the definitions of the spectra locations. The BOXES list can be updated in accordance with the actual data using the UPDBOXES command.

The 1D spectra files are created using the EXTRACT command and the wavelength calibration of the arc images to take place via the ARC command. These commands have many options and are described in detail below. Alternatively the following routines may be useful to perform variations on the standard analysis:

• SKYSUBTRACT performs the sky-subtraction for each spectrum. It can be run in either automatic or, by use of the logical argument PLOT, interactive mode. The algorithm differs somewhat from that used by EXTRACT.
• FLUX uses a fluxing standard contained in an FSPEC list to correct for the spectral response of the detector/instrument combination. It can be multiplied ( /MUL) or divided ( /DIV). The spectrum and standard are assumed to have the same dispersion and spectral range.
• SCRUNCH transforms the data to a linear wavelength scale.
• BOXIMAGE cuts out a given spectrum and puts it into a smaller external 2D image. This might be required if another package is to be used for the subsequent calibration.

  The header items from the parent image are propogated, and information regarding the details about the box is also recorded.

• HORNE is an option to perform optimal extraction using the Horne (P.A.S.P., 1986) algorithm. This algorithm is particularly appropriate for spectra which have only small tilts of the object spectrum, such as those produced by LDSS-2.

  It is intended to be performed on images which have already been sky subtracted.

• Flatfielding
• Arc subpackage
  • ARC command interface
• Using the Extract command