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Wavelength calibration

In order to find the relationship between pixel position, in a given spectrum, and wavelength we use one or more dispersed, multiaperture images of standard wavelength calibration lamps. The procedure is to identify strong lines of known wavelength in each spectrum and to use this information to fit a low order polynomial relating pixel position to absolute wavelength. In order to get a reasonably good fit, ten or more lines should be identified, ranging over the whole spectral range.

The facility in LEXT for doing this is ARC. When invoked from the command line, the user enters a sub-package from which a smaller number of ARC specific commands can be issued. The reader is advised to refer to the on-line HELP for full details, but here we consider the main steps.

Before entering ARC either the whole image, or the best box should be displayed on the image display. On entering ARC the first command to try is HE, which lists the available commands (the full HELP must be accessed from the LEXT prompt). The command CP takes the user through several options which determine the functioning of the algorithms used later, for example, the size of the search box used in centroiding a spectral line. Once these have been set up, the next step is to use the CU option to interactively identify several lines in one spectrum. The cursor positioning is only to get a first approximation, and the software then searches for the centre of the line within the fitting box. For each line a value of the wavelength, usually taken from a table or plot of standard lines, must be input. This information is all stored in a LIST of type LIPOS.

Once several lines have been identified, the FI option is then used to perform the wavelength fit to these positions. The user is prompted for the order of the polynomial and the results are stored in a LIST of type COEFS. If this looks acceptable we are now in a position to perform the same procedure for all the spectra in the image. This is done automatically with the FA option, which uses the earlier results as a template for searching for lines. Usually this routine will find all the lines identified in the first spectrum, although the user should follow the progress, as shown graphically on the image display, and go back later to any dubious fits.

The wavelength calibration coefficients thus derived, and stored in the COEFS list, are then picked up automatically when using EXTRACT so that the final spectra are wavelength calibrated. Of course, if you are intending to perform extraction in a future LEXT session, then this list should be written to disk before exiting.