The wavelength calibration is done using a reference feature list, usually provided by an arc-lamp exposure. The routine allows any candidate features to be identified and used as 'knowns' for the calibration (as position/intensity pairs).
These features may then be manually identified using a reference lamp atlas. Facilities are provided for adding/deleting lines and altering the degree of polynomial fit performed. A typical interactive plot appears below.
An automatic line identifier is included which works by searching the suppled line list for multi-line 'features'. This search can be optimised by constraining the space to be searched in terms of permissible wavelength and/or dispersions (in pixels/Angstrom). In addition, the program will automatically constrain the search range further if it can determine the central order number and wavelength (by looking in the data frame header).
As soon as two orders have been successfully calibrated, the search range for the remaining orders is re-evaluated to take this into account. In general the automatic method will be most useful when you are unsure of the exact wavelength range covered.
When the level of doubt is such that the wavelength scale may decrease from left to right across the frame, then the software may be instructed to automatically check for this 'reversed' condition. The parameter TUNE_REVCHK controls this process and defaults to NO to avoid wasting CPU time.
Finally, the software is flexible as to the vertical orientation of the orders, i.e., higher wavelength orders may be at the top or bottom of the frame (for echelle data).
Calibration may be performed using either 1 or 2 (before and after) arc frames at present. See ARC_FRAMES for details.
Options are presented in a menu form and selected by typing a one or two-character string, followed by carriage return. The following options are supported:
Martin Clayton