Wavelength calibration

Simple statements such as `the wavelength of channel n is so many Angstroms' are not as unambiguous as they appear. This statement is really a slightly simplified version of `the wavelength range covered by channel n is from so many Angstroms to so many Angstroms' but some precision has been lost in the simplification. It presumably means `the wavelength of the centre of the range covered by channel n is so many Angstroms'-or does it mean that `the wavelength of the start of the range covered by channel n is so many Angstroms'?

These notes are intended to explain the conventions used by Figaro. Please note that they are all somewhat arbitrary and in some cases are historical rather than ideal. However, they are consistent and they are precisely defined. Hopefully, these notes will also serve as a brief guide to the use of Figaro for wavelength calibrations. If they seem a trifle pedantic, this is because experience has shown that the whole subject is a real can of worms.

Arc-fitting is also available with Specdre (arc spectrum axis calibration) which is now part of Figaro. It is reputed to be far superior to that described in this section. It is currently only useable with Specdre commands but not with other Figaro commands.

Another alternative for arc-fitting is Molly, a semi-interactive wavelength calibration program written by Tom Marsh. It allows the first of a series of similar arcs to be calibrated interactively, then calibrates the rest automatically. In the right circumstances it can be a great time-saver.