The number of wavelength reference frames to be used.
These will usually be arc spectra. The default is a single frame, but it is possible to use two frames which bracket the object exposure in time. In this case it is assumed that the shift between exposures has been due to smooth variation, an assumption which it is usually not possible to verify.
Note: the names of all frames must be supplied in response to the arc prompt when locating the reference features. For example:
ARC=,
Martin Clayton