When calibration files are reduced they are listed in an index file in the directory containing the
reduced data. Where separate index files are used by imaging and spectroscopy recipes
(e.g. index.flat_im and index.flat_sp) the spectroscopy files will be used. Two of the index files
used include more information than just the filename of the calibration frames.
The index.iar file indexes not only the .iar files generated by Figaro iarc containing the wavelength
calibration of every row but also the wavelength range (in Angstroms) to which the spectra should be
scrunched. The filename and the minimum and maximum wavelengths are separated by
colons.
Similarly, the index.offset file contains both the name of the file from which the offset value was
measured and the offset itself separated by a colon.
There is also one index file, index.arlines, which is not stored in the reduced data directory but in
the calibration directory ($ORAC_DATA_CAL). This is the index of the arc lines lists (see section 9.2), which
are static calibration files.