Wavelength calibration

Automatic wavelength calibration is significantly easier with a grism based spectrometer such as UIST than it would be with a grating based spectrometer such as CGS4. This is because the wavelength calibration of each grism should be constant to within a pixel or two, whereas the central wavelength of a grating can be varied continuously over a wide range.

For each grism there is at least one file in $ORAC_DATA_CAL/grisms/ listing arc lines (there may be more than one if the grism can sensibly be calibrated with either lamp). These files have the extension .lis and are in the format of the arlines.lis file produced by the FIGARO arc task. The appropriate file is found by the calibration system and copied to $ORAC_DATA_OUT/arlines.lis where it is used by iarc to generate a wavelength calibration for each row of the image.

These arc line lists will need to be re-measured after any change which will shift the spectra on the array. This includes changes in the optical alignment of UIST (removing and replacing the grism wheels, slit wheel, camera wheel or the array). It may also be necessary to generate new calibrations if the x-shift values in ifu_profile.dat are changed significantly.