The identification of the emission lines of an image corresponding to a comparison lamp (hereafter, arc image) is done to perform the calibration in wavelength of the spectra.
Firstly, we use identify (load noao, imred and specred packages) to identify the emission lines (around 15 features) of the middle fibre of the arc image.
Secondly, we use reidentify (load noao, imred and specred packages) to identify the emission lines of the remaining fibres of the arc image taking the middle fibre like reference. So, we will have a arc image with the identification of the emission features for all fibres. We can use this arc image like reference to identify the emission lines of the whichever arc image using reidentify.
After that, we use setjd (load noao, imred and specred packages) to set geocentric Julian day, the heliocentric Julian day and the ljd parameter (which is a constant value for all images taking during the same observing night).
Then, we assign one or two reference arc image for each object image with refspectra (load noao, imred and specred packages).
And, finally, we can perform the dispersion correction using the dispcor task (load noao, imred and specred packages).