Echelle Spectrograph Wavelength Calibration

The mirror carriage in the instrument rotator has two operating positions, one of which permits both TV field acquisition and comparison illumination, while the other provides TV viewing of the slit while the starlight is entering the spectrograph. This mirror is operated remotely, either from the Telescope Control System or by initiating a comp exposure using the Arcon software (or asking for an object exposure after having taken a comp exposure).

The normal wavelength calibration source is a Th-Ar lamp. The best atlas for identifying lines in its spectrum is:

"A CCD Atlas of Comparison Spectra: Thorium-Argon Hollow Cathode"
by Daryl Willmarth, KPNO report.

You can have your very own endless line list, suitable either for framing, deleting, or use with IRAF.

There is also a quartz lamp on the same port as the Th-Ar lamp. A manually operated prism selects between the two; one or the other can be remotely controlled during the night, but you cannot remotely switch between them.

Manually operated neutral density filters can be inserted in front of the two lamps: they cut the light by a factor of 0.3 or 0.1.

WARNING!

There is a comparison lens in the spectrograph which is driven over the slithead to simulate the telescope beam during comparison exposures. This is manually operated. There is no point in using it unless you are using the long slit mode, with a mirror replacing the cross-dispersing grating. It is important to make sure that it is out of the beam when you are actually observing... there seem to be little elves around the place who are always putting it in.