The calibration lamps are located in the ``photon torpedo'' and their output is fed through the center of the triple wheels via fibers. The fibers illuminate the back of the entrance hatch. The CuAr is designed to illuminate well only the central 20 arcsec, since it is expected to be used only for echelle mode. (In fact it can be useful even in low-D mode to get calibration lines in the blue.)

The hatch must be closed to take calibration lamp spectra. A red light beam on the GUI represents the lamp light.

10.1. Echelle Mode

For echelle mode observations, the CuAr hollow cathode lamp provides a large number of unsaturated, unblended lines in every order. Since the lamp power is spread over many lines, individual lines are faint and a total exposure time of 300 sec or more is required for good S/N in the lines. The Web provides line ID plots and lists for echelle mode.

Fig. 13. 300 second CuAr spectrum in echelle mode.

10.2. Low-D Mode

For the low-D mode, a combinations of Hg-Ar, Ne and Xe lamps provide reasonably good coverage over the full wavelength range. The Xe lines in the blue are faint and may require longer exposures. Long CuAr exposures can also provide some lines in the blue, although with illumination only over the central 20 arcsec or so of the 8 arcminute-long slit.

The Web contains information such as line ID plots and lists and wavelength fits.