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The comparison sources are mounted on a post to the right of the slit head, and are selected by a rotatable mirror. The following sources are currently available:
The Fe-Ar hollow cathode is less rich than the Th-Ar lamp and may be useful with the lower dispersion RC gratings. The Ne-Ar lamp has few bright lines shortward of 5800 Å so is used exclusively for red work. For high dispersion work the Th-Ar lamp may be required to obtain a sufficient number of lines in the spectral region of interest. Wavelength plots of these lamps may be found in the appropriate CCD Atlas of Comparison Spectra... which are kept at the coudé spectrograph. A new Web-based ThAr atlas is now available with higher resolution and more line identifications.
The QI lamp is used with the CCD detector to provide flat field illumination for calibration of small scale detector response.
A set of neutral density filters is available at the coudé spectrograph for controlling lamp brightness. These mount on filter holders just in front of the source exit window and can be rotated in and out of the beam. For CCD observing, use the diffusers instead of n.d. filters to avoid periodic fringing in the data.
The other optical element required for the comparison sources consists of a small mirror which can be manually moved over the slit to send the light into the spectrograph. This mirror has two modes of operation: