Next: 3.2 CCD Characteristics
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The introduction of CCD ( C harge C oupled D evice) detectors at the coude spectrograph in 1982 initiated the employment of these low noise linear devices in medium to high resolution spectroscopy. This technology enables observers to obtain data of higher signal to noise and larger dynamic range than previously obtainable with conventional photographic or image tube techniques. The resulting data has the added convenience of being in digital form, ready for subsequent analysis.
The CCD Universal dewars can be mounted either on camera 5 or 6. Table 16 may be consulted for the operating characteristics of the CCDs on these cameras. Section 2.6 should be consulted for additional information concerning these cameras. There are also a number of gratings used in the 4 m RC spectrograph which are usable on the coudé spectrograph with a special adapter. These gratings are approximately the size of gratings ``C'' and ``D" and use the small collimator. Table 5 lists useful parameters for these gratings. It is important to note that their use depends upon their scheduling at the 4 m and requires advance notice. An 8" x 15", 31.6 g/mm, 63 echelle grating became available for use in the fall of 1988. On camera 5, this grating yields a demagnification of ~12 and a resolution of ~100,000; in other words a very large slit can be used and still obtain high resolution. On camera 6 the demagnification is ~3 and the resolution is ~200,000. Cross-dispersion of the echelle is accomplished by the use of a grism located ~1 meter below the slit and allows the recording of 15 -- 20 orders per integration. As with the RC gratings, the grisms are ``borrowed" and their use depends upon the scheduling of the Cryogenic camera.
Observers wishing to measure precision radial velocities may want to use a fiber optic image scrambler. This device consists of 1 fiber for the star and two fibers to monitor a comparison lamp. The stellar fiber reduces the effect of guiding errors and refraction, while the comparison fibers monitor the small motions of the CCD during the night. An IRAF program (DOFIBERS) is available for reduction of this type of data. The main disadvantage of the fiber is its throughput; about 60% of that of a slit.
An image slicer is available which may be useful for certain types of observing on camera 6. The slicer is of the Bowen-Walraven type which produces 10 slices stacked end-to-end, each 40 wide and 0.5 mm tall. The slicer is constructed of quartz for good uv transmission. The entrance aperature is about 500 square (3.6 arc-sec with coude feed). §3.6 contains more details on the use of the image slicer.
With the aid of the IRAF routines on the Sun computer, observers can leave the mountain with data that has been reduced; that is free of instrumental effects. These reductions include subtraction of bias counts, division by flat spectra to eliminate small scale detector response, and compression to a one dimensional spectrum. Wavelength calibration, continuum rectification and analysis can also be done as need and time allow.