In theory any of the Universal dewars could be mounted on the echelle cameras, but in practice T2KB is generally used due to its large format, high quantum efficiency, and low readout noise. If a need exists for small pixels, TI5 could be requested, and a slit width narrow enough to realize the higher resolution would be needed. The operating characteristics of these devices are given in table 8 and a graph of quantum efficiency vs. wavelength is shown in figure 9. As new CCD evaluation is ongoing at NOAO, prospective users may want to inquire about the newest devices available.
Table 8: CCD Characteristics
The CCD can be operated either with or without on-chip summation of pixels, in either the row or the column direction (or both), as appropriate to the requirements of the scientific program. The readout noise per summed pixel is approximately the same as that of one pixel even though the charges from several pixels per resolution element are added together.
Table 9 provides approximate values for limiting magnitude, resolution, signal-to-noise, and spectral coverage with the echelle-CCD system. Since these quantities are wavelength dependent, the values given are only typical. The following values assume the use of the 31.6 g mm-1 echelle grating, a 226 g mm-1 cross disperser, a slit width yielding 2 pixel resolution, 1.5 arc-second seeing, and on-chip summation in the direction perpendicular to the dispersion only (15 x 30 pixels for TI). More specific information for a particular spectral region may be calculated as in §2.11.
Table 9: Echelle-CCD Spectrograph Performance
The TI CCD is usually aligned with orders along columns which allows placement of orders between the few bad columns on each chip. For some applications, the orders may be aligned along the rows of the CCD, or spectral lines may be oriented parallel to rows or columns.
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