The echellette mode of the spectrograph uses a rather coarse (180 l/mm) grating ruled at a high blaze angle (23.90) to yield 1--2Å resolution for multi-order spectroscopy. A sketch of the optical path is contained as Figure 2.6 (from Carswell et al. 1975, ApJ, 196, 351). With the 60 quartz cross-dispersion prism and a 1200 pixel CCD, 8 spectral orders provide continuous coverage from the atmospheric limit to approximately 8800Å in the format shown in Figure 2.7. Maximum slit length depends on the spectral region of interest, reaching a minimum between orders 4 and 5. Thus, a special decker is generally substituted for the standard slit plate for echellette spectroscopy, providing for slits of 16 in length with a wide selection of widths. See chapter 3.1 for details regarding the echellette decker.
Figure 2.6: Schematic of echellette mode.
Figure 2.7: Cross-dispersed formats.
Conversion to or from the echellette mode requires considerable mechanical adjustments to the spectrograph and therefore can only be carried out by qualified technical staff during the day. Should you wish to take advantage of the substantial benefits of this mode of observing, you must notify the technical staff well in advance of this request!