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!