McDonald Laser Ranging Station
White Dwarf Stars and the Age of the Galaxy
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 Figure 2 End view of EXES in cross-dispersed mode. Once again, optical elements are in green while structural (or unused optical) elements are in blue. Holes in walls are denoted by closely spaced blue lines. Fore-optics (a-h) are best seen in Figure 1. After passing through the slit wheel (h), the light reflects from flat mirror (i), which directs it into the echelon chamber. It then reflects (out of the plane of the figure) from flat (j) to an off-axis paraboloid (k), which sends collimated light onto the echelon (m). Light dispersed by the echelon returns to the paraboloid (k), which refocuses it as it passes into the cross-dispersion chamber after reflecting off of flat (n). It then reflects (again out of the plane) from flat (p) to paraboloid (r), to the echelle grating (s: difficult to read), through lenses (t), from flat (u), to the detector array (v), as in Figure 3. Flat (q) is folded out of the light path between flat (p) and paraboloid (r). The echelon chamber collimator/camera mirror is a 100 cm focal length paraboloid, used 5.7º off-axis. The echelon (m) (seen end-on) is a 1-m long, 10-cm wide, R10 diffraction grating, with a 7 mm groove spacing. It is used in very high order, ranging from 2500 at 5.6 µm to 500 at 28 µm. |
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