EXES
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Figure 1
Side view of EXES in long-slit mode. In the drawing, light rays are in red, optics are in green, and structural elements are in blue.
Focused light enters the dewar at f/20 through the entrance window/field lens (a), which images the telescope pupil onto the cold
stop (b). An off-axis ellipsoidal mirror (c) reimages the focal plane at f/10 (with a 2:1 image scale reduction). Mid-infrared light
reflects from a dichroic flat (d), while near-infrared light passes through and reflects off of flat (e) to an InSb guide detector (f).
The mid-infrared light passes through a filter wheel (g) containing 12 1-inch filters and is focused onto a slit wheel (h). Folding
flat (i) is removed from the light path, and components j-p are not used, in long-slit mode. Folding flat (q) directs the light toward
an off-axis paraboloid (r), which reflects collimated light to the grating (s). Dispersed light is refocused by the paraboloid (r), through
a reducing lens (t), and off of a fold mirror (u), onto the detector array (v).
Window (a) is 45 cm focal-length, 5 cm diameter, plano-convex, CsI field lens. Mirror (c) is an ellipsoid with 75 cm and 37.5 cm
focal distances, used 7.5º off-axis. Mirror (r) is a 75 cm focal length paraboloid, used 6.9º off-axis. Grating (s) is either a 31.6 gr/mm,
63º blaze angle echelle or a 12 gr/mm, 12º blaze angle grating used in first order at 17-29 µm and in third order
at 5.5-8 µm. (The two
gratings are mounted back-to-back.) The f/3.3 CsI meniscus lens pair, which is used at 17-29 µm, is shown. For observations at 5-14
µm they would be replaced by a ZnSe meniscus, which focuses the light at f/6. (If the detector has 50 µm pixels, the required optics
would be f/10 and f/5 so that focal reduction would only be needed for the long wavelength region.) A typical mechanism (the slit
wheel drive) and representative mechanical components are shown.
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