4.1.2 Optics

The optics consist of an entrance window/lens, fore-optics, three wheels housing the slits, deckers and filters, an echelon chamber, and a cross-dispersion chamber. The entrance window/lens (2 inches diameter) forms an image of the SOFIA telescope secondary at the liquid helium cold stop within the fore-optics. The fore-optics, including the entrance window, changes the incoming f/19 beam to f/10. After coming to a focus, the beam expands through a pupil (at the cold stop) to an ellipsoidal mirror. The light is redirected off two flat mirrors to a focus at the slit plane.

As the beam comes to a focus, it passes through the slit/filter cassette. This consists of three wheels on a common axle containing (i) filters to isolate grating orders, (ii) deckers to determine the length of the slit, and (iii) slits of different widths. The filter wheel has 12 slots, and these will be loaded with specific filters for each cooldown cycle based on the planned observations. Broader Filters for use in the low-resolution configuration are included in 4 of the decker wheel slots. The decker wheel has a total of 11 features, which include continuously variable length slits, fixed length slits, pinholes, and an open position. The continuously variable slit length is provided by a cutout on the decker wheel that gets larger as a function of angle. The smallest size is about 4.5'' and the largest about 45''. The slit length depends on the wavelength and the instrument configuration. With that caveat, slit lengths can range from 1'' to 180'' on SOFIA.The slit wheel contains six slits. On SOFIA, EXES will typically use four of them (Table 4). There is also a wide 9.400 slit intended for flux calibration.

Table 4: EXES observing configurations, modes, slits and spectral resolutions

Configuration Available Modes Available Slit Widthsa
arcseconds
Resolving Power b
High_Medium nod on slitc, nod off slit, map 1.4 112,000
    1.9 86,000
    2.4 67,000
    3.2 50,000
High_Low nod off slit, map 1.4 112,000
    1.9 86,000
    2.4 67,000
    3.2 50,000
Medium nod on slit, nod off slit, map 1.4, 1.9, 2.4, 3.2 5,000-20,000d
Low nod on slit, nod off slit, map 1.4, 1.9, 2.4, 3.2 1,000-3,000d

a 1.400 slit unavailable >12 μm, 1.900 slit unavailable >16 μm, 2.400 slit unavailable >21 μm
b Observers must check the most recent resolving powers as a function of slit width and wavelength at http://irastro.physics.ucdavis.edu/exes/etc/
c On-slit nodding not possible at all wavelengths. Observers must check this at http://irastro.physics.ucdavis.edu/exes/etc/
d Resolving power is a strong function of wavelength and slit width

After passing through the slit wheel, the beam hits a flip mirror mechanism, which is used to choose between instrument resolution configurations (Table 4) by either directing the beam into the echelon chamber (high-resolution) or into the cross-dispersion chamber (medium- and low-resolution). In the high-resolution configuration, the beam enters the echelon chamber and expands to an off-axis hyperboloid mirror that serves as both collimator and camera mirror for the echelon grating. The dispersed light, focused by the hyperboloid, bounces off a flat into the cross-disperson chamber.

The cross-dispersion chamber is conceptually similar to the echelon chamber. The light expands from the input to an off-axis paraboloid that again serves as both collimator and camera mirror. The collimated beam is sent to the cross-dispersion grating which disperses the light in the plane of the grating. The camera mirror sends the light to our detector. When operating in single-order, long-slit spectral configurations -- our medium and low resolution science configurations -- the light never enters the high-resolution echelon chamber.

There is a wheel in front of the detector, which provides a lens for imaging the pupil through the instrument, and a dark slide for isolating the detector. The wheel would also be available for including transmissive optics to adjust the plate scale on the detector, if desired.