EXES Instrument Design

I. EXES Instrument Description

  The Echelon-cross-Echelle Spectrograph (EXES) operates in the 4.5 ‒ 28.3 μm wavelength region, at high (R ≈ 50,000 ‒ 100,000), medium (R ≈ 5000 ‒ 20,000) and low (R ≈ 1000 ‒ 3000) spectral resolution. The instrument uses a 1024x1024 Si:As detector array. High resolution is provided by an echelon ‒ a coarsely-ruled, steeply-blazed aluminum reflection grating ‒ along with an echelle grating to cross-disperse the spectrum. The echelon can be bypassed so that the echelle acts as the sole dispersive element. This results in single order spectra at medium- or low-resolution depending on the incident angle.

II. Thermal Design

  EXES is a liquid helium cooled instrument. The cryostat is approximately 24 inches in diameter and 72 inches long. There are two cryogen reservoirs, one for liquid nitrogen and one for liquid helium. These are at the forward end, as mounted on SOFIA, with the entrance window on the aft end toward the telescope. There are three layers of radiation shielding within EXES - a vapor cooled shield tied only to the cryogen fill tubes, one attached to the liquid nitrogen reservoir, and the third attached to the liquid helium reservoir. All optics except for the entrance window/lens are attached to the liquid helium level. Baffling tubes connected to the liquid nitrogen level reduce thermal emission impinging on the internal optics. Within the liquid helium level, the optics are all tied to a rigid optics box constructed out of aluminum, and the detector headerboard is isolated with G10 fiberglass and actively maintained at a uniform temperature. Cryogen hold times for liquid oxygen/hydrogen are safely over 24 hours.

III. 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 slit length depends on the wavelength and the instrument configuration. The slit wheel contains six slits. On SOFIA, EXES will typically use four of them (see table below). There is also a wide 9.4" slit intended for flux calibration.

EXES observing configurations, modes, slits and spectral resolutions
Configuration     Available Modes Available Slit Widthsa
  Resolving Powerb
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
2.4 67,000
3.2 50,000
Medium nod on slitc, nod off slit, map 1.4, 1.9, 2.4, 3.2 5,000-20,000d
Low nod on slitc, nod off slit, map 1.4, 1.9, 2.4, 3.2 1,000-3,000d

  After passing through the slit wheel, the beam hits a flip mirror mechanism, which is used to choose between instrument resolution configurations (see table above) 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 mirror, bounces off a flat mirror into the cross-disperson chamber.

  The cross-dispersion chamber is conceptually identical to the echelon chamber. The light expands from the input to an off-axis paraboloid mirror 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.

IV. Detector

  The detector is a Raytheon Vision Systems Si:As array with 1024x1024 pixels. The array is mounted in a separate enclosure to reduce scattered light. The headerboard is thermally isolated from the rest of the optics box to permit active temperature control of the array. The photon fluxes in the low-resolution configuration will be significantly above the level intended for the array, and it is expected that only a subsection of the array will be clocked out in this configuration for a faster read-out (as well as in any imaging configuration). It is expected that a quarter to half the array will be utilized in these configurations, so the effective slit length is about 60''.