2.1 Specifications

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2.1   Specifications

2.1.1   Instrument Overview

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.

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2.1.1.1   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 is actively maintained at a uniform temperature.

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2.2.1   Performance

EXES delivered performance appears consistent with expectations over the flight series so far. There are some variations from observation-to-observation, but we believe the values quoted here are fair estimates of what is typical. The angular resolution of EXES will match that achieved by the telescope. For the latest sensitivities, observers are recommended to consult the online Exposure Time Calculator (ETC) at http://irastro.physics.ucdavis.edu/exes/etc/. The ETC also provides the slit length as a function of wavelength and instrument configuration (and therefore whether on-slit nodding is possible or not), as well as the wavelength coverage in a single setting and echelon orders that can be targeted.

The wavelength coverage ranges from 4.5–28.3 μm. There are three resolution regimes—high, medium and low—with the exact resolving power depending on wavelength, grating angle and slit width. Generally, the resolution will be higher at shorter wavelengths in each regime. The high-resolution configurations use the echelon grating and will achieve R = 50,000–100,000. If the cross disperser echelle angle is 35–65°, the configuration is called HIGH_MED and if 10–25° it is called HIGH_LOW. For these high-resolution configurations, there is non-continuous spectral coverage in high-resolution configuration for λ > 19 μm, but the central wavelength can be tuned so that lines of interest do not fall in the gaps. The Medium configuration will use high angles on the echelle grating to achieve R = 5,000–20,000, and the LOW configuration will use low angles to achieve R = 1,000–3,000.

The HIGH_MED configuration slits are 4.5–45 arcsec long, and the HIGH_LOW slits are 1–12 arcsec long. The shorter slits in HIGH_LOW allow for more orders to be packed onto the array, thus increasing the instantaneous wavelength coverage, while maintaining the high spectral resolution (see Figure 2-6 for an example). In the Medium and Low configurations the slit lengths vary from 25 to 180 arcsec depending on the number of rows to be read out.

The sensitivity of the instrument is shown in Figures 2-1 through 2-4 for the HIGH_MED, Medium, and Low configurations for both point sources and extended sources. The Noise Equivalent Flux Density for S/N of 10σ in a clock-time (Note that the other instruments in this Handbook report sensitivities based on the total time on-source, not the clock-time. The latter includes the total time on-source + applicable overheads, excluding target acquisition and instrument set-up time.) of 900 seconds is plotted as a function of wavelength. These values have been calculated for a point source assuming image quality between 2 and 4 arcsec (FWHM) and the narrowest of the available 1.4 to 3.2 arcsec slits, both of which vary with wavelength, and take into account estimated instrument efficiency. They assume an altitude of 41,000 feet, 40° elevation, and 7 μm precipitable water vapor.

Figure 2-1.
High Resolution and High-Low mode plots

Figure 2-1. The minimum detectable point source flux as a function of wavelength for the EXES HIGH_MED (top) and HIGH_LOW (bottom) configurations, assuming the conditions mentioned above. It takes into account that on-slit nodding is not possible at all wavelengths. The vertical dotted lines show the boundaries between the slit widths used (1.4", 1.9", 2.4", 3.2"). For precise numbers at individual wavelengths and for the HIGH_LOW configuration please use the online ETC.

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2.1.2.1   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 that 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 four 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 arcsec and the largest about 45 arcsec. The slit length depends on the wavelength and the instrument configuration. With that caveat, slit lengths can range from 1 to 180 arcsec on SOFIA.The slit wheel contains six slits. On SOFIA, EXES will typically use four of them (Table 2-1). There is also a wide 9.400 slit intended for flux calibration.

Table 2-1: Observing Configurations, Modes, Slits, and Spectral Resolutions

Configuration Available Modes Available Slit Widthsa (arcseconds) Resolving Powerb
HIGH_MED 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 2-1) 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.

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2.1.2.2   Detector

The detector is a Raytheon Vision Systems Si:As array with 1024x1024 pixels. The detector material is bonded to a SB 375 multiplexer. 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 configuration will be significantly above the level intended for the array. This prevents observations at longer wavelengths and/or with wider slits. When photon fluxes allow, 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''. There are other potential work-arounds that reduce the instrument sensitivity and may require extra overheads to prepare for and recover from low resolution observations. Proposers interested in low resolution observations should contact the instrument team to discuss their goals and possible options.

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