EXES is a high-resolution, mid-infrared spectrograph mounted on NASA's Stratospheric Observatory for Infrared Astronomy (SOFIA), the world's largest flying telescope.
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.
Echelon grating cross-dispersed by (or replaced by) an echelle grating gives us Echelon-cross-Echelle Spectrograph or EXES.
The combination of EXES's high spectral resolution and SOFIA's access to infrared radiation from space provides an unprecedented ability to study celestial objects at wavelengths unavailable from ground-based telescopes.
In particular, high spectral resolution enables the study of molecular hydrogen, water vapor, and methane from sources such as molecular clouds, protoplanetary disks, interstellar shocks, circumstellar shells, and planetary atmospheres.
EXES is a Principal Investigator class instrument. EXES's Principal Investigator is Dr. Matt Richter of the University of California, Davis.
It is not required to contact us before submitting a proposal to SOFIA, but we will be able to improve proposals before submission. All EXES data collection and reduction will be done by the EXES team.
It is expected that preparation of results for publication will be a collaboration of General Investigators and the EXES team.
EXES completed its first two commissioning flights in April 2014. More are scheduled for late in Cycle 2, along with a night of GI observations.
Cycle 3 observations will start in March 2015. 8 proposals have been accepted, awarded a total of 36.4 hours of observing time with EXES during Cycle 3.
1. Science Configurations
EXES has four different science configurations: high-medium, high-low, medium, and low. The terms relate to the gratings used and the dispersion provided by each.
The EXES high-medium configuration is our standard high resolution mode. It uses the echelon for high spectral resolution and the echelle at high angles for cross dispersion.
This mode has roughly 1% spectral coverage and a slit length generally sufficient to allow nod_on_slit observing of point sources.
The EXES high-low configuration is intended for maximizing spectral coverage in high resolution. It also uses the echelon for high spectral resolution, but the echelle cross-disperser is used at low angles.
This mode has roughly 4% spectral coverage, but the slit length is short and may even be smaller than the image FWHM.
Medium mode uses just the echelle at high angles for single order, R=5000 to 20,000 spectroscopy. The slit is up to 160" long and the wavelength coverage is roughly 1%.
Low mode uses just the echelle at low angles for single order, R=1000 to 3000 spectroscopy. The slit is up to 160" long and the wavelength coverage is roughly 4%.
Low mode may result in very high photon fluxes and we are working on finding the best strategy for making it work with our detector array.
2. Observing Modes
EXES has two basic observing modes: nod and map. The nod mode can be further divided into nodding on slit and nodding off slit.
In nod mode, the target is nodded between two points so that the paired data can be differenced to remove sky and telescope emission.
Where possible, nodding on the slit is better because it improves the efficiency and provides better sky subtraction. Nod mode always starts with the target at the center of the slit.
For nodding on the slit, the first move is roughly 25% of the slit length and subsequent nods are half the slit length.
For nodding off the slit, the target is at the center of the slit and then moved to the off position, which can be specified in RA and Dec as might be required in complex regions.
alpha Tau during nod observation:
one slit position with sky lines and star both visible (left) after subtraction (right)
In map mode, the slit is initially centered on the target. The telescope is then moved to a blank sky position. This could be a specified position separated from the map or the initial points of the map.
Once at the start of the map, the telescope is moved, typically by half a slit width, across the object. Motions are generally perpendicular to the slit to produce a stripe.
At the end of the map, the telescope returns to the blank sky position for three additional exposures. To completely map objects larger than the slit length, multiple stripes are required.
Spectral maps of Jupiter: C2H2 map (left) and CH4 map (right)
[ Click HERE for more details about EXES from SOFIA's instrument page ]