Scientific/Technical Abstract:

To take advantage SOFIA's unique potential for high-resolution spectroscopy in the mid-infrared, the Echelon-Cross-Echelle Spectrograph (EXES) will operate in three spectroscopic modes (R~10⁵, 15000, and 4000) from 4.5 - 28.3 microns.

EXES will use a 1024² Si:As IBC detector. High dispersion is provided by an echelon, a coarsely-ruled, steeply-blazed aluminum reflection grating. Using the echelon requires an echelle grating to cross-disperse the spectrum, resulting in continuous wavelength coverage of ~10 cm^-1 for a slit length of ~10", resulting in R=10⁵. Optionally, the echelon can be bypassed so that the echelle or a low order grating acts as the sole dispersive element. This results in a single order spectrum with slit lengths of roughly 100" and 50" for the medium and low resolution modes respectively. The low resolution grating also serves as a slit positioning camera when it is rotated face on.

One of the greatest advantages of SOFIA will be its ability to study molecules which are blocked by the Earth's atmosphere. 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 Performance Summary:

The instrument sensitivity and resolution summaries are provided to permit estimating feasibility of scientific investigations. The EXES performance summaries show the expected system performance for Full Operational Capability, which may differ from that during commissioning.

EXES Angular Resolution

The angular resolution of EXES will match that achieved by the telescope. For standard observing, the slit width will be set to 1.6 times the image FWHM, though the slit width can be adjusted at the observer's discretion. Available slit widths range from 0.8" to 3.2".

EXES Spectral Resolution

Wavelength coverage ranges from 4.5 - 28.3 µm. EXES uses a 1024 x 1024 Si:As IBC detector. Spectra can be obtained using three different set-ups; low-resolution, long-slit mode; medium-res, long-slit mode; and high-res, cross-dispersed mode. The long-slit modes have a slit length of approximately 50"-100" while the cross-dispersed mode has a 5"-20" slit length. The resolution is roughly R=4000, 15000, or 10⁵ for the low-, medium-, and high-res modes respectively. The corresponding free spectral range for each mode is 10000 km/s at low-res and 2500km/s for medium- and high-res.

Below is a plot of the spectral resolution vs. wavelength. The resolving power plotted corresponds to the FWHM of the instrument line spread function for a monochromatic line from a point source. Note that there is non-continuous spectral coverage in high-res mode for λ > 19 µm. The central wavelength can be tuned so that the gaps in spectral coverage do not fall on lines of interest.

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EXES Sensitivities

The figure below plots the "minimum detectable line flux" (MDLF) for a 4σ detection with 15 minutes of on-source integration versus wavelength. The MDLF is plotted for an unresolved line from a point source. The MDLF scales roughly as (S/N) / sqrt(t), where t = net integration time.

Reference points for the MDLF for a 4σ detection with 15 minutes of on-source integration at 10 and 20 µm wavelengths are:

The sensitivity estimates assume that SOFIA is diffraction limited at λ > 15 µm. Line measurements in bright continuum sources may take longer to reach the same (S/N). Atmospheric transmission may preclude measurements at some wavelengths and reduce sensitivity at others. Further details for particular wavelengths of interest are available from the SI team; see contact information above.

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EXES Observation Preparation and Data Handling

Once the observatory has been fully commissioned, additional information will be provided, including a full accounting of overheads associated with particular instrument set-ups and observing strategies; information on preparing observations using the SPT; and details regarding data formatting, calibration, and reduction.

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Disclaimer

All sensitivity and resolution data are preliminary, and based on anticipated performance of the observatory and the instrument.  Actual performance of the SOFIA telescope and instrument combination will be established after flight operations begin.  Telescope performance is expected to be upgraded during the first two years, and instrument performance may be upgraded, or additional modes or capabilities may be added.

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Additional References:

Richter et al., "Status of the Echelon-cross-Echelle Spectrograph for SOFIA," Ground-based and Airborne Instrumentation for Astronomy III, Ian S. McLean, Suzanne K. Ramsay, & Hideki Takami, Editors, Proc. SPIE 7735, 77356Q (2010), DOI: 10.1117/12.856515 [pdf]

Richter et al., "Development and future use of the echelon-cross-echelle spectrograph on SOFIA," Ground-based and Airborne Instrumentation for Astronomy, Ian S. McLean & Masanori Iye, Editors, Proc. SPIE 6269, 62691H (2006), DOI: 10.1117/12.670559 [pdf]