An Update on the Echelon-cross-echelle Spectograph for Sofia

To take advantage of 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, 104, and 3000) from 5 -28 microns.

EXES will use a 1024x1024-element Si:As BIB 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 5 cm-1 and a slit length of 10" at R=1E5. 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 length of roughly 90" and R=1E4 or 3000, 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 planetary atmospheres, protoplanetary disks, interstellar shocks, circumstellar shells, and molecular clouds.

We present an instrument development status update and preliminary test results on performance of the detector array.