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German REceiver for Astronomy at Terahertz Frequencies (GREAT)
The German REceiver for Astronomy at Terahertz Frequencies (GREAT) is a modular heterodyne instrument with multiple configurations that provide high-resolution spectra (up to R = 108) in several frequency windows between 0.4900–4.7448 THz. The front-end unit consists of two independent dewars that operate simultaneously, each containing one of the following channels:
The upGREAT Low Frequency Array (LFA) is a 14-pixel array with two polarizations (Horizontal and Vertical with 7 pixels each) with 6 pixels arranged in a hexagonal pattern around the central pixel. Both polarizations of the array (LFAH and LFAV) can be tuned to one simultaneous or two independent frequencies in the 1.835-2.007 THz range. Tuning the LFA polarizations to two independent frequencies is offered on a best effort basis. The [OI] 145 µm line can can only be observed in V polarization, which has an additional tuning range from 2.060 to 2.065 THz.
The upGREAT High Frequency Array (HFA) is a 7-pixel array arranged with 6 pixels in a hexagonal geometry around a central pixel. The local oscillator is a Quantum Cascade Laser with a narrow tuning range limited to the [OI] 63 µm frequency of 4.74477749 THz (with a tuning range of roughly + 250 km/s to -100 km/s in LSR velocity).
4GREAT consists of four co-aligned (to within a few arcsec) pixels at four different frequencies. The tuning ranges of these pixels are: 491-635 GHz (4G1), 890-1092 GHz (4G2), 1240-1525 GHz (4G3), and 2490-2590 GHz (4G4).
The GREAT instrument uses eXtended bandwidth Fast Fourier Transform Spectrometers (XFFTS) as backends. Each XFFTS has a bandwidth of 4 GHz and 16,384 channels, thus providing a resolution of 244 kHz (note that the useful bandwidth depends on the tuning and the atmospheric transmission). The beam size of GREAT is diffraction limited (14.1” at 1.9 THz).
Examples of GREAT scientific observations and summary of specifications: GREAT Flyer
Information applicable to the observing cycles can be found in their respective versions of the Observer's Handbook. Be sure that you are referencing the correct version of the handbook!
Use when citing SOFIA/GREAT results.
"GREAT: The SOFIA High-frequency Heterodyne Instrument"
Heyminck, S., et al., 2012, A&A, 542, L1.
"GREAT: A First light instrument for SOFIA"
Heyminck et al., 2008, ProcSPIE, 7014, 701410.
Graf et al., "GREAT: the German first light heterodyne instrument for SOFIA," Infrared Spaceborne Remote Sensing and Instrumentation XV, Marija Strojnik-Scholl, Editor, Proc. SPIE 6678, 66780K (2007), DOI: 10.1117/12.768027
Wagner-Gentner et al., "GREAT optics," Millimeter and Submillimeter Detectors for Astronomy II, Jonas Zmuidzinas, Wayne S. Holland, & Stafford Withington, Editors, Proc. SPIE 5498, 464 (2004), DOI: 10.1117/12.551236
Güsten, et al., "GREAT: the first-generation German heterodyne receiver for SOFIA," Airborne Telescope Systems, Ramsey K. Melugin & Hans-Peter Roeser, Editors, Proc. SPIE 4014, 23 (2000), DOI: 10.1117/12.389122 [pdf]