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9.1 GREAT Instrument Overview
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GREAT is a dual channel heterodyne instrument that will provide high resolution spectra (up to R=108) in several frequency windows in the 1.5 ‒ 5 THz range. The front-end unit consists of two independent dewars, each containing a set of mixers, labeled L1, L2, Ma, Mb, and H, sensitive to ''Low'', ''Medium'' and High'' frequencies.
L1 operates in the range 1.262 ‒ 1.396 THz (237.7 ‒ 214.9 μm) and 1.432 ‒ 1.523 THz (209.5 ‒ 197.0 μm). The gap matches frequencies blocked by the atmosphere. L2 operates in the range 1.800 ‒ 1.910 THz (166.7 ‒ 157.1 μm). The M channel requires two local oscillators. Ma in the range 2.495 ‒ 2.519 THz (120.2 ‒ 119.1 μm) and Mb in the range 2.67 ‒ 2.68 THz (112.28 ‒ 111.86 μm). The H channel operates at 4.745 THz (63.18 μm).
The upGreat Low Frequency Array (LFA) is the first of two second-generation receivers for the GREAT instrument. The LFA is a 14-pixel array (2 polarizations × 7 pixels) arranged in a hexagonal geometry with a central beam. The current tuning range of the LFA is restricted to within several GHz of the [CII] transition at 1900.536 GHz.
During Cycle 4, L1, L2, H, and the LFA will be configurations L1-LFA and L2-H. The H channel range is currently limited by the tuning range of the quantum cascade laser local oscillator and, therefore, only the velocity range -25 ‒ +90 km/s around the 4744.77749 GHz [O I] line will be available for Cycle 4. With the negative velocity setup, the useable velocity range is -140 ‒ -30 km/s.
The GREAT instrument uses Fast Fourier Transform (FFT) spectrometers as backends. Each XFFTS spectrometer has a bandwidth of 2.5 GHz and 64,000 channels, providing a resolution of 44 kHz. The beam size is close to the diffraction limit - about 16' at 160 μm.