9.1 GREAT Instrument Overview

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 1.25 ‒ 1.39 THz and 1.42 ‒ 1.52 THz. The gap matches frequencies blocked by the atmosphere. L2 operates in the range 1.81 ‒ 1.91 THz. The M channel requires two local oscillators. Ma in the range 2.49 ‒ 2.56 THz and Mb in the range 2.67 ‒ 2.68 THz. The H channel operates at the [OI] frequency of 4.74477749 THz (+/- 100 km/s LSR velocity).

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 +/- 5 GHz of the [CII] transition at 1900.536 GHz.

During Cycle 5, two configurations are offered: L1-LFA and L2-H. 

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.

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