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 1.262 ‒ 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.

9.1.1 Observing Modes

Four observing modes are supported by GREAT.

  • Position switching (PSW): In this mode the telescope nods between a target and nearby emission-free reference position. This mode should be used for observing one or more positions on an extended source. The reference position should ideally be less than 30' away from the source positions.
  • Beam switching (BMSW): In this mode, the secondary mirror chops between the source and a reference position less than 10' away at a rate of about 1 ‒ 2 Hz. The telescope nods between these positions at a slower rate. This mode results in better sky cancellation than position switching, but can be used only for compact targets, where the chop/nod throw would move the beam off-source.
  • Raster mapping: In this mode, the telescope points to an off position, followed by several on-position observations. The user must specify the map positions, the off-position for a PSW observation or the chopper set-up for a BMSW observation, and the totl integration time per map position. This mode is ideally used for small maps or strips, where relatively long exposure times (> 30 s) per map point are required.
  • On-the-fly mapping (OTF): In this mode the telescope scans along a line of constant latitude (a row) with the backends continuously integrating the incoming signal and recording an average after the telescope has moved a fraction (typically 1/3) the beam size. Once the end of the row is reached, the telescope moves off-source to a reference position. Then, the telescope steps half a beam width in longitude and scans the next row. This process continues until a map of the desired size has been observed. The whole map is repeated until the required sensitivity is reached.