9. Instruments VI: GREAT



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.

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.

9.2 Planning GREAT Observations

The Guide to Observation Planning with GREAT (GOPG) describes the GREAT instrument and the available observing modes. The information required to estimate observing times in each mode, along with examples on how to do so, are also contained in the GOPG.

Note that the sensitivity calculations for PSW and BMSW modes are the same. In the case of OTF mapping, the sensitivity reached depends on the scan rate and the overlap between successive rows. Information on how to plan OTF mapping observations can be found in the Guide to Planning OTF Maps with the upGREAT Array document.

If the frequency of interest has not already been used by GREAT, please contact the GREAT team to ensure that the line is feasible. There may be gaps where the broadband Local Oscillators do not provide enough power to pump the mixers.

9.2.1 Estimating Exposure Time

The exposure times for GREAT observations may be estimated using the Online Calculator.