4GREAT - The multicolor extension of GREAT

The last addition to the GREAT instrument platform is the 4GREAT channel, which corresponds to a four-color single-beam fully independent tunable receiver system.

A paper describing the 4GREAT system and its first science results in detail is in preparation.

After the HERSCHEL Space Observatory ceased operations in 2013, certain frequency windows, only observable from space (due to atmospheric absorption) were not available any more. Also, due to the cancellation of SOFIA’s CASIMIR (CAltech Submillimeter Interstellar Medium Investigations Receiver) instrument, the sub-THz range was uncovered by SOFIA. With SOFIA’s flight altitudes (up to 13 km), the success of the upGREAT channels and the enhanced cooling capabilities of SOFIA in mind, the idea of 4GREAT was born. The goals of its design were

  • to incorporate the 1st generation channels GREAT-L1 (1.25 – 1.5 THz) and GREAT-Ma (2.49 – 2.52 THz), which would stream-line GREAT’s channel suite and facilitate the handling of the instrument, because less channel swaps would be needed, and

  • to extend the frequency coverage to sub-THz ranges and “re-open” (two) former Herschel/HIFI windows, for which spare detector devices of HIFI were still available.

4GREAT incorporates four individual receiver (sub-)systems (4G-1, 4G-2, 4G-3 and 4G-4), covering different frequency bands, co-aligned on sky. It is operated in parallel to the upGREAT/HFA channel within the GREAT platform infrastructure.

Frequency band

Astronomical lines of interest

4G-1

492 – 627 GHz

[CI], HDO, CH, H218O(110-101), NH3, CO, ArH+, HCl, CS, HCN and HCO+

4G-2

893 – 1073 GHz

CO, 13CO, HDO, H218O(202-111), H2S, OH+, NH, CS, HCN and HCO+

4G-3

1.24 – 1.52 THz

[NII], OD, CO, 13CO, H2D+, SH, NH, CS, HCN and HCO+

4G-4

2.49 – 2.69 THz

18OH, CO, (HD)

Table 1: Frequency coverage and important astronomical lines for the 4GREAT receiver systems. Due to instrument constrains the HD line in 4G-4 is not available for routine observations.

Transmission and 4GREAT bands.

Frequency bands and atmospheric transmission

The figure shows the 4GREAT frequency bands, as well as the coverage of the upGREAT channels and a comparison with ALMA bands. The atmospheric transmission for SOFIA (altitude: 43.000 ft = 13.1 km, pwv: 10 µm) is represented in light blue, and for ALMA (5000 m, 200 µm) in gray.

4GREAT liaisons

  • Dr. Oliver Ricken
  • Dr. Carlos Duran
  • Dr. Nicolas Reyes
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