The New GREAT SOFIA Scenario With 4GREAT

The German REceiver for Astronomy at Terahertz frequencies has been successfully in service onboard SOFIA since 2011. GREAT, with its modular approach, is composed of a group of cryostats containing detectors for different frequency bands between 1.25 and 4.7 THz. Individual cryostats are then grouped in pairs. 4GREAT, a new member of the GREAT constellation, is a four-color single-pixel- detector. Two channels, CH1 and CH2, are implemented using spare flight mixers of Herschel-Heterodyne Instrument for the Far-Infrared: HIFI band 1 (492– 635 GHz) and band 4 (892-1100 GHz). The third channel -CH3- makes use of the GREAT L1 detector (1.2 – 1.5 THz), while the fourth –CH4- covers the entire frequency range of GREAT Ma,b (2.5-2.7 THz), using a variation of one the upGREAT HFA mixers. The channels are all co-aligned on sky in a single closed-cycle cooled cryostat. 3 Channels of 4G have been commissioned by July-2017, while the fourth one has been recently integrated, making the system ready for the next observing campaign. It will be used simultaneously with the upGREAT-HFA (an array of 7 pixels at 4.745 THz covering the line of atomic oxygen [O I]), allowing multiple frequency band observations of astrophysically important transitions including ammonia (NH3), the methylidyne radical (CH), ground-state isotopic water H218O, hydroxyl (OH), and carbon monoxide among many others.