upGREAT: The Extension of GREAT into Heterodyne Arrays

The upGREAT high resolution (λ/Δλ ≥ 107) spectrometers channels are the second generation receivers for the GREAT project on board SOFIA. The two new receivers are based on 7-pixel sub-arrays configured in a hexagonal pattern around a center pixel.

The low frequency array (LFA) has 2 x 7 pixels (dual-polarization) and presently covers the frequency range of 1.83 – 2.07 THz. This allows to observe the [CII] and [OI] lines at 158 µm and 145 µm. Ultimately, its frequency range will extend from 1.9 – 2.5 THz.

The high frequency array (HFA) currently uses one 7-pixel sub-array (one polarization) targeting at the [OI] line at 63 µm. The channel design allows an easy upgrade with the second sub-array for dual-polarization operation in the future. Two quantum cascade laser local oscillators, built by two different groups (KOSMA and DLR-Pf), are available for the HFA.

Both array channels use a closed-cycle Pulse-Tube cooler. Early 2017, NASA completed the development, integration and testing of a dual-channel closed-cycle cryocooler system, with two independently operable He compressors on board SOFIA. Since then, both upGREAT channels can be operated in parallel and constituted the prime GREAT configuration until the full commissioning of the 4GREAT channel in mid-2018.

The addition of the upGREAT channels to GREAT’s receiver suite imposed major changes to its hardware components. Due to the parallel operation of 21 pixels, GREAT was required to expand its IF processor and FFT spectrometers channels drastically. The 1st generation GREAT-channels were single-pixel receivers only. Furthermore, an optical de-rotator – or K-mirror – was implemented into GREAT’s optics system.

Please refer to the GREAT webpages for a table with current receiver and performance parameters.

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