# The upGREAT dual frequency heterodyne arrays for SOFIA

## TL;DR

The paper reports on the performance and capabilities of the upGREAT dual frequency heterodyne array receivers on SOFIA, highlighting their design, operational success, and recent upgrades for high-resolution far-infrared observations.

## Contribution

It introduces the upGREAT dual-channel array system, detailing its design, operational achievements, and recent enhancements for simultaneous multi-frequency observations on SOFIA.

## Key findings

- Successful deployment of dual-frequency arrays on SOFIA.
- Capability to observe key far-infrared spectral lines.
- Integration of dual-channel cryocooler system for parallel operation.

## Abstract

We present the performance of the upGREAT heterodyne array receivers on the SOFIA telescope after several years of operations. This instrument is a multi-pixel high resolution (R > 10^7) spectrometer for the Stratospheric Observatory for Far-Infrared Astronomy (SOFIA). The receivers use 7-pixel subarrays configured in a hexagonal layout around a central pixel. The low frequency array receiver (LFA) has 2x7 pixels (dual polarization), and presently covers the 1.83-2.06 THz frequency range, which allows to observe the [CII] and [OI] lines at 158 um and 145 um wavelengths. The high frequency array (HFA) covers the [OI] line at 63 um and is equipped with one polarization at the moment (7 pixels, which can be upgraded in the near future with a second polarization array). The 4.7 THz array has successfully flown using two separate quantum-cascade laser local oscillators from two different groups. NASA completed the development, integration and testing of a dual-channel closed-cycle cryocooler system, with two independently operable He compressors, aboard SOFIA in early 2017 and since then, both arrays can be operated in parallel using a frequency separating dichroic mirror. This configuration is now the prime GREAT configuration and has been added to SOFIA's instrument suite since observing cycle 6.