Performance of a Highly Sensitive, 19-element, Dual-polarization, Cryogenic L-band Phased-array Feed on the Green Bank Telescope

Abstract

A new 1.4 GHz 19-element, dual-polarization, cryogenic phased array feed (PAF) radio astronomy receiver has been developed for the Robert C. Byrd Green Bank Telescope (GBT) as part of FLAG (Focal L-band Array for the GBT) project. Commissioning observations of calibrator radio sources show that this receiver has the lowest reported beamformed system temperature (T sys ) normalized by aperture efficiency (η) of any phased array receiver to date. The measured T sys /η is 25.4 ± 2.5 K near 1350 MHz for the boresight beam, which is comparable to the performance of the current 1.4 GHz cryogenic single feed receiver on the GBT. The degradation in T sys /η at ∼ 4 (required for Nyquist sampling) and ∼ 8 offsets from the boresight is, respectively, ∼ 1% and ∼ 20% of the boresight value. The survey speed of the PAF with seven formed beams is larger by a factor between 2.1 and 7 compared to a single beam system depending on the observing application. The measured performance, both in frequency and offset from boresight, qualitatively agree with predictions from a rigorous electromagnetic model of the PAF. The astronomical utility of the receiver is demonstrated by observations of the pulsar B0329+54 and an extended H ii region, the Rosette Nebula. The enhanced survey speed with the new PAF receiver will enable the GBT to carry out exciting new science, such as more efficient observations of diffuse, extended neutral hydrogen emission from galactic in-flows and searches for Fast Radio Bursts.

Enabling discoveries: a review of 30 years of advanced technologies and instrumentation at the National Science Foundation

“…CASPER powered spectrometers include the Allen Telescope Array Fly's Eye radio transient search, 155 the Green Bank Ultimate Pulsar Processing Instrument, 156 the workhorse back-end system for the GBT, 157 the versatile GBT astronomical spectrometer (VEGAS), and many others. CASPER-based correlators and beamformers have been used for the Precision Array for Probing the Epoch of Reionization (PAPER), 158 Hydrogen Epoch of Reionization Array (HERA), 159 Large Aperture Experiment to Detect the Dark Ages (LEDA), 160 and the Focal L-band Array for the GBT (FLAG) 161 discussed below, as well as the Smithsonian Astrophysical Observatory's submillimeter array (SMA) Wideband Astronomical ROACH2 Machine (SWARM), 162 the Breakthrough Listen, 163 and other projects to search for extratrerrestrial life. CASPER hardware was used for the Very Long Baseline Array (VLBA), 164 and it was subsequently used for the millimeter wavelength Very Long Baseline Interferometry (VLBI) as part of the EHT.…”

Model for a Noise-Matched Phased Array Feed

“…The dipole array, balun, ground plane, and the transmission line together are referred to as the PAF. Further details of the PAF are available in [24]. The reference plane marked on the figure corresponds to that shown in Fig.…”

“…The LNAs are located at the end of the matching network. The LNAs used for the measurements were cryogenic silicon germanium heterojunction bipolar transistor (SiGe HBT) amplifiers cooled to about 15 K [24], [27]. The i th LNA has an input impedance of Z L N A i ∼ 50 and gain of ∼ 38 dB.…”

“…A summary of the novel aspects of our PAF model and its implementation are presented in Section III. We then compare the model predictions with measurements made with a 1.4 GHz, 19-element dual-polarized PAF on the Green Bank Telescope (GBT) 1 [24]. The results of the comparative study are given in Section IV.…”

An International Survey of Front-end Receivers and Observing Performance of Telescopes for Radio Astronomy

“…The GBT has an extremely flexible suite of instrumentation, including single-and dual-feed heterodyne receivers covering 290 MHz to 92 GHz. The telescope is currently equipped with four multi-pixel systems: FLAG, a 19 element, dual-polarization cryogenic PAF operating at 1.4 GHz that was in its commissioning phase at the time of this review (Roshi et al 2018); KFPA, a seven pixel, dual-polarization feed horn array operating at 18 -26 GHz (Morgan et al 2008); ARGUS, a 16-pixel single-polarization feed horn array operating in the 85-116 GHz range (Sieth et al 2014); and MUSTANG-2, a 223-feed horn dual-polarization bolometer camera with 20 GHz bandwidth centered at 90 GHz (Stanchfield et al 2016).…”