GBT Metrology Improvements:
The GBO has received NSF-MSIP funding to implement a laser metrology system for measuring the surface of the GBT precisely and quickly.
The 2008 panels of the primary surface of the GBT can be adjusted in real time to maintain its parabolic shape. At present, the surface is measured using “out-of-focus” holography, which takes ~30 min but remains valid for many hours at night. During the day, however, thermal gradients in the antenna backup structure can vary on time scales approaching 1 hour, requiring calibration measurements at least this often. This is extremely inefficient, and as a result, observations at 3mm are rarely made during the day.
Recent advances in commercial technology have made it possible to purchase a laser scanner, that if mounted near the GBT focus, will produce a hundred million angle and range measurements of the surface every few minutes. Any given measurement has a range uncertainty ~2 mm, but the data can be averaged to reduce the errors to tens of microns on the relevant angular scales. Measured surface distortions can then be corrected using the active surface.
Measurement of the GBT surface in real time will allow operation at the highest frequencies night or day, doubling the available telescope time at 3mm wavelength, and increasing the observing efficiency for all projects that operate above 25 GHz.
This project takes advantage of the GBT active surface and large collecting area, which makes it the largest telescope in the world operating at mm wavelengths.
Wide Band Pulsar Receiver:
GBO is developing a new ultra-wide-band receiver for the GBT that will instantaneously cover ~0.7 — 4 GHz.
Radio pulses from pulsars are dispersed by their passage through ionized gas in the interstellar medium. Measurement of pulses at widely-spaced frequencies is necessary to characterize and remove dispersive effects. This currently requires use of two separate GBT receivers, often on different days.
The new receiver will allow these measurements to be made simultaneously with a single instrument, improving the signal-to-noise for pulsar observations, and improving observing efficiency for high-precision pulsar timing programs.
This receiver takes advantage of the collecting area, sensitivity, and relative freedom from interference afforded by the GBT at the Green Bank Observatory, which makes it a premier instrument for pulsar science worldwide.
The ARGUS+ Project:
This will increase the speed of the GBT by a factor of 10 for mapping key molecular transitions in the 3mm band. These transitions provide critical information on the processes that regulate star formation over many scales. Argus+ will routinely produce sensitive maps of molecular species with a spatial dynamic range of 10^4 to 10^5.
Argus+ takes advantage of the unique angular resolution and sensitivity of the GBT in the 3mm band.
Argus+ consists of a 144-pixel camera and detector, data reduction software, and data archive. The camera is a 9-times copy of the existing Argus receiver with improved amplifiers and thus has low technical risk.
There is significant involvement of the scientific community in designing and executing legacy surveys. There is student team involvement in the surveys and production of materials for exhibits at the Green Bank Science Center.
Digitizing the IF/RF:
The GBO is developing wide-band digital systems to increase the range of frequencies detected at any instant, and to improve capabilities for removing interfering signals.
Over much of the GBT’s operational range, only a fraction of the frequencies covered by a receiver can be sent at any time from the telescope to the lab for analysis. This arises from the restricted bandwidth of the existing analog IF and fiber systems.
Advances in digital technology are now making it feasible to directly digitize bandwidths up to 8 GHz at RF or IF after a single down-conversion. The GBO is beginning a project to enable direct digital sampling of its receivers’ output to increase the instantaneous bandwidth by several factors, bypassing most analog components. This will also increase the dynamic range of the data, opening the possibility of real-time excision of interfering signals.
These improvements will shorten the time needed for many measurements, provide greater instrumental stability, improve accuracy of data in the presence of interfering signals, and lower power consumption. It will open up the full power of many GBT receivers that now operate with restricted capabilities.
Upgraded L band receiver:
The GBT receiver covering L band (1.15-1.73 GHz) is in high demand for observations of pulsars and the 21cm line in Galactic and extragalactic sources. It is now about 20 years old.
A straightforward upgrade using modern components will reduce the zenith system temperature from the current 18 K to 14 K or less This will improve the receiver ’s reliability and performance for all projects, and can reduce the time needed for many observations by as much as a factor of 0.6.
This upgrade uses known technology and takes advantage of the GBT’s clean optics to achieve new records in receiver sensitivity.
Phased Array Feed Receiver for Multibeam 21cm Measurements:
Working with partners including NRAO, Green Bank has developed a cryogenically cooled phased array feed (PAF) receiver that operates between 1 and 1.6 GHz, called FLAG.
Feed horns at the Gregorian focus of the GBT for 21cm wavelength are so large that it would be impractical to develop multi-pixel receivers using this technology. Phased array feed technology replaces the monolithic feedhorns with a series of small antennas in the focal plane, whose output is then combined in phase to emulate a feedhorn. A dual-polarization receiver working at 21cm, FLAG, has now been developed using this technology. Its 19 dipoles produce seven beams within a field of view of ~20′ with a Tsys < 20 K over an instantaneous bandwidth of 30 MHz for spectroscopy and ~150 MHz for continuum. A partnership funded by the NSF between the GBO, West Virginia University, NRAO, and Brigham Young University, has developed a Beamformer system which is critical to digitizing and forming beams from the FLAG signals.
FLAG has been tested but has not been developed to the point where it can be offered routinely, but the GBO will implement this as the first of a suite of phased-array feed technology receivers that will eventually replace all of its current receivers.
FLAG offers increases in mapping speeds of a factor ~5 over the current single-pixel receiver. It is well matched to the GBT’s clear aperture and relatively low sidelobe levels to provide high sensitivity for HI 21cm mapping projects and wide-field pulsar observations.
3mm Point Source Receiver:
The GBT currently has the Argus 16-pixel array for observations at the upper frequencies of the 3mm atmospheric window This is excellent for mapping, but only detects a single polarization. A dual-polarization receiver with optimized signal and reference pixels would speed up observations of compact sources by a factor of at least 2.
This receiver will provide maximum sensitivity for observations of radio sources of a few arc-seconds or smaller, such as stars and the compact cores of star-forming regions, in the important lines of CO and its isotopologs. This project takes advantage of the unique sensitivity of the GBT at 3mm. The receiver will be modeled on the existing W band receiver that covers the lower 3mm band frequencies, uses existing technology, and is thus low risk.
More information concerning future planning at the Green Bank Observatory at ASTRO2020.