Richard M. Prestage

The Green Bank Telescope

The Robert C. Byrd Green Bank Telescope of the National Radio Astronomy Observatory is the worlds premiere single-dish radio telescope operating at centimeter to long millimeter wavelengths. This paper describes the history, construction, and main technical features of the telescope.

Out-Of-Focus Holography at the Green Bank Telescope

Mullard Radio Astronomy Observatory, Cavendish Laboratory, Cambridge CB3 0HE, UKthe date of receipt and acceptance should be inserted laterAbstract. We describe phase-retrieval holography measurements of the 100-m diameter Green Bank Telescope using astro-nomical sources and an astronomical receiver operating at a wavelength of 7mm. We use the technique with parameterizationof the aperture in terms of Zernike polynomials and employing a large defocus, as described by Nikolic et al. (2006). Individualmeasurements take around 25minutes and from the resulting beam maps (which have peak signal to noise ratios of 200:1) weshow that it is possible to produce low-resolution maps of the wavefront errors with accuracy around λ/100.Using such measurements over a wide range of elevations, we have calculated a model for the wavefront-errors due to theuncompensated gravitational deformation of the telescope. This model produces a significant improvement at low elevat ions,where these errors are expected to be the largest; after applying the model, the aperture efficiency is largely independent ofelevation. We have also demonstrated that the technique can be used to measure and largely correct for thermal deformationsof the antenna, which often exceed the uncompensated gravitational deformations during daytime observing.We conclude that the aberrations induced by gravity and thermal effects are large-scale and the technique used here is particu-larly suitable for measuring such deformations in large millimetre wave radio telescopes.Key words. telescopes

A GPU-Based Wide-Band Radio Spectrometer

The Graphics Processing Unit (GPU) has become an integral part of astronomical instrumentation, enabling high-performance online data reduction and accelerated online signal processing. In this paper, we describe a wide-band reconfigurable spectrometer built using an off-the-shelf GPU card. This spectrometer, when configured as a polyphase filter bank (PFB), supports a dual-polarization bandwidth of up to 1.1 GHz (or a single-polarization bandwidth of up to 2.2 GHz) on the latest generation of GPUs. On the other hand, when configured as a direct FFT, the spectrometer supports a dual-polarization bandwidth of up to 1.4 GHz (or a single-polarization bandwidth of up to 2.8 GHz).

Holographic Measurement and Improvement of the Green Bank Telescope Surface

We describe the successful design, implementation, and operation of a 12 GHz holography system installed on the Robert C. Byrd Green Bank Telescope (GBT). We have used a geostationary satellite beacon to construct high-resolution holographic images of the telescope mirror surface irregularities. These images have allowed us to infer and apply improved position offsets for the 2209 actuators which control the active surface of the primary mirror, thereby achieving a dramatic reduction in the total surface error (from 390 μm to ~240 μm, rms). We have also performed manual adjustments of the corner offsets for a few panels. The expected improvement in the radiometric aperture efficiency has been rigorously modeled and confirmed at 43 GHz and 90 GHz. The improvement in the telescope beam pattern has also been measured at 11.7 GHz with greater than 60 dB of dynamic range. Symmetric features in the beam pattern have emerged which are consistent with a repetitive pattern in the aperture due to systematic panel distortions. By computing average images for each tier of panels from the holography images, we confirm that the magnitude and direction of the panel distortions, in response to the combination of gravity and thermal gradients, are in general agreement with finite-element model predictions. The holography system is now fully integrated into the GBT control system, and by enabling the telescope staff to monitor the health of the individual actuators, it continues to be an essential tool to support high-frequency observations.

Portable telescope control system project

The portable telescope control system (PTCS) project is a collaboration between the Anglo-Australian Observatory and the Joint Astronomy Centre, Hawaii. The project aims to develop telescope control software in a form which is portable between a wide range of computer systems, and which can easily be adapted to different telescopes. PTCS uses the DRAMA software environment which provides a high degree of operating system independence. The PTCS design is based on tested algorithms used in existing successful telescope control systems. The initial version of PTCS is now being interfaced to an EPICS based drive servo system for use with the James Clerk Maxwell sub-millimeter telescope on Mauna Kea.

A dual-polarization InSb receiver for 461/492 GHz

A dual-polarization InSb hot-electron bolometer-mixer receiver has been built for the James Clerk Maxwell Telescope, for operation at 461 and 492 GHz (the frequencies of theJ=4→3 rotational transition of CO and of the3P1→3P0 transition of neutral carbon). Receiver noise temperatures of 500K have been obtained at 461 GHz, in observing bandwidths of 3 MHz. The receiver was designed as a “common-user” or facility instrument. Here we describe those aspects of the design and construction which enabled this goal to be realized.

Measuring and Correcting Wind-Induced Pointing Errors of the Green Bank Telescope Using an Optical Quadrant Detector

Wind-induced pointing errors are a serious concern for large-aperture high-frequency radio telescopes. In this paper, we describe the implementation of an optical quadrant detector instrument that can detect and provide a correction signal for wind-induced pointing errors on the 100 m diameter Green Bank Telescope (GBT). The instrument was calibrated using a combination of astronomical measurements and metrology. We find that the main wind-induced pointing errors on timescales of minutes are caused by the feed arm being blown along the direction of the wind vector. We also find that wind-induced structural excitation is virtually nonexistent. We have implemented offline software to apply pointing corrections to the data from imaging instruments such as the MUSTANG 3.3 mm bolometer array, which can recover ~70% of sensitivity lost due to wind-induced pointing errors. We have also performed preliminary tests that show great promise for correcting these pointing errors in real time using the telescopes subreflector servo system in combination with the quadrant detector signal.

The Green Bank Telescope: current status and early results

The National Radio Astronomy Observatory Green Bank Telescope (GBT), the worlds largest fully steerable telescope, is now undergoing commissioning and early scientific operation. The GBT has many innovative design features that advance imaging quality, sensitivity, and versatility. These include an unblocked aperture, an active surface, and a six-degree of freedom Gregorian subreflector. The GBT has an advanced laser rangefinder metrology system, which will measure the position of the active surface panels, and also guide the precision pointing of the telescope. Early commissioning results have confirmed the performance of the telescope, and exciting scientific discoveries are already being made. This paper describes the various features of the telescope in more detail, and presents the latest results.

The versatile GBT astronomical spectrometer (VEGAS): Current status and future plans

The VEGAS multi-beam spectrometer (VEGAS) was built for the Green Bank Telescope (GBT) through a partnership between the National Radio Astronomy Observatory (NRAO) and the University of California at Berkeley. VEGAS is based on a Field Programmable Gate Array (FPGA) frontend and a heterogeneous computing backend comprised of Graphical Processing Units (GPUs) and CPUs. This system provides processing power to analyze up to 8 dual-polarization or 16 single-polarization inputs at bandwidths of up to 1.25 GHz per input. VEGAS was released for “shared-risk” observing in March 2014 and it became the default GBT spectral line backend in August 2014. Some of the early VEGAS observations include the Radio Ammonia Mid-Plane Survey, mapping of HCN/HCO+ in nearby galaxies, and a variety of radio-recombination line and pulsar projects. We will present some of the latest VEGAS science highlights.

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

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 (