Sivasankaran Srikanth

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.

Hybrid UTD-MM analysis of the scattering by a perfectly conducting semicircular cylinder

A hybrid UTD-MM technique which combines the uniform geometrical theory of diffraction (UTD) and the method of moments (MM) is employed to analyze efficiently the problem of electromagnetic diffraction of transverse electric (TE) and transverse magnetic (TM) waves by a perfectly conducting semicircular cylinder. An analysis of this problem is useful for understanding the coupling between the mechanisms of edge and convex surface diffraction. The accuracy of the numerical results for the far-zone fields based on this solution is established by comparison with an independent formally exact MM solution.

A compact full waveguide band turnstile junction orthomode transducer

This paper describes the development, construction, and measurement of a full waveguide band (8–12 GHz) orthomode transducer (OMT). The design uses a turnstile junction at the input, two 90° H-plane bends in each of four waveguide arms, and two E-plane Y-junctions. The outputs are in WR90 waveguides. Measured return loss is better than 19 dB and cross polarization lower than −50 dB in the 7.8–12.2 GHz range. Three large aluminum blocks form the main body of the OMT. The device has a diameter of 8.0”, height of 2.7”, and weighs 2.5 lbs.

ALMA band 2 optics: Preliminary results

The National Radio Astronomy Observatory is currently developing the ALMA Band 2 cartridge, to be completed by mid-2016. The receiver will cover the 67–90 GHz band, with dual linear polarization. An orthomode transducer (OMT) based on turnstile junction has been developed and measured. The measured input return loss is better than 20dB. The optics comprises of a linear taper corrugated horn and a bi-hyperbolic lens that matches the waist of the horn to that of the telescope. The lens mounts on the 300K plate and serves as the vacuum barrier for the cryostat. The window for band 2 is offset from the center of the cryostat by 255mm, which results in a feed/ lens tilt of 2.48° to the cryostat axis. The feed is cooled to 15K, while the lens is at 300K. The minimum distance between the lens and the feed aperture is 83 mm and the maximum size of the lens is 92mm.

THE GBT 67–93.6 GHz SPECTRAL LINE SURVEY OF ORION-KL

We present a 67--93.6 GHz spectral line survey of Orion-KL with the new 4 mm Receiver on the Green Bank Telescope (GBT). The survey reaches unprecedented depths and covers the low-frequency end of the 3 mm atmospheric window which has been relatively unexplored previously. The entire spectral-line survey is published electronically for general use by the astronomical community. The calibration and performance of 4 mm Receiver on the GBT is also summarized.

A 4mm spectroscopic dual-beam receiver for the Robert C. Byrd green bank radio telescope

With a 100-meter aperture, and recent improvements to its surface accuracy and servo system upgrades, the Robert C. Byrd Green Bank Telescope is the most sensitive telescope operating at 90 GHz. A dual-feed heterodyne receiver is developed for observations at the lower frequency end of the 3-4mm atmospheric window (67 to 93 GHz). The science goals are primarily molecular spectroscopic studies of star formation and astrochemistry both internal and external to the Milky Way galaxy. Studies of the structural and physical properties of star-forming, cold-cloud cores will be revolutionized with molecular spectroscopy of the deuterium and other important species within the band. Essential for spectroscopy is the ability to remove slow gain and atmospheric variations. An optical table external to the cooled components rotates into the path of either beam an ambient temperature load, an offset mirror for viewing an internal cold load, or a quarter-wave plate that produces circular polarization for VLBI observations. A composite waveguide window comprised of HDPE, Zitex, and z-cut quartz provides a high-strength, low-loss medium for transmission of the signal to the cooled corrugated feed horn. An orthomode transducer separates the polarization components which are amplified by low noise HEMT amplifiers. Warm W-band MMIC amplifiers are required to compensate a negative gain slope and to reduce noise contributions from the down conversion to the GBT IF frequencies. Initial science results and receiver performance during commissioning observations will be presented along with details of the component design.

Towards optics design for the next generation very large array

The science goals set by the Science Working Group for the ngVLA include study of star and planet formation, imaging the Milky Way and nearby galaxies, study of galaxy formation, cosmology etc. In order to achieve these targeted studies, the ngVLA proposal calls for about 300 antennas spread over a large geographical area, mainly in the western New Mexico region. The proposed array will consist of a compact core comprising of about 30% of the total with the rest on long baselines and some on the outer core. The sensitivity goal calls for 10 times that of the VLA, with antenna aperture in the 12 to 25-meter range. The goal is to provide continuous frequency coverage from 10–50 GHz and 75–116 GHz with maximum sensitivity and option to cover down to 1.2 GHz with some marginal performance reduction.

ALMA band 2 optics: Design, constraints, implementation and measurements

This paper describes the development, construction, and measurement of the optics of a receiver operating between 67-90 GHz prototyped by the National Radio Astronomy Observatory (NRAO). The intent is to install this receiver on the Atacama Large Millimeter/submillimeter Array (ALMA) as a new band increasing the capability of ALMA. ALMA is an international radio telescope in the Atacama Desert of northern Chile [1]. The array consists of 54 12-meter and 12 7-meter high precision antennas operating in the 0.3 to 10 mm wavelength range. The instrument provides both interferometric and total-power astronomical information on atomic, molecular and ionized gas and dust in our galaxy, the solar system and the nearby to high-redshift universe.

Short backfire antenna with concentric sleeves for highly sensitive radio astronomical receivers

A broadband 800-MHz short backfire antenna (SBA) integrated with a cryogenic vacuum chamber is designed for astronomical applications. The cryostat offers the low-temperature condition to front-end components because of low-noise demands for the astronomy observations. In order to overcome the impacts from the cryostat, the SBA adopts a pair of concentric sleeves to replace the conventional double-discs design. It features a dual-mode response and enhances the bandwidth. The simulated results show the VSWR less than 2.2 and the gain of 11-15 dBi over the band of 700-945 MHz.