Search

One of the most promising technologies to enable and enhance large survey capabilities for radio astronomy is the use of focal plane phased array antenna feeds, or more simply, phased array feeds (PAFs). PAFs allow for full and continuous coverage of the telescope’s field of view (FoV), and combined with cryogenic amplifiers, can result in survey speed improvements several orders of magnitude better than current multiple-feed-horn cameras. In order to locate cryogenic PAF elements and amplifiers at the telescope focal plane, a radio-frequency transparent vacuum vessel is required. Unlike typical radomes, the transmission properties must be exceptionally good when dealing with weak astronomical signals. The dome must also be sufficiently strong to carry the mechanical load on the vacuum vessel due to atmospheric pressure. Furthermore, the thermal loading on the internal cryogenic stages from the dome must be manageable for the cooling system. We have solved these problems by using a combination of welded polyethylene sheet to maintain the vacuum integrity and a closed-cell rigid foam to transfer the mechanical load to the opposite side of the vessel (a welded aluminum structure). The PAF elements and amplifiers operate at 20 K, while the foam transfers the mechanical load through an 80 K temperature stage, which also serves as a low-temperature radiation shield for the 20 K sections. The poor thermal conductivity of the foam, combined with G10-CR thermal standoffs on the opposite side, ensures the 80 K stage is sufficiently thermally isolated from room temperature conduction. The radiative loading is reduced via the usual employment of multi-layer insulation. In order to facilitate instrument maintenance and future upgrades, a modular PAF element mechanical strategy is employed. The design is such that a PAF-element-amplifier unit can be replaced without accessing the 20 K stage owing to the use of a “cryo-clamp” that uses materials with different coefficients of thermal expansion to tightly hold the units when cold. Cooling is supplied by three two-stage Gifford-McMahon cryo-coolers. This paper presents these design details for the cryostat of the ALPACA (Advanced L-Band Phased Array Camera for Arecibo), an instrument currently being designed and built for the 305 m radio telescope of the Arecibo Observatory in Puerto Rico.

Brigham Young University (BYU), in collaboration with Cornell University, is developing the Advanced Cryogenic L-band Phased Array Camera for Arecibo (ALPACA) as a user provided facility instrument on the Arecibo 305 m radio telescope. This instrument will consist of a fully cryogenic 69-element phased array feed (PAF) and real-time digital beamformer back end capable of producing 40 simultaneous dual polarized beams with approximately 305 MHz of instantaneous bandwidth at 1.4 GHz. The target design goal is to operate with a system noise temperature of 25 K. We report on simulation results and a preliminary overview of the design and progress of the instrument’s development.

A wide-field receiver for the Arecibo Telescope is currently in development, the Advanced L band Phased Array Camera for Arecibo (ALPACA). The Arecibo optical system provides a new challenge for design optimization of phased array feeds. We report on results from a simulation and design optimization study aimed at determining the element layout and geometry that maximizes the receiver sensitivity, field of view, and survey speed.

The authors have developed a new family of dual-polarized compact range feeds with a bandwidth ratio of 3:1. These feeds are also suitable for use as near-field probes.

We present the novel design of microfabricated, silicon-substrate based mirrors for use in cryogenic Fabry-Perot Interferometers (FPIs) for the mid-IR to sub-mm/mm wavelength regime. One side of the silicon substrate will have a double-layer metamaterial anti-reflection coating (ARC) anisotropically etched into it and the other side will be metalized with a reflective mesh pattern. The double-layer ARC ensures a reflectance of less than 1% at the surface substrate over the FPI bandwidth. This low reflectance is required to achieve broadband capability and to mitigate contaminating resonances from the silicon surface. Two silicon substrates with their metalized surfaces facing each other and held parallel with an adjustable separation will compose the FPI. To create an FPI with nearly uniform finesse over the FPI bandwidth, we use a combination of inductive and capacitive gold meshes evaporated onto the silicon substrate. We also consider the use of niobium as a superconducting reflective mesh for long wavelengths to eliminate ohmic losses at each reflection in the resonating cavity of the FPI and thereby increase overall transmission. We develop these silicon-substrate based FPIs for use in ground (e.g. CCAT-prime), air (e.g. HIRMES), and future space-based telescopes (e.g. the Origins Space Telescope concept). Such FPIs are well suited for spectroscopic imaging with the upcoming large IR/sub-mm/mm TES bolometer detector arrays. Here we present the fabrication and performance of multi-layer, plasma-etched, silicon metamaterial ARC, as well as models of the mirrors and FPIs., Comment: Presented at SPIE Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation III, June 14, 2018

We have developed a dual polarized compact range feed with a cross-polarization level of −30 dB from 10.7 GHz GHz to 13 GHz.

We have developed a fully cryogenically cooled phased array feed (PAF) prototype camera and tested it on the Arecibo Radio Telescope in the summer of 2013. The camera consists of 19 dual-polarized elements on a hexagonal grid. The array elements are modified sleeve dipoles specially designed to be reinsertable, to include a noise injection coupling port and to operate at 18 K. Experimental results for observations on the Arecibo Telescope demonstrate performance of the cryogenic amplifiers, digital beamforming, and sensitivity over the PAF field of view, and provide an important proof of concept for a full-scale PAF receiver.

This paper presents the current concept design for ALPACA (Advanced L-Band Phased Array Camera for Arecibo) an L-Band cryo-phased array instrument proposed for the 305 m radio telescope of Arecibo. It includes the cryogenically cooled front-end with 160 low noise amplifiers, a RF-over-fiber signal transport and a digital beam former with an instantaneous bandwidth of 312.5 MHz per channel. The camera will digitally form 40 simultaneous beams inside the available field of view of the Arecibo telescope optics, with an expected system temperature goal of 30 K.

We present the details of the optical design, corrector system, mechanical layout, tolerances, pointing requirements, and overall performance of the sub-millimeter wavelength Large Balloon Reflector telescope (LBR).

This paper presents the results of the optical design tradeoff study that result in a reduction in complexity, size and cost of the structure for the sub-millimeter 25 m class CCAT telescope. Four optical configurations are presented; dual reflector Cassegrain and Gregorian options, and Gregorian Nasmyth and quasi-Nasmyth options. All configurations are shown to have diffraction limited performance.

We are presenting a novel ultra-wideband, dual polarized, antenna design based on a non-planar quasi self-complementary (QSC) approach. An important characteristic of this QSC antenna is an input match better than -10 dB over a decade of bandwidth. We have demonstrated the concept by fabricating a QSC prototype antenna that works from 400 MHz to 4.0 GHz. We have measured the prototype performance in terms of input impedance and radiation patterns, from which the antenna directivity, beamwidth, polarization, and aperture efficiency for a 65° illumination angle were calculated. From these measurements we confirmed the ultra-wideband characteristics of the QSC feed prototype. In particular, we looked for the presence of resonances by measuring the radiation patterns with 3 MHz resolution, over the decade of bandwidth, in both polarizations, and none was found. The measured directivity of the QSC antenna feed is 10 dBi. The half beam-width, at -10 dB level, varies between 60° and 70°, with a frequency average value of 65°. The measured beam-integrated cross-polarization inside the -10 dB level of the patterns is better than -10 dB. The phase center location does not move with frequency.

This paper provides an update of the current efforts towards the development of a conceptual design for a large format cryogenic phased array camera instrument for the 305 m Arecibo radio telescope.

Refractive optical elements are widely used in millimeter and sub-millimeter astronomical telescopes. High resistivity silicon is an excellent material for dielectric lenses given its low loss-tangent, high thermal conductivity and high index of refraction. The high index of refraction of silicon causes a large Fresnel reflectance at the vacuum-silicon interface (up to 30%), which can be reduced with an anti-reflection (AR) coating. In this work we report techniques for efficiently AR coating silicon at sub-millimeter wavelengths using Deep Reactive Ion Etching (DRIE) and bonding the coated silicon to another silicon optic. Silicon wafers of 100 mm diameter (1 mm thick) were coated and bonded using the Silicon Direct Bonding technique at high temperature (1100 C). No glue is used in this process. Optical tests using a Fourier Transform Spectrometer (FTS) show sub-percent reflections for a single-layer DRIE AR coating designed for use at 320 microns on a single wafer. Cryogenic (10 K) measurements of a bonded pair of AR-coated wafers also reached sub-percent reflections. A prototype two-layer DRIE AR coating to reduce reflections and increase bandwidth is presented and plans for extending this approach are discussed.

We describe the Short Wavelength Camera (SWCam) for the CCAT observatory including the primary science drivers, the coupling of the science drivers to the instrument requirements, the resulting implementation of the design, and its performance expectations at first light. CCAT is a 25 m submillimeter telescope planned to operate at 5600 meters, near the summit of Cerro Chajnantor in the Atacama Desert in northern Chile. CCAT is designed to give a total wave front error of 12.5 μm rms, so that combined with its high and exceptionally dry site, the facility will provide unsurpassed point source sensitivity deep into the short submillimeter bands to wavelengths as short as the 200 μm telluric window. The SWCam system consists of 7 sub-cameras that address 4 different telluric windows: 4 subcameras at 350 μm, 1 at 450 μm, 1 at 850 μm, and 1 at 2 mm wavelength. Each sub-camera has a 6’ diameter field of view, so that the total instantaneous field of view for SWCam is equivalent to a 16’ diameter circle. Each focal plane is populated with near unit filling factor arrays of Lumped Element Kinetic Inductance Detectors (LEKIDs) with pixels scaled to subtend an solid angle of (λ/D)2 on the sky. The total pixel count is 57,160. We expect background limited performance at each wavelength, and to be able to map < 35(°)2 of sky to 5 σ on the confusion noise at each wavelength per year with this first light instrument. Our primary science goal is to resolve the Cosmic Far-IR Background (CIRB) in our four colors so that we may explore the star and galaxy formation history of the Universe extending to within 500 million years of the Big Bang. CCAT's large and high-accuracy aperture, its fast slewing speed, use of instruments with large format arrays, and being located at a superb site enables mapping speeds of up to three orders of magnitude larger than contemporary or near future facilities and makes it uniquely sensitive, especially in the short submm bands.

Low-loss lenses are required for submillimeter astronomical applications, such as instrumentation for CCAT, a 25 m diameter telescope to be built at an elevation of 18,400 ft in Chile. Silicon is a leading candidate for dielectric lenses due to its low transmission loss and high index of refraction; however, the latter can lead to large reflection losses. Additionally, large diameter lenses (up to 40 cm), with substantial curvature present a challenge for fabrication of antireflection coatings. Three anti-reflection coatings are considered: a deposited dielectric coating of Parylene C, fine mesh structures cut with a dicing saw, and thin etched silicon layers (fabricated with deep reactive ion etching) for bonding to lenses. Modeling, laboratory measurements, and practicalities of fabrication for the three coatings are presented and compared. Measurements of the Parylene C anti-reflection coating were found to be consistent with previous studies and can be expected to result in a 6% transmission loss for each interface from 0.787 to 0.908 THz. The thin etched silicon layers and fine mesh structure anti-reflection coatings were designed and fabricated on test silicon wafers and found to have reflection losses less than 1% at each interface from 0.787 to 0.908 THz. The thin etched silicon layers are our preferred method because of high transmission efficiency while having an intrinsically faster fabrication time than fine structures cut with dicing saws, though much work remains to adapt the etched approach to curved surfaces and optics < 4" in diameter unlike the diced coatings.

We have developed a fully cryogenically cooled, 19-element phased array feed (PAF), prototype camera for the Arecibo Radio Telescope. The 19 PAF elements are dual polarized dipoles over a ground plane, and they sit behind a 70 cm diameter vacuum window transparent to RF.

Motivation: The “New Worlds, New Horizons”, 2010 Astronomy Decadal Survey stated that the pursue of phased array technologies to gain full access of the instantaneous field of view (FOV) of a telescope optics, will enable gains in mapping speed of orders of magnitude in current and future radio telescopes. To realize this goal, we have developed a fully cryogenically cooled 19-element, dual polarized, phased array prototype camera for the Arecibo radio telescope that works at L-Band (1.2 to 1.7 GHz). One key aspect of the design is a 70cm diameter vacuum window consisting of a Kapton membrane supported by radio-transparent foam capable of withstanding loads in excess of 200 psi. In order to support the ~5 tons of atmospheric pressure on the window, without having unreasonably high thermal loads, a detailed structural and thermal analysis and simulation was performed, resulting in a trade-off between structural and thermal loads in the cryostat design.

This paper will give an overview of the unique mechanical and optical design of the DVA-1 telescope. The rim supported carbon fibre reflector surfaces are designed to be both low cost and have high performance under wind, gravity, and thermal loads. The shaped offset Gregorian optics offer low and stable side lobes along with a large area at the secondary focus for multiple feeds with no aperture blockage. Telescope performance under ideal conditions as well as performance under gravity, wind, and thermal loads will be compared directly using calculated radiation patterns for each of these operating conditions., 2012 International Conference on Electromagnetics in Advanced Applications (ICEAA), 2012-09-2 - 2012-09-7, Cape Town, WP, South Africa

Summary form only given. The cost of an antenna element is a key parameter in maximizing the performance per cost ratio of an array. Most current radio telescope array antennas were not designed for high-volume production but generally “only” for performance since typically the production numbers were not large enough to (a) justify the additional design effort and (b) qualify for discount production in terms of amortized tooling costs and quantity purchase. The Allen Telescope Array (ATA) was one of the first radio telescope antenna arrays to be designed for high-volume antenna production.

The U.S. design concept for the Square Kilometer Array (SKA) program is based on utilizing a large number of 15 meter dish antennas. The Technology Development Project (TDP) is planning to design and build the first of these antennas to provide a demonstration of the technology and a solid base on which to estimate costs. This paper describes the performance of the selected optics design. It is a dual-shaped offset Gregorian design with a feed indexer that can accommodate corrugated horns, wide band single pixel feeds or phased array feeds.

We are developing the next generation instrumentation for the Arecibo radiotelescope, consisting of an L-band focal phased array feed (PAF) with an effective field of view (FOV) of approx 20 arcmin in diameter. In 2010 and in part of 2011, as part of this development, we made a feasibility study to address two issues, in particular, necessary to implement this technology: first, to determine the available FOV of a PAF system with the Arecibo shaped optics, and second, the development of a suitable cryogenic concept with the potential of reaching a receiver temperature of 35K. We are presenting the results of this feasibility study and the system concept design of the Arecibo cryogenic phased array feed (AO-PAF).

To prepare for a permanent wide-field, L-band phased array feed on the Arecibo radio telescope, a focal plane characterization experiment was conducted from July-August 2010. Experimental and numerical results indicate that a continuous field of view equivalent to 40 beams overlapping at half power is achievable.

The U.S. design concept for the Square Kilometre Array (SKA) program is based on utilizing a large number of small-diameter dish antennas in the 12 to 15 meter diameter range. 12The Technology Development Project (TDP) is planning to design and build the first of these antennas to provide a demonstration of the technology and a solid base on which to estimate costs. The latest considerations for selecting both the optics and feed design are presented.

The Square Kilometer Array (SKA) is the next generation of meter and cm-wave radio telescope with a goal sensitivity of 50 to 100 times higher than any other existing instruments. Very exiting science missions for this instrument include planetary formation, astrobiology, the origin and evolution of cosmic magnetism, primordial galaxy evolution, cosmology, and dark energy [1]. The SKA will operate from 100MHz to 25 GHz, with milliarcsec resolution at the shortest wavelengths, and with an architecture that will provide multiple simultaneous fields of view (FOV). The proposed reference design for the SKA consists of a large number of reflector antennas (12 to 15m in diameter) with ultra-wide band single pixel feeds. Some of the wide band feed candidates includes the ATA Feed [2], the eleven feed [3], a modified version of the quad-ridge feed [4], and the QSC Feed [5]. The QSC (Quasi Self Complementary) feed is a dual polarized antenna with 3-D log-periodic arms over a ground plane in a quasi-self-complementary configuration, currently being developed at Cornell University, with an instantaneous bandwidth in excess of 10:1. We already have fabricated a 400 MHz to 4.0 GHz QSC feed prototype and verified an input impedance match better than −11 dB over a decade of bandwidth [6]. Precise characterization of feed polarization, directivity, beam size is required in order to optimize the SKA reflector optics necessary to attain the high sensitivity and very low noise requirements for this instrument. We have measured the radiation patterns of the QSC feed prototype at Satimo's facilities in Kennesaw, GA, USA from 400 to 4 GHz in 3 MHz steps to spot any resonances.

The Cornell Caltech Atacama Telescope (CCAT) is a 25m-class sub-millimeter radio telescope capable of operating from 300GHz up to 1.5 THz. The CCAT optical design is an f/8 Ritchey-Chretien (RC) system in a dual Nasmyth focus configuration and a 20 arc-min FOV (diffraction limited imaging performance better than 0.31" at the edge of the field). The large FOV is capable to accommodate up to 1200x1200 (Nyquist Sampled) Pixels at 200 microns, with better than 96% Strehl ratio. The telescope pedestal assembly is a counterbalanced elevation over azimuth design. The main reflector surface is segmented and actively controlled to attain diffraction-limited operation up to 200 microns. A flat Mirror located behind the main reflector vertex provides the optical path relay to either of the two Nasmyth platforms and to a bent-Cassegrain focus for surface calibration. We present the imaging characteristics of the CCAT over the 20arc-min FOV at 200 microns at the Nasmyth focal plane, as well as the positioning sensitivity analysis of CCAT's 3.2m-diameter sub-reflector given in terms of the telescope optical performance, antenna pointing requirements and sub-reflector chopping characteristics.

The Cornell Caltech Atacama Sub-millimeter Telescope (CCAT) is proposed to have 25m-diameter primary segmented active surface capable of diffraction-limited operation in the wavelength range between 200 microns to 1mm. The active surface design layout is composed of 162 "pie-shaped" segments, each fitted with three actuators that provide piston and tilt/tip control for segment positioning and orientation. We present a performance analysis for five types of segment positioning errors, e.g., piston, tilt/tips, radial and azimuth displacements, and twist errors. From these only the first two, segment piston and tilt/tip errors, are directly controllable by the actuator system. Segment tilt/tip motions may indirectly compensate radial and azimuth segment positioning errors. Residual segment twists introduce quadric phase distribution errors across the face of the segments that cannot be compensated by a simple 3-actuator/segment active surface control system. We have obtained Ruze's coefficients that relate the standard deviation of each segment positioning error type with the overall Strehl ratio of the telescope at 200 microns.

The frequency specifications of the Square Kilometre Array (SKA) call for an optimum operation of the antenna elements from 25 down to 100 MHz. The current 12 m diameter US-SKA design is specified from 500 up to 25 GHz, with an upper goal of 35 GHz. At the low frequency end of the band (i.e., 100 MHz), a 12 m reflector antenna is about four wavelengths in diameter. Then, the question is: how well can you do, at this low frequency end of the specified band of operation for the SKA, with a symmetric reflector configuration using an ultra-wide-band prime focus feed? This paper presents the analysis of the antenna performance, in terms of Aeff/TA, of three symmetric configurations of the 12 m US-SKA antenna design between 100 and 200 MHz.

In the fabrication of high-performance, low-cost secondary reflectors for radio telescopes, it is a significant challenge to avoid introduction of low-order surface errors such as astigmatism or coma. This arises primarily because low-order surface errors are easily induced by support structure placement or simple thermal variations in the manufacturing process. It is, of course, possible to bring these errors to within the required tolerance, but if an active primary reflector is present, it may be possible to relax the requirements on the secondary and perhaps lower its cost. In this paper, we take the Large Millimeter-wave Telescope (LMT/GTM) as an example system. We model the effects of correcting a deformed sub-reflector by using the existing segmented active primary. The sub-reflector deformation patterns employed are low-order (e.g., astigmatism or coma), but are allowed significant excursions from the nominal surface figure. For each case, we demonstrate the best theoretical performance, using the active primary to correct for the errors. Additionally, to determine whether such an approach would be practical, we also demonstrate the likely performance improvement that could be achieved using brief measurements on an astronomical source. In this approach, we introduce varying amounts of known low-order deformation patterns into the active primary and seek the combination that results in the maximum signal. Finally, we compare this result to the theoretical maximum and make recommendations on the practical utility of the approach.