The Arizona Radio Observatory: A New 12-M Telescope and Other Developments
Robert W. Freund*1, Lucy M. Ziurys2, Eugene F. Lauria3, George P. Reiland4
lThe University of Arizona, Tucson, Arizona 85721-0065, USA, (520)626-7714, (520)621-5554 FAX, [email protected], 2(520)621-6525, [email protected], 3(520)626-1098, [email protected], 4(520)626-6744, [email protected]
Abstract
The Arizona Radio Observatory (ARO) operates two radio telescopes in the millimeter and sub-millimeter wavelength region. Recently, one of these instruemnts, the aging, former NRAO 12 meter telescope on Kitt Peak, was replaced with one of the 12 m ALMA prototype antennas. The high performance design of this new instrument, including its 20 microns surface and sub-arcsecond pointing, will expand observational capabilites at the ARO. New receivers and backends are also in development at ARO. A dual polarization sideband separating SIS mixer covering the 0.7 mm atmospheric windw, ranging from 385 GHz to 500 GHz, has been installed as a facility instrument on the Sub-Millimeter Telescope (SMT) on Mount Graham. A prototype Band 2 receiever, based on HEMT amplifier technology, has been built and tested on the sky. These new additions and other developments will be presented.
1. Introduction
Astronomical observations at millimeter and sub-millimeter wavelengths are routinely performed at the two telescopes operated by the Arizona Radio Observatory (ARO) at the University of Arizona. One telescope, the 10-meter diameter Sub-Millimeter Telescope (SMT), is located at an exceptional site at 3186.0 m on Mount Graham, 160 km east of Tucson, Arizona. The other telescope, the 12-Meter telescope, is located at 1914 m on Kitt Peak, 80 km west of Tucson. Both observatories employ high sensitivity near quantum limited heterodyne receivers matched to the atmospheric characteristics above their respective site. Backend instrumentation include stable, multi-resolution spectrometers to support molecular line observations, and classical continuum detector systems. The SMT facility IF system and all its radiometers support 4 GHz wide channels. The antenna at the SMT, shown in Figure 1, uses a high precision reflector attached to honeycomb panels supported on a thermally stable carbon-fiber reinforced plastic (CFRP) structure. Excellent performance is obtained from the panel’s 15 µm RMS surface accuracy permitting observations well into the sub-millimeter wavelength region, even as short as 300 µm, unaffected by diurnal temperature variations.
The venerable 12-Meter antenna on Kitt Peak, originally constructed by the National Radio Astronomy Observatory’s (NRAO) in 1967 and subsequently upgraded in 1984, was recently retired and removed from the site. It was replaced by one of the two 12-m diameter ALMA evaluation antennas. Early last year, ARO obtained this antenna from the European Southern Observatory (ESO) and moved it from the Very Large Array (VLA) site west of Socorro, Figure 1. The ARO 10-meter New Mexico, to Kitt Peak, Arizona. The new antenna will enable the ARO to SMT on Mount Graham, continue supporting astronomical research at the longer millimeter wavelengths Arizona. well into the future.
The ARO emphasizes instrumentation, infrastructure, and observational techniques that enable coherent detection and identification techniques of extremely weak signals in spectrally confused regions. Both telescopes have played important roles in millimeter VLBI, and are participating in the Event Horizon Telescope (EHT) and other projects.
2. A New 12-m Telescope on Kitt Peak
In the summer of 2002, the construction of the two evaluation antennas of the Atacama Large Millimeter Array (ALMA) Project began at the National Radio Astronomy Observatory’s Antenna Test Facility at the Very Large Array (VLA) in New Mexico. Nine years later, their jobs complete, the community was notified that the antennas were no longer required by the Project and would become available. Last March, the ARO obtained ownership of the high performance European Southern Observatory’s (ESO) evaluation antenna. A plan was quickly developed that involved separating the antenna into two large pieces suitable for truck transport to Kitt Peak, the 650 kilometers west of the VLA. The plan involved separating the reflector from its supporting pedestal/receiver cabin. One difficulty for transport was that the reflector backing structure and receiver cabin were both constructed from CFRP, and thus major pieces were held
978-1-4673-5225-3/14/$31.00 ©2014 IEEE together by glue. Fortunately, the reflector was connected to the receiver cabin primarily with multiple bolts, as well as numerous CFRP interconnecting vanes that had to be cut. Most of the external protrusions such as platforms, stairs, cable wraps, thermal insulation and the prime focus quadrapod were removed from the Figure 3. The new ARO 12-Meter structures. Their removal Antenna on Kitt Peak, Arizona. was necessary to reduce the Figure 2. ESO 12-meter evaluation overall dimensions so that antenna located at the VLA site, NM. the pedestal and reflector could fit under the highway bridges that would be encountered along the way. To accomplish this removal, most of the telescope wiring was removed, one of the larger tasks of the project. Shown in Figure 2 is the ESO evaluation antenna located at the VLA site before disassembly and the start of the transportation process. Figure 3 shows the antenna within the 12-Meter observatory dome without its quadrapod shorting after arriving on Kitt Peak in December 2014.
A transportation company that specialized in moving heavy and bulky items, Precision Heavy Haul of Phoenix, AZ, provided the customized vehicles, special moving fixtures and expertise. During the second week of last November, the 9 ton reflector was removed from its 75 ton pedestal. Then the pedestal was fitted into its custom transportation fixture and lifted onto a special trailer to begin its trip to Kitt Peak. After traveling for one week, the pedestal arrived safely at the observatory on Kitt Peak. During a snow storm, the pedestal was lifted into the 12-Meter dome and bolted to its new foundation. Two weeks later, the process started again, with the reflector being loaded onto its specialized trailer. A week later, the reflector was slid through the opening in the 12-Meter dome with about 1cm to spare on either side. It was placed onto the waiting receiver cabin completing the transportation phase of the relocation.
The last step in the relocation process is to restore the antenna to its pre-move state. The external platforms, stairs, cable wraps, insulation have already been installed, and the prime-focus quadrapod will follow. All the removed wiring will be restored and some of the power wiring will be modified to accommodate North American power standards rather than the European or Chilean power for which it was designed. New software will be developed to interface with the telescope’s computer. The existing dual polarization 3 mm SBS receiver and facility IF system will be the first radiometer components to be installed on the new antenna. Commissioning with this instrumentation package will commence during late spring with observations resuming following the monsoons at the end of the summer. Later, the new 4 mm (Band 2) receiver is be installed, based on a prototype tested on the old 12 M before decommissioning. After upgrading the existing 2 mm receiver, a copy of the ARO 1.3 mm receiver will be constructed to support the expanding needs of the VLBI community.
Coherent Receivers
The goal of the ARO is to provide stable, high 385-500 GHz Receiver, with grid, IF = 5 GHz sensitivity, single sideband heterodyne receivers 2/25/14 utilizing SIS devices to its user community. Receivers with these characteristics permit the 350 detection and unambiguous identification of spectral 300 line emissions at levels limited only by the available 250
(K) V-pol. T LSB integration time. Observing programs requiring 200 rssb integration times as long as 100 hours are not T 150 V-pol. T USB uncommon. To maximize the system sensitivity, all 100 H-pol. T LSB ARO receivers detect both orthogonally polarized 50 H-pol. T USB signals. Very Long Baseline Interferometry (VLBI) 0 programs are also supported at ARO facilities. The 400 420 440 460 480 500 520 ARO telescopes now support receivers in all 6 LO Frequency (GHz) atmospheric windows from 4 mm to 0.4 mm; 4 - 3, 2, 1.3, 0.8, 0.7, and 0.4 mm. Figure 4. Noise temperatures of the 0.7 mm SBS dual polarization receiver. Three different SIS single-sideband dual polarization receivers operate on Kitt Peak. They cover the 2 atmospheric windows, from 65 GHz at the low end to 170 GHz at the high end. Even though all 3 receivers are single sideband, only the 3 mm receiver employs sideband separating (SBS) mixers that reject energy from the unwanted image band, thus improving the spectroscopic sensitivity. The 2 mm receiver employs waveguide tuning to filter out the unwanted sideband, The new 4 mm receiver will achieve SSB operation through sideband-separation at E-band, already demonstrated on the Band 2 prototype. At the SMT, the 1.3 mm receiver operates from 210 GHz to 283 GHz. The 0.8 mm and 0.4 mm atmospheric windows are cover by receivers using a pair of double sideband (DSB) mixers. The frequency ranges covered are 275 – 370 GHz and 620 - 720 GHz respectively These 3 SMT receivers use wire grids for polarization separation.
The most recent ARO receiver utilizes a new SIS material technology in its mixer design to cover the 0.4 mm atmospheric window. It operates from 385 GHz to 500 GHz. and uses 2 – unbalanced SBS mixers to receive the 2 orthogonally polarized signals. The devices were fabricated with an AlN insulating layer sandwiched between 2 - Nb superconducting films on a Si on insulator (SoI) substrate. The mixer circuit design was performed in collaboration with the Coordinated Development Laboratory (CDL) at the NRAO and the SIS mixer junctions were provided by the Microfabrication Laboratory at the University of Virginia. The ARO plans to replace the unbalanced LO mixer blocks with a balance circuit design. This change will reduce the LO power requirements and simultaneously improve the sensitivity by reducing the LO noise. A plot of recently measured receiver temperatures of the 4 individual receiver channels is shown in Figure 4. Currently, a Figure 5. The ARO 0.7 mm SBS dual wire grid is used to separate the 2 orthogonally polarized signals. A polarization receiver containing an OMT. waveguide orthomode transducer (OMT) is under development to replace the wire grid. A few developmental versions have already been fabricated and evaluated but, as yet, do not provide the required level of performance. A picture of the inside of the receiver’s Dewar with one of the early prototype OMT is shown in Figure 5. The availability of a high sensitivity SBS receiver in this relatively unexplored band promises to yield new and exciting discoveries.
All ARO sideband separating receivers employ two double sideband (DSB) SIS mixing elements installed in a sideband separating circuit as shown in Figure 6. Only 2 SMT receivers utilize the sideband separating mixer configuration, the 1.3 mm and 0.7 mm receivers, while the other receivers at 0.8 mm and 0.4 mm utilize the classic DSB mixer. Work has already begun to upgrade the 0.8 mm DSB mixer with a pair of sideband separating SIS mixers to detect orthogonal linearly polarized signals. This work is being performed in collaboration with the NRAO CDL and the UVA Microfabrication Laboratory that will again provide the SIS mixer elements. These junctions will Figure 6. Schematic diagram of a sideband use the classic SiO insulator sandwiched between two separating mixer configuration. layers of Nb film all placed on a quartz substrate.
The ARO has obtained complete 3 mm SBS mixer assemblies for the dual polarization receiver covering the 84 – 115 GHz range from the Herzberg Institute for Astrophysics (HIA), operated by the National Research Council of Canada. The assembled 1.3 mm SBS mixer blocks for a dual polarization receiver covering the frequency range of 210 – 275 GHz were obtained from NRAO’s CDL and the assembled 602 to 720 GHz DSB mixers were acquired from Space Research Organization of the Netherlands. These 3 mixer designs were originally developed for ALMA.
4. Future Plans
With the 0.7 mm receiver installed at the SMT as a facility instrument, the 0.8 mm receiver is the next existing receiver to be upgraded at the SMT. The new receiver will cover the same frequency range as the existing receiver and will contain 2 SBS mixers to support dual polarization observations. Its performance is expected to be better than the ALMA Band 7 but will be developed by ARO. As before, the junctions will be obtained from the University of Virginia’s Microfabrication Laboratory. A collaboration has been established with the NRAO CDL on some aspects of the mixer circuit and block design.
Early observations with the new ARO 12-m telescope will use the existing 3 mm SBS receiver with HIA mixers. We have also developed a prototype 4 mm (Band 2) receiver using both a microwave integrated circuit (MIC) and a monolithic microwave integrated circuit (MMIC) low noise amplifiers (LNA) obtained from the NRAO. The LNAs were coupled with a SBS mixer configuration using two Schottky barrier diode mixers. The system was tested at the old 12 m in conjunction with previous generation SIS mixers. The performance of the amplifiers was comparable to the SIS devices. The prototype will be modified for the new 12 m and will use the MIC amplifiers with the Schottky mixers - neither which require cooling to 4 K, simplifying the refrigeration requirements.
Because of the heterodyne nature of all ARO receivers and the careful attention given to stability and noise performance derived from a low phase noise local oscillator systems, ARO telescopes continue to be involved in millimeter wavelength VLBI programs. An upgrade program has been initiated at the SMT to simplify the instrumental conversion at the start of a VLBI program to reduce the setup time. The turnkey environment will enable quick activation of a VLBI observation. A second 1.3 mm receiver will be built using the same SIS SBS mixers as the current SMT receiver, to be installed at the new ARO 12-Meter telescope, adding further baseline capability. The SMT remains an integral node of a growing multi-organizational global millimeter VLBI network. With the addition of the ARO 12- Meter telescope, increasingly more accuracy measurements become possible.
Other future planes include increasing the bandwidth, frequency resolution and flexibility of the ARO spectrometer at both facilities. The demand for both higher resolution and wider bandwidth are growing, especially at the SMT. A 4 channel spectrometer utilizing high speed digital technology is planned.
5. Scientific Directions
The science at ARO focuses primarily on spectral-line observations, along with mm/sub-mm VLBI. The high sensitivity and stability of the ARO receivers and backends has allowed for deep integrations, which has enabled the detection of numerous weak lines. Among the scientific projects conducted at ARO which exploit the instrumental capabilities include detections of new interstellar molecules, such as FeCN, AlO (Figure 7), AlOH, and HSCN, and KCN [e.g. 1.2], studies of isotope ratios through molecular isotopologues (CN, 13CN, C15N, etc [e.g. 3]), and weak molecular emission from evolved planetary nebulae, such as the Ring, Dumbbell, and Helix Nebulae [4, 5]. ARO is continuing its goal of providing observers with all band coverage with the high sensitivity that heterodyne receivers provide, orthogonal polarization capability, simultaneous sideband availability, and 4 GHz of instantaneous bandwidth for each of the 4 IF channels. A program is planned to replace the DSB mixers in the existing 600 to 720 GHz receiver with sideband separating mixers, providing another band with sideband separation capability.
6. Conclusions
The ground-based studies of the millimeter and sub-millimeter sky are being advanced by the high sensitivity and high stability of the ARO suite of heterodyne SIS receivers and the flexible resolution of its spectrometers, as well as a new high performance 12 m antenna With the installation of the facility receiver that covers the 0.7 mm atmospheric window, ARO receivers now provide complete frequency coverage from 70 GHz to 720 GHz. Millimeter and sub-millimeter wavelength astronomy has a long and fertile future at the University of Arizona.
7. References
Figure 7. Discovery of a new interstellar 1. E. D. Tenenbaum. and L. M. Ziurys,, “Millimeter Detection of AlO molecule, AlO, made using the ARO (X2Σ+): Metal Oxide Chemistry in the Envelope in VY Canis Majoris,” facilities ( from Tenenbaum & Ziurys 2009 Astrophysical Journal Letters, 694, 2009, pp. L59-L63.
2. L.N. Zack, D.T. Halfen, and L.M. Ziurys, “Iron-Containing Molecules in Circumstellar Envelopes: Detection of FeCN 4 (X Δi) in IRC+10216,” Astrophysical Journal (Letters), 733, 2011, pp. L36-L40.
3. G.R Adande and L.M. Ziurys, “Millimeter-Wave Observations of CN and HNC and their 15N Isotopologues: A New Evaluation of the 14N/15N Ratio Across the Galaxy,” Astrophysical Journal, 744, 2012, pp.194 – 209.
4. J.L. Edwards and L.M. Ziurys, “The Remarkable Molecular Content of the Red Spider Nebula (NGC 6537),” Astrophysical Journal (Letters), 770, 2013, pp. L5-L9.
+ 5. L.N. Zack and L.M. Ziurys “Chemical Complexity in the Helix Nebula: Multi-Line Observations of H2CO, HCO , and CO,” Astrophysical Journal, 765, 2013, pp. 112-126.