An aperture-synthesis interferometer at Ooty for operation at 327 MHz

A 3.5-km long interferometer consisting of the 530 by 30 m Ooty radio telescope, two 13.5-m diameter dishes, and a 5-m diameter dish are described. For generating coherent local-oscillator signals for the interferometer, a simple phase-stable radio link has been developed which uses the sum-frequency method proposed for the VLA antenna. The interferometer, expected to be in operation in 1973, will provide a resolution of about 0'.8×5'.5 arc at 327 MHz and will be able to detect a radio source of flux density 2 × 10^-28. Wċm^-2Hz^-1with a S/N ratio of 5 to 1. A proposed 10-km long synthesis interferometer consisting of the Ooty telescope and 36 dishes of 5-m diameter is also described. At 327 MHz, this instrument will provide beamwidths of 15" by 3" arc and 15" by 20" arc at declinations of 0° and 30°, respectively. It will have a sensitivity of about 10^-28Wċm^-2Hz^-1for a 9.5-h observing period.     

The Table Mountain 8-mm wavelength interferometer

A two-element radio interferometer operating at 8.33- mm wavelength has been developed at the Jet Propulsion Laboratory's Table Mountain Observatory near Wrightwood, CA. The interferometer employs a 5.5-m and a 3-m diameter antenna on an east-west baseline of 60 or 120 m, yielding fringe spacings at transit of 28" or 14", respectively. The broad intermediate-frequency bandpass of 100- 350 MHz and the system noise temperature of 500 K provide high sensitivity for the measurement of continuum sources. The interferometer has been used for high-resolution studies of the planets and the Sun, and it is currently being adapted to study solar flare emissions at high spatial and time resolution.     

A new southern hemisphere synthesis radio telescope

An earth-rotational synthesis radio telescope is now operating at a wavelength of 21 cm at Fleurs, near Sydney, Australia. The telescope produces a map of a 1.5 square-degree region of sky in an observing time of 12 to 16 h. In the first stage the resolution is 40" and the sensitivity (5 times rms noise fluctuations) is about 70 × 10^-29. A full range of scanning angles is available for mapping sources between declinations 80°S and 30°S, but mapping of a lower quality is possible at lower declinations. The telescope consists of two independent arrays used consecutively. One is in an east-west line and the other is in a north-south line. Each consists of two 13.8-m-diam paraboloids and thirty-two 5.8-m paraboloids arranged as a compound grating interferometer. The paraboloids track a source for 8 h and the phases of signals from each paraboloid are rotated continuously to compensate for the movement of the source across the sky. 64 cosine and 64 sine Fourier components of the brightness distribution over the source are extracted simultaneously and each is recorded every minute. After an observation of several hours, a digital computer makes the necessary transformations and produces a map.     

Interferometric measurement at 1415 MHz of radiation pattern of paraboloidal antenna at Dwingeloo radio observatory

The complete radiation pattern of a 25-m diameter radio telescope has been measured at 21-cm wavelength to a level of 60 dB below the main beam response. Strong cosmic radio sources with known flux density were used as a signal source. The measurement employed an interferometer consisting of the radio telescope and a 7.5-m diameter reference antenna. By changing the orientation of the telescope, while keeping the reference antenna pointed at the source, the output voltage of the interferometer varies proportional to the field strength pattern of the telescope. This method provides a large improvement in sensitivity over a total power measurement. The paper discusses the theoretical basis of the technique. It describes the layout of the interferometer, the electronic equipment, and the methods of observation and reduction of the data. Some 19 000 points in the pattern were measured. The pattern is displayed by contour plots of equal level. The derived main characteristics of the telescope [1968 AD] at 1415 MHz are as follow: gain49.0 pm 0.2dB, absorption area282 pm 8m², aperture efficiency 0.56, half-power beamwidth0.57 times 0.62deg. The accuracy and detail allow astronomers to reliably correct radio astronomical observations for the effect of radiation in the sidelobes, spillover region, and feed support scatter lobes. Theoretical calculations of pattern parameters may be checked quantitatively.     

The 100-meter radio telescope at Effelsberg

The 100-m fully steerable radio telescope of the Max-Planck-Institut für Radioastronomie (MPIfR) has now been in service for over a year. The telescope was designed to allow observations in the decimeter and centimeter wavelength regions, with the vastly improved resolution given by the 100-m diameter. During construction of the telescope measurements were made at a wavelength of 75 cm. Since the completion of the adjustments, tests and observations were made at 11-cm and 2.8-cm wavelengths. In particular, at the 2.8-cm wavelength, tests have fully confirmed the design goals. In fact, we hope to extend the operational range of the telescope at least to the wavelength of 1.2 cm. A number of problems, new in this field, had to be solved in the construction of an instrument of this size and precision. Measuring methods are described which allowed adjustment of the surface to ±1 mm. A computer-controlled servo loop allows setting of the telescope to 5". Experience of operation under various conditions is described.     

A 45-m telescope with a surface accuracy of 65 μm

The Nobeyama 45-m telescope has been upgraded for higher frequency and higher sensitivity observations. The surface accuracy of the antenna was improved from 0.2 mm rms to 65 μm rms using a radio holography method at the prime focus. Pointing accuracy was also improved by replacing a large Gregorian subreflector cabin with a smaller cassegrain subreflector in 1985 to reduce wind loading effects, and by installation of a new master collimator with multi-pole resolvers which were calibrated to an accuracy of 0.4 arcsec rms in 1988. Four SIS receivers using Nb/AlO_x/Nb array junctions are now available on the telescope for 40, 80, 100, and 150-GHz bands. These receivers achieved quite low noise and wide-band tunability. The 2×2 multi-beam receiver for 115 GHz is very powerful for mapping observations. These improvements of telescope performance and these receivers have significantly increased the astronomical observation capability of the telescope in the wide frequency range of 40 to 150 GHz     

Coma correction of a wobbling subreflector

Radio astronomical observations at millimeter wavelengths with Cassegrain telescopes often use on-source/off-source measurements with a wobbling subreflector to eliminate the influence of the Earth's atmosphere. The tilt of the subreflector produces the required beam throw, but also a coma-like wavefront deformation noticeable as a degradation of the beam pattern and a reduction of the main beam power. We demonstrate, with measurements made with the IRAM 30-m telescope at 1.3 mm wavelength (230 GHz) of the quasi-point-like source Mars and the extended source Saturn, that it is possible to eliminate (to a large extent) this wavefront deformation when using a near-focus corrector, on the 30-m telescope conveniently located on the co-rotating Nasmyth mirror     

Hologram-based compact range for submillimeter-wave antenna testing

A hologram-based compact antenna test range (CATR) is being developed to overcome challenges met in antenna testing at submillimeter wavelengths. For the first time, this type of CATR has been built for testing of a large reflector antenna at submillimeter wavelengths. The CATR is based on a 3-m computer-generated hologram as the focusing element. This paper discusses the design and the construction of the CATR, and the verification of the CATR operation with quiet-zone tests done for the CATR prior to the antenna testing. Assembly of the CATR, testing of the 1.5-m reflector antenna at 322 GHz, and the disassembly were all done within two months in 2003. The quiet-zone field measurement results are analyzed in this paper. The CATR was concluded to be qualified for antenna testing. The antenna testing is described in a separate paper.     

Design parameters and measured performance of the IRAM 30-mMillimeter Radio Telescope

The “Millimeter Radio Telescope” (MRT) is operated by the Institute for Radio Astronomy in the millimeter range (IRAM) and is located at 2850-m altitude in the Sierra Nevada, near Granada, Spain. It is a reflector antenna of 30-m diameter with a surface accuracy of 0.08 mm and a pointing accuracy of better than 2 arcsec. The telescope is equipped with sensitive receivers for the atmospheric windows between 0.8- and 7-mm wavelength. The authors describe the optics layout of the receiver and calibration system, which allows simultaneous observations at a number of frequencies. The special design aspects of the antenna, in particular the control of thermal deformations and the achievement of a high reflector and pointing accuracy are described. The authors compare the design computations with the characteristics of the telescope, derived from several years of operation and optimization. The success of the design is demonstrated by observational experience. The authors conclude the paper with a short review of some of the astronomical results obtained with the telescope     

Astigmatism in reflector antennas: measurement and correction

Investigates astigmatism is Cassegrain antennas from cross scans through in-and-out-of-focus diffraction images of extended radio sources. The use of extended sources is of interest for large (mm-wavelength) telescopes where the bright planets subtend a substantial fraction of the beam. The experimental and theoretical results presented refer to the IRAM 30-m telescope and measurements at 7.3 mm-1.3 mm wavelength; however, extrapolation to other telescopes is evident. The authors report on an experiment to correct the astigmatism (phase) at a position close to the focal plane of the telescope, i.e. the flat surface of the corotating Nasmyth mirror     

Upgrading the Haystack radio telescope for operation at 115 GHz

The Haystack 37-m radio telescope has been upgraded for operation at frequencies up to 115 GHz. A unique deformable subreflector with active actuator control has been developed to correct for gravitational distortion effects including astigmatism and deflections associated with the particular reflector surface. Active thermal compensation of the surface has also been implemented to conduct for both thermal lag effects and circularly symmetric gravitational deviations. Holographic mapping of the antenna surface deviations was achieved using 12-GHz geostationary satellite transmissions, which required the use of special techniques to correct for diffraction and multiple reflection effects involving the space-frame radome that covers the antenna. Realignment of the antenna surface utilized a finite-element structural model to translate surface deviations to motions of the unusual adjustment mechanisms on the antenna. The currently measured rms surface deviation (Dec. 23, 1992) is 210 μm. The telescope has been equipped with a two-channel cryogenically cooled 3-mm SIS receiver, covering the range from 84 to 115 GHz. A new flexible digital spectrometer has been constructed for spectral line astronomy. Configurations can range from a widest bandwidth coverage of 160 MHz at 512 lags to 0.66 MHz at 4096 lags. Examples are given of surface holographic maps and radio measurements of aperture efficiency, pointing, and other performance parameters     

空间碎片探测与测距复合系统光学望远镜

针对空间碎片探测与测距复合系统地面验证演示实验中,工作环境10~30 ℃、光学基台的尺寸限制（不超过450 mm×400 mm）以及光学望远镜尾部安装导致重心远离安装面的问题,提出了空间碎片探测与测距复合系统光学望远镜的设计。使用ANSYS有限元分析软件对光学望远镜建立了有限元模型,针对环境温度10~30 ℃、尾部安装状态下、光轴方向和垂直光轴方向1 g （g=9.8 m/s2）重力加速度工况下进行了分析。分析结果表明:光学望远镜整机一阶模态为133 Hz动态刚度较好,重力为光轴方向时主次镜间距最大变化量0.01 mm,重力为垂直光轴方向时主次镜间距最大为0.007 mm,光学望远镜系统波像差RMS值为λ/15,次镜最大倾角1.93″,具有较好的力、热稳定性,可以满足光学天线装校、检测以及外场实验验证过程中的指标要求。在光学望远镜装校完成后,使用ZYGO干涉仪对其像质进行检测,在重力垂直于光轴方向、环境温度10、20、30 ℃条件下进行检测,结果显示:系统波像差RMS值分别为0.097λ、0.075λ及0.1λ,整机光学望远镜系统波像差RMS值在最低温与最高温度时均优于λ/10均满足系统使用要求。     

Adaptive optics and Keck Observatory

Since Galileo used his first telescope in 1609, ground-based astronomy has been severely limited by the image blurring resulting from turbulence in the Earth's atmosphere. Adaptive optics (AO) was first proposed in the 1950s (Babcock, 1953) as a means of correcting this image blurring. Unfortunately, existing technology was not up to the task, and AO was not demonstrated on an astronomical telescope until the early 1990s. Twin 10-m diameter Keck telescopes were the first of the generation of large 8-10 m telescopes to be completed in the 1990s, and Keck Observatory has participated at the forefront of the application of AO to astronomy on these large telescopes. The Keck II NGS AO system saw first light in 1999 (Wizinowich et al., 2000 ), and the Keck interferometer, which requires AO-corrected wavefronts from both telescopes/ saw first light in early 2001. Another first for a large telescope was the first light of the Keck II laser guide star (LGS) AO system in 2003. AO systems similar to those at Keck are in use or under development at other observatories around the world. This article discusses the Keck AO facilities, some of the technical challenges for AO in astronomy, and the new era this technology has introduced for astronomy.     

Supercontinuum generation in non-silica fibers

The development of super continuum sources is driven by the requirements of a wide range of emerging applications. This paper points out how non-silica fibers are of benefit not only because their broad mid-IR transparency enables continuum generation in the 2–5 μm region but also since the high intrinsic nonlinearity of the glasses reduces the power-threshold for devices at wavelengths below 2 μm. For these glasses, the material zero-dispersion wavelength is typically shifted to long wavelengths compared to silica so dispersion tailoring is key to creating sources based on practical, near-IR, solid state pump lasers. We show how modeling work has produced fiber designs that provide flattened dispersion profiles with high nonlinearity coefficients and zero-dispersion wavelengths in the near-IR. Building on this flexibility, modeling of the pulse dynamics then demonstrates how coherent mid-IR supercontinuum sources could be developed. We also show the importance of the second zero-dispersion wavelength using bismuth fibers as an example. Nonlinear mode-coupling is shown to be a factor in larger core fibers for high-power applications. Demonstrations of supercontinuum in microstructured tellurite fibers, all-solid lead–silicate (SF57) fibers and in bismuth fibers and tapers are then reported to show what has been achieved experimentally using a range of materials and fiber geometries.     

The Allen Telescope Array: The First Widefield, Panchromatic, Snapshot Radio Camera for Radio Astronomy and SETI

The first 42 elements of the Allen Telescope Array (ATA-42) are beginning to deliver data at the Hat Creek Radio Observatory in northern California. Scientists and engineers are actively exploiting all of the flexibility designed into this innovative instrument for simultaneously conducting surveys of the astrophysical sky and conducting searches for distant technological civilizations. This paper summarizes the design elements of the ATA, the cost savings made possible by the use of commercial off-the-shelf components, and the cost/performance tradeoffs that eventually enabled this first snapshot radio camera. The fundamental scientific program of this new telescope is varied and exciting; some of the first astronomical results will be discussed.      

Design and performance analysis of adaptive optical telescopesusing lasing guide stars

The use of adaptive optical systems using electrically deformable mirrors to compensate for turbulence effects is discussed. Since these systems require bright reference sources adjacent to the object of interest and can be used only to observe the brightest stars, artificial guide stars suitable for controlling an adaptive imaging system must be created in the upper atmosphere by using a laser to excite either Rayleigh backscattering in the stratosphere or resonance backscattering in the mesospheric sodium layer. The design requirements of a laser-guided adaptive telescope, as well as the expected imaging performance, are discussed in detail. It is shown that a 2-m ground-based laser-guided telescope can achieve imaging performance levels at visible wavelengths nearly matching the theoretical imaging performance of the Hubble Space Telescope (HST). The required lasers can be either bought off the shelf or built with current technology. The laser power requirement for the Rayleigh guide star approach is on the order of 82 W for zenith viewing when the atmospheric seeing cell diameter is 20 cm. For the same conditions the laser power requirement for the Na guide star approach is on the order of 14 W. Both systems will achieve near diffraction limited imaging with a Strehl ratio of ~0.67 and an angular resolution of approximately 0.07 arcsec for an observation wavelength of 0.5 μm     

Low-power all-optical broad-band switching device using ytterbium-doped fiber

All-optical photonic switching has been demonstrated at 1310 nm and 1545 nm using an all-fiber Mach-Zehnder interferometer incorporating ytterbium-doped silica fiber. The switching was achieved with 980 nm excitation from a laser diode, with a power-length product of 3.7 mW/spl middot/m and 5.8 mW/spl middot/m at 1310 nm and 1545 mm respectively, thus giving low power all-optical switching at the preferred telecommunications wavelengths.     

A modern giant Yagi

A giant HF divided-boom Yagi antenna designed, constructed, and demonstrated by radio amateurs is described. The antenna, designed using the method-of-moments MININEC wire code, consists of 13 widely-spaced elements disposed on six towers, in a fixed-azimuth application. Overall antenna dimensions are 10 m by 100 m by 25 m above the ground: a structure roughly 0.5×5.0×1.0 wavelengths in the WARC-allocated 20-m amateur band. The elements are arranged for a considered balance among forward gain, sidelobe level, impedance level, bandwidth, and structural wind survival and construction economies, with an EM emphasis on forward gain. The antenna operates at a center frequency of 14.150 MHz (λ=21.15 m). The operating principle of the antenna is reviewed, and some of the errors to which it is prone are examined