J. Richard Fisher

Cosmicflows-2: The Data

Cosmicflows-2 is a compilation of distances and peculiar velocities for over 8000 galaxies. Numerically the largest contributions come from the luminosity-line width correlation for spirals, the Tully-Fisher relation (TFR), and the related fundamental plane relation for E/S0 systems, but over 1000 distances are contributed by methods that provide more accurate individual distances: Cepheid, tip of the red giant branch (TRGB), surface brightness fluctuation, Type Ia supernova, and several miscellaneous but accurate procedures. Our collaboration is making important contributions to two of these inputs: TRGB and TFR. A large body of new distance material is presented. In addition, an effort is made to ensure that all the contributions, both our own and those from the literature, are on the same scale. Overall, the distances are found to be compatible with a Hubble constant H 0 = 74.4 ? 3.0?km?s?1?Mpc?1. The great interest going forward with this data set will be with velocity field studies. Cosmicflows-2 is characterized by a great density and high accuracy of distance measures locally, falling to sparse and coarse sampling extending to z = 0.1.

THE EXTRAGALACTIC DISTANCE DATABASE: ALL DIGITAL H I PROFILE CATALOG

An important component of the Extragalactic Distance Database is a group of catalogs related to the measurement of H I line profile parameters. One of these is the All Digital H I catalog which contains an amalgam of information from new data and old. The new data result from observations with Arecibo and Parkes Telescopes and with the Green Bank Telescope, including continuing input since the award of the NRAO Cosmic Flows Large Program. The old data have been collected from archives, wherever available, particularly the Cornell University Digital H I Archive, the Nancay Telescope extragalactic H I archive, and the Australia Telescope H I archive. The catalog currently contains information on {approx}15, 000 profiles relating to {approx}13, 000 galaxies. The channel-flux per channel files, from whatever source, is carried through a common pipeline. The derived parameter of greatest interest is W {sub m50}, the profile width at 50% of the mean flux. After appropriate adjustment, the parameter W{sub mx} is derived, the line width that statistically approximates the peak-to-peak maximum rotation velocity before correction for inclination, 2V {sub max}sini.

Full-sampling array feeds for radio telescopes

Phased array feeds offer the possibility of more efficient use of large radio astronomy reflector antennas by providing more closely spaced beams over a wide field of view and higher aperture efficiency in each beam than have been realized with horn feeds. This paper examines the array design constraints imposed by complete sampling of the fields near the reflector focus. In particular, array element spacing must be less than 1 (lambda) for large F/D reflectors and less than about 0.7 (lambda) for F/D < 0.5. This rules out conventional horns as array elements and sets a limit on the array bandwidth. The receive-only case of radio astronomy permits the use of number of signal combining techniques that do not degrade system sensitivity. Because practical arrays are of finite extent, and unwanted noise from the antenna surroundings is largely coherent between the elements, neither field conjugate nor maximal-ratio diversity methods of array weight optimization can be used. A modified form of field matching is a good starting approximation, however. Correction of reflector errors is examined briefly.

Beamforming and imaging with the BYU/NRAO L-band 19-element phased array feed

An experimental 19-element L-band phased array feed was installed on the Green Bank 20-Meter Telescope in October 2007 and July 2008 to measure sensitivity and effciency and demonstrate signal processing algorithms for array calibration, multiple beam formation, imaging, and adaptive spatial filtering methods for interference mitigation. System noise performance was characterized using a warm absorber/cold sky Y-factor setup. The peak beam aperture effciency was 69% and the minimum beam equivalent system temperature was 66K. With a single reflector pointing, a high sensitivity image of a field of view approximately two half-power beamwidths in diameter can be produced. Measured figures of merit compare well to numerical simulations, indicating that complicating effects such as mutual coupling are understood well enough to enable the next phase of array feed development to proceed on firm grounds.

Bayesian detection of radar interference in radio astronomy

L-Band observations at the Green Bank Telescope (GBT) and other radio observatories are often made in frequency bands allocated to aviation pulsed radar transmissions. It is possible to mitigate radar contamination of the astronomical signal by time blanking data containing these pulses. However, even when strong direct path pulses and nearby fixed clutter echoes are removed there are still undetected weaker aircraft echoes present which can corrupt the data. In a previous paper we presented an algorithm to improve real-time echo blanking by forming a Kalman filter tracker to follow the path of a sequence of echoes observed on successive radar antenna sweeps. The tracker builds a history which can be used to predict the location of upcoming echoes. We now present details of a new Bayesian detection algorithm which uses this prediction information to enable more sensitive weak pulse acquisition. The developed track information is used to form a spatial prior probability distribution for the presence of the next echoes. Regions with higher probability are processed with a lower detection threshold to pull out low level pulses without increasing the overall probability of false alarm detection. The ultimate result is more complete removal, by blanking the detected pulse, of radar corruption in astronomical observations.

Phased Array Feed Calibration, Beamforming and Imaging

Phased array feeds (PAFs) for reflector antennas offer the potential for increased reflector field of view and faster survey speeds. To address some of the development challenges that remain for scientifically useful PAFs, including calibration and beamforming algorithms, sensitivity optimization, and demonstration of wide field of view imaging, we report experimental results from a 19 element room temperature L-band PAF mounted on the Green Bank 20 Meter Telescope. Formed beams achieved an aperture efficiency of 69% and a system noise temperature of 66 K. Radio camera images of several sky regions are presented. We investigate the noise performance and sensitivity of the system as a function of elevation angle with statistically optimal beamforming and demonstrate cancelation of radio frequency interference sources with adaptive spatial filtering.

Combating Pulsed Radar Interference in Radio Astronomy

Only a small fraction of the radio spectrum can be reserved exclusively for passive scientific measurements, so radio astronomers must expect to observe some natural radiation at frequencies allocated to an increasing number of active users of the spectrum. This paper presents a scheme for removing pulsed transmissions of air surveillance radar from astronomical data based on the temporal and amplitude characteristics of the radar signals. Two filter schemes for maximizing the separation of the radar signals from normally distributed noise are compared on the same real data. The goal is to remove as little of the useful data as possible. Removal of both strong- and weak-signal interference to the level of detectability in a 5 s integration and a scheme for compensating for the frequency dependence of the excision algorithm are demonstrated.

Phased array antenna design and characterization for next-generation radio telescopes

Instrumentation for radio astronomical observations is currently undergoing a transition from large reflectors with single-pixel feeds to multi-pixel phased arrays. Research efforts aimed at realizing the benefits of phased arrays for highly sensitive, calibrated astronomical measurements have opened up interesting challenges for antenna designers. This paper surveys recent theoretical and experimental results on characterization of beam equivalent noise temperature and efficiency, impedance matching to minimize front end noise, and beamforming and calibration algorithms for phased array feeds.

Excision of Distance Measuring Equipment Interference from Radio Astronomy Signals

Distance measuring equipment (DME) signals transmitted by aircraft in flight present a strong source of interference in radio astronomy bands around 1 GHz. In this paper, we present a method for greatly suppressing this interference without significant loss of radio astronomy data. The method exploits the known structure of the DME waveform to perform a time/frequency domain blanking on the aggregate signal. Results are presented that illustrate the effectiveness of the procedure. Efficient implementation of the blanking algorithm is also considered.

Development of cryogenic phased array feeds for Radio Astronomy antennas

The National Radio Astronomy Observatory and Brigham Young University are jointly working to develop phased array feed systems for use on radio astronomy reflector antennas. This paper describes recent progress in development of a cryogenically cooled L-band PAF receiver. Noise test results for dipole elements and cryogenic low-noise amplifier assemblies are presented. The results indicate it will be possible to achieve PAF system noise temperature competitive with the best single-beam radio astronomy receivers.