## BIMA Memoranda Series - Abstracts

101. , M.C.H. Wright
available as:
PDF, gzipped PostScript

We simulated imaging for the 23-antenna CARMA telescope in three compact configurations using a model image of Saturn. Simulated uv data sampled by the heterogeneous array of 10.4, 6.1, and 3.5 m antennas were used to make mosaic images. Three different Maximum Entropy deconvolutions were used. The best image fidelity was obtained using a joint deconvolution of the interferometer and single dish data. Using the single dish data as a default image, provides a total flux estimate and low spatial frequencies unsampled by the interferometric mosaic. Both methods give higher image fidelity than just using the interferometer data with a total flux estimate. We analyze two problems: i) sampling the short spatial frequencies, ii) Nyquist sampling the large scale structure, and show that the image fidelity is improved by a higher uv sampling density. Using the interferometer spacings between 3.5 and 6.1 m sampled by the 3.5 m antennas further improves the image fidelity because the interferometer observations provide better quality data for these short spatial frequencies, than can be obtained from the joint deconvolution with the 10.4 m single dish data.

100. Polarimetry with the BIMA Interferometer: Observations to Data Reduction , Brenda Matthews
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This memo covers the basics of BIMA polarimetry observing, including writing polarimetry proposals, templates of observing scripts (to include both continuum and line polarimetry together), and then data reduction using MIRIAD. Some knowledge of interferometry and MIRIAD is assumed.

99. Stability of BIMA Antenna Positions , J. R. Forster
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This memo discusses temporal changes of up to 3mm in the positions of BIMA antennas observed at HCRO. The motions occur on timescales of days to months, and appear to be related to seasonal temperature changes. Position changes observed over a three month period in the spring of 2001 are shown for illustration, and possible sources of the instability are discussed. Ground stability at the 0.1mm level will be required at Cedar Flat in order to avoid regular monitoring and correction of initial pointing and baseline solutions for CARMA.

98. J-Miriad: Java Wrappers for MIRIAD Methods , G.R. Harp, M.C.H. Wright,
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A new set of Java wrappers for the MIRIAD input-output routines provides access to MIRIAD data from Java and Jython (Jython is a dialect of Python). Originally developed as part of the correlator for the Allen Telescope Array, these wrappers are general purpose and are available for application in other software projects where Java or Python is used. In this article we describe the architecture of these wrappers and demonstrate their use with some examples.

97. CARMA Spectral Line Correlator Requirements , M.C.H. Wright,
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We review the expected sensitivity and spatial dynamic range of the CARMA array during the next 10 years in comparison with the ALMA, ACA, SMA and IRAM arrays. The scientific capabilities of the CARMA array can be greatly enhanced at modest cost by adding a full polarization spectral line correlator.

96. Evaluation of the GILDAS Package for ALMA Off-line Data Processing , F. Gueth, S. Guilloteau, R. Lucas and J. Pety
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This document presents the results of an evaluation of the GILDAS software Package for the purposes of ascertaining its compliance with the ALMA Offline Data Processing Requirements given in ALMA-SW Memo 18. This provides a useful index and description of the data processing currently used for millimeter wavelength arrays.

95. Evaluation of the MIRIAD Package for ALMA Off-line Data Processing , M.C.H. Wright and Peter Teuben
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This document presents the results of an evaluation of the MIRIAD software Package for the purposes of ascertaining its compliance with the ALMA Offline Data Processing Requirements given in ALMA-SW Memo 18. This provides a useful index and description of the data processing currently used for millimeter wavelength arrays.

94. A CANbus Replacement for the BIMA Antenna Telemetry , Alberto D. Bolatto
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This memo briefly describes the existing implementation of the BIMA telemetry and discusses in detail a conversion to CANbus, necessary for the merging of BIMA into CARMA. This implementation (by no means final) uses a VME board equiped with a microcontroller Phillips XAC3 to replace the current telemetry receiver, keeping the rest of the hardware intact. I also discuss the possible structure of the embedded software, as well as the limitations impossed by the hardware and the data rates on the field bus.

93. Characterizing BIMA Proposals 2000-2003 , Robert Gruendl and M.C.H. Wright, April 2003
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In this memo, we characterize BIMA proposals for the period 2000-2003 into 8 categories: Science, Antenna Configuration, Help Requested, Special Needs, Observation Type, Polarization, Receiver Band, and Mosaicing. We present the data as tables and pie charts for each category. Most, ~90%, of the proposals requested (and observed) spectral line data. Most of the proposals requested antenna configurations, receiver bands, and observing modes in the regime where the BIMA telescope works well without and special requirements for instrument or weather.

92. Allen Telescope Array Imaging , M.C.H. Wright, Nov 2002
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In this memo, we review the imaging requirements, calibration procedures, and processing options for the Allen Telescope Array. The array has excellent uv coverage and a very high data rate which will rapidly saturate conventional human data reduction and imaging. The array should produce final, calibrated images as its normal output. Imaging with the ATA was simulated for a range of sources. The performance of MIRIAD running on modern PC's appears adequate for the ATA, but not by a large factor. Efficient software is required.

91. Compact Configuration Evaluation for CARMA , M.C.H. Wright, Sep 2002
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In this memo, we compare the imaging performance of three compact configurations for the CARMA telescope. We discuss ring and spiral configurations. MIRIAD scripts simulate imaging with heterogeneous arrays, and pipeline the results into tables. We present tables of beam FWHM, brightness sensitivity and sidelobe levels for source declinations from +60 to --40 degrees. Mosaic images for source diameters $32''$, and $40''$ are analyzed. The residual imaging errors are characterized by the recovered flux, peak flux density and the sidelobe levels. We present tables of the sidelobe levels and image fidelity. The most compact configurations provide the best brightness sensitivity, but suffer from antenna shadowing. Spiral configurations are more versatile and give better imaging fidelity for the source models considered.

90. Antenna Position Calibration , M.C.H. Wright, 17 May 2002
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This memo summarizes the antenna position calibration used at Hat Creek, and discusses its application to the CARMA and ALMA telescopes. Observations of quasars over wide range of HA and DEC provide data from which the antenna positions are determined. If the antenna positions are in error by many wavelengths, 2pi phase ambiguities may make this process difficult, especially when the atmospheric phase coherence is poor on long baselines. In this case the antenna positions can be first determined from the phase difference between the sidebands in a double sideband system, or across a wideband single sideband. In addition to the antenna positions, there may be axis offsets on the antennas which can be fitted from the quasar data or measured mechanically. These offsets should be stable and not change when the antennas are moved.

89. Polarization Stability of the BIMA Array at 1.3mm , G. Bower, M.C.H. Wright, J.R. Forster   17 May 2002
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We summarize measurements of linear polarization leakage terms made with the BIMA array at 1.3 mm over a 2 year period beginning in 2000. The individual antenna leakage terms show a median rms of 1.5% over this period. If these errors are stochastic, then the error in measured polarization is 0.5%. There is no apparent change in the leakage terms with time. The occurrence of the occasional bad calibration solution argues for the use of a mean set of leakage terms rather than frequent recalibration. There is no apparent reason to calibrate more frequently than once per array. The polarization fraction and position angle measured for 3C 279 agrees with that found at 86 GHz with BIMA and with that found at 22 and 43 GHz with the VLA.

88. BIMA Optical Pointing Project I. The STV Video Camera , J. Swift   26 March 2002
available as:
PostScript and PDF

This memo presents the first steps toward a functional optical pointing system at BIMA. The specifications of the STV video camera mounted on the optical pointing telescopes of the Hat Creek interferometer are shown. The sensitivity of the system has been empirically determined (zeropoint = 15.9 mag) and simulations have shown that a signal to noise of approximately 15 is needed to determine a centroid accurate to a 1" with 95% confidence. This means that a pointing solution can be obtained in a random field (stellar density averaged over the celestial sphere) with an integration time of a few seconds.

87. Tests of a planar L-band orthomode transducer in circular waveguide , G. Engargiola and R.L. Plambeck,  26 February 2002
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This note describes tests made in March 2000 of a planar L-band orthomode transducer. The transducer consists of 4 probes symmetrically arranged in a circular waveguide and two wideband baluns, made from back-to-back slotline-to-microstrip transitions. We designed this device to model the performance of a millimeter wave transducer where probes and baluns are microfabricated with SiON thin-film dielectric deposited on a quartz substrate. In the L-band model, probes and baluns are printed on 0.010 3thick Duroid 6006, mounted on a 0.25 3 thick piece fiberglass and placed flush against the waveguide. Each pair of opposite probes is coupled through a balun to a 50 ohm microstrip output. Measurements show that, from 1.2 1.9 GHz, the orthomode transducer has an output return loss less than 18 dB and crosspolarization less than 35 dB. It should be possible to scale this design to cover the 80-115 GHz or 210-270 GHz bands.

86. Preliminary Thoughts for Compact and Intermediate Configurations for CARMA , Tamara T. Helfer, Jan 2002
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Here are some very preliminary thoughts on the design of compact and intermediate configurations for CARMA. I propose that we should design 15- and 23-element configurations for D array, and 15-element configurations for the larger arrays. The most compact configuration can reasonably expect to be $\ga$50m in diameter, in accord with Lee's previous assumptions. I show a sample, non-optimized 23-element configuration that utilizes all minimum separations available among the six different pairs of primary elements. The uniformly weighted beams for this configuration are $\sim$11\arcsec. Given the 0.3--0.4\arcsec\ beams for A array, this suggests that we should aim for intermediate configurations that yield uniformly weighted beams of $\sim$3.5\arcsec\ and $\sim$1.1\arcsec\ if we design four main configurations. I suggest that we do design four (rather than five) configurations, and try to share 3--5 pads among adjacent arrays where possible. Using resources that we save by sharing pads (and by eliminating a configuration), I also suggest that we design hybrid DC, CB, and BA configurations that utilize e.g. 10-12 fixed stations from each of the D, C, B, and A arrays, for observing low-declination sources. F. Boone is currently modifying his configuration optimization code to allow for heterogeneous arrays; this may be a very useful tool for CARMA array design.

85. CARMA Calibration Requirements , M.C.H. Wright, Jan 2002
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84. CARMA Sensitivity and Correlator Requirements , M.C.H. Wright, Jan 2002
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83. WASP and APHID Microcontroller Command Interface, A. I. Harris, Nov 27, 2000
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The WASP and APHID spectrometers contain Intel 80C251SB microcontrollers to manage the spectrometer's real-time and other hardware tasks. The microcontroller accumulates data from the spectrometer hardware, sends signals to the telescope's chopping secondary and beamswitch (nodding'') inputs, and sets the spectrometer's instrument state. It communicates with a host computer or terminal using simple commands and a serial interface. This memorandum describes the microcontroller commands.

82. The Overlap of the Astronomical and WVR Beams , A. G. Gibb, A. I. Harris, Oct 24, 2000
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The variation of the overlap of the astronomical and water-vapor radiometer beams with height in the atmosphere is investigated for the current BIMA setup, and is compared with similar calculations for OVRO and a proposed system for ALMA. The findings are that at BIMA, the 3-mm and 1.3-cm beams essentially do not overlap above a height of ~1 km. The linear separation of the beams is ~35 m at a zenith height of 4 km. The dominant factor in determining the beam overlap is the angular offset between the beams, with the overlap decreasing very rapidly with increasing offset. Under conditions where the dominant source of water-vapor fluctuations has a scale greater than the beam separation, the small beam overlap will generally not pose a serious problem to correcting the phase.

However, we do find that the current 28-arcmin beam offset at BIMA potentially places a limit on the measurement of path length differences achievable with the APHID water vapor radiometer, although this is dependent on the height of the dominant source of fluctuations in the water vapor content. For a number of reasons, the beam separation will not be a problem for an instrument which has recently been proposed for ALMA. Finally, we also suggest that, if possible, future instruments should have beams which are offset in azimuth to minimize the error introduced by looking through atmospheric lines of sight of unequal length.

81. MIRIAD Benchmarks. I. Linux-Pentium and Solaris-Sparcs , P. J. Teuben, August 1, 2000
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We introduce the MirStone, a dimensionless measure of how well a standard set of MIRIAD programs perform on a given machine. It is simply computed from the elapsed time a benchmark script takes on an otherwise idle machine. It is normalized to be roughly unity on a typical late 20th century desktop (Ultra-300 or Pentium-400). A MirStone is defined as 5 divided by the elapsed time (in minutes) the benchmark takes to run. This paper introduces the benchmark, and compares Solaris and Linux configurations in the late 20th Century. This paper is expected to evolve as better hardware becomes available.

80. Precision Radiometry and the APHID 22 GHz Water Line Monitor , A. I. Harris, May 3, 2000
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Fluctuating amounts of water vapor over an antenna in an interferometer add phase shifts to the instrument, shifting the fringe pattern on the sky to decrease signal correlation. Optically thin line radiation from the same water vapor that causes the decorrelation can be accurately measured, however, allowing an estimate of the column of water above individual antennas. Knowledge of the water column allows an equivalent but opposite phase to be inserted in the data processing; this is the radio equivalent of adaptive optics. This memo explores the instrumental requirements for a multi-channel radiometer capable of measuring pathlength differences to 35 µm, a goal for interferometry at 1 mm wavelength. An examination of linear and nonlinear error sources shows that residual amplifier or detector nonlinearity is likely to be the most significant instrumental limit for atmospheric phase correction.

79. Measurements of the 1mm Receiver Optics , Dick Plambeck, March 2000
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When 1mm receivers were first installed on the BIMA antennas in October 1997, the aperture efficiencies were found to be unexpectedly poor. Lab measurements of the 1mm receiver optics finally identified the problem - two of the optical components in each dewar, a lens at the feedhorn aperture and a flat 50 K window used as an infrared filter, had been machined from defective rods of extruded Teflon (PTFE). These rods were slightly porous along their central axes, leading to a radial gradient in refractive index which corrupted the receiver beam pattern. When the defective components were replaced with lenses and windows fabricated from Teflon plate, the measured beam patterns closely matched those predicted by Gaussian beam analysis, and the aperture efficiencies improved dramatically. The new lenses were antireflection-coated by drilling a grid of holes into each surface, rather than by cutting circular grooves into the surface. In this way the refractive index of the antireflection layer is insensitive to polarization. This memo summarizes the optical layout of the BIMA antennas and receivers, gives a brief history of the 1mm aperture efficiency problem, and describes lab tests and theoretical modelling of the 1mm optical components. Section 7 discusses the antireflection coating on the new lenses.

78. Atmospheric Phase Correction using Water Vapor Radiometers , M.C.H. Wright, January 2000 (updated February 2001)
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We discuss the calibration and use of water vapor radiometers used for atmospheric phase correction on the BIMA array. Over intervals of a few minutes the WVR corrected data are consistent with the thermal noise of 100 microns. On longer timescales the calibration is currently limited by variations in the WVR gain and the atmospheric scale factor.

77. Error Correction in Quadrature Down-Converters , W. L. Urry, November 11, 1999
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The requirements for greater noise immunity from outside sources, such as satellites, in radio astronomy require systems with greater dynamic range so that the interfering signal doesn't wash out the very quiet signals from deep space. One of the classic methods for bringing a high frequency RF signal down to a lower more manageable IF frequency range is the quadrature down-converter. While quadrature down-converters are relatively simple, they suffer from sideband separation problems. Loud signals in one sideband can leak into the other and the amount of this leakage depends upon how much time and money is spent upon the circuit. Typically one can expect sideband rejection levels of only 30 dB below the interference even with components of reasonably high precision. The errors in a quadrature down-converter are easily identified and their effects on the power spectrum can be eliminated, digitally, at the low frequency IF, resulting in a circuit with superior side-band separation. These errors can be identified using the signal being measured so that no separate time consuming calibration is required. As the astronomic measurement is taking place an error function can be determined and used to servo the very slowly changing corrections required as the circuit ages or changes with temperature. As the cost of digital electronics drops it becomes economic to follow a relatively inexpensive quadrature down-converter with digital electronics designed to compensate for it's short- comings.

76. On the Use of Shifted m-sequences for Phase Switching Or Why not Complex Walsh Functions? W. L. Urry, August 9 1999
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Recently Eric Keto of the Smithsonian Astrophysical Observatory ( Technical Memo Number 134 ) suggested using four level, shifted m-sequences as an alternative to two level Walsh functions used for phase switching in interferometer arrays. Phase switching is used to reduce DC level shifts, to separate the sidebands, and it is used to provide cross-talk and interference rejection. In order for cross-talk and interference rejection to work, it is necessary for all of the phase demodulation functions for each of the baselines to be orthogonal. It is also necessary for all of the antenna phase modulators to be orthogonal. As bigger arrays are designed and the need continues for shorter integrations, there is a need for more efficient switching schemes. The requirement for baseline and antenna orthogonality places severe constraints on the selection of switching functions. The crucial question that must be answered in judging any modulation scheme is how efficient it is in supporting the largest antenna array in the fewest steps. We examine m-sequences, normal two level Walsh functions, and two kinds of complex Walsh functions. It appears that, for large systems, even an ideal solution may produce a set of phase modulation functions that are too long for short integrations.

75. Astronomical Imaging with the One Hectare Telescope M.C.H.Wright, December 1998
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74. Measurements of a scale-model ortho-mode transducer , Douglas Bock, June 30 1999
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This memo describes measurements made of a scale-model ortho-mode transducer to evaluate its suitability for use at millimeter wavelengths. The device consists of four probes placed in a single cross-sectional plane of a cylindrical waveguide. Each pair of probes is driven in anti-phase with a 180 degree hybrid. A good 50 ohm match with a return loss of 20 dB and polarization isolation of 40 dB was achieved with a fixed backshort in a 40% bandwidth. An implementation in the 3-mm band is suggested.

73. Image Fidelity , M.C.H. Wright, May 27 1999
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This memo discusses the imaging properties of the combined BIMA \& OVRO arrays. The 15-antenna array with 8 GHz bandwidth and a system temperature 100 K will have a sensitivity 1 mJy min$^{-0.5}$. For extended sources the image fidelity depends on the source brightness distribution, and data sampling. For mosaic observations the heterogeneous array comprised of 10.4 and 6.1 m antennas will have three different primary beam types. We compare the image fidelity obtained as a function of the $uv$ sampling, the pointing sampling, and the image complexity, and compare these results with imaging by a homogeneous array of 8 m antennas.

72. A New Interferometer for Monitoring Atmospheric Phase Fluctuations

Water vapor in the Earth's troposphere introduces extra electrical path in the propagation of radio signals through the atmosphere. The distribution of water vapor is irregular and distorts the wavefronts of incoming radio waves, limiting the angular resolution that can be achieved with ground-based telescopes. The level of fluctuations depends both on the location of the site and on the prevailing atmospheric conditions. The ability to measure the fluctuations is therefore important when choosing a site for a new instrument, and for scheduling observations of existing telescopes.

Existing phase monitors are radio interferometers that monitor monochromatic beacon tones from geostationary communications satellites at a frequency of ~12 GHz. They have a classical heterodyne design based on two satellite receiving antennas; each has a frontend for amplifying and downconverting the incoming signals using a local oscillator that is phase-locked to a common reference frequency. In addition to multiple phase-locked loops these instruments require expensive phase-stable cabling to reduce the effects of thermal drift.

The new system uses two consumer 18" digital satellite TV dishes to monitor satellite TV broadcast signals over a bandwidth of 500 MHz (12.2 to 12.7 GHz). The novel design eliminates the need for phase-locked loops and thermally stable components, and uses a pair of Gilbert Cell multipliers to perform the broadband correlation. A phase monitor has been been built and deployed at the site of the Berkeley-Illinois-Maryland Association Millimeter Array in Northern California, and has been operating successfully since June 1998, measuring the difference in electrical path length for parallel lines of sight to the satellite separated by a baseline of 100 m. With a hardware cost of approximately $4000, it is much cheaper than previous instruments, and the low power requirements and high reliability make the system suitable for site testing in remote locations. Plots showing current conditions at the site and a detailed description of the instrument can be found at http://bima2.hcro.berkeley.edu/phasemon/phasemon.html. 71. A New Correlator Architecture Eases Wiring Problem , W. L. Urry Nov. 13, 1998 available as: PostScript A big problem in correlator design is the large number of signal wires that must be distributed to the correlators. A 10 antenna system has 45 baselines. Each antenna must supply signal to 9 correlators for a total of 90 signal paths. If each antenna has a single digital sampler that samples to two bits then there are 180 signal pairs that must be carefully designed to arrive at the proper place at exactly the correct time. If an extra antenna is added there are 220 signal pairs to deal with and the whole signal distribution system must be redesigned. By a simple strategy it is possible to limit the signal paths from each antenna to two. It is also possible to expand the system indefinitely without the need to redesign the distribution of signals. 70. Fast-switching cycle times for BIMA's A array , R.L. Akeson Oct 1998 available as: PostScript Fast switching cycle times to minimize atmospheric phase fluctuations are calculated for conditions applicable to BIMA A array observing. The optimal cycle time is defined as having the lowest total noise from thermal and atmospheric contributions. It is assumed that decorrelation from the atmosphere can be partially corrected using observations of a test calibrator. Guidelines are given for cycle times for various array, source and atmospheric parameters. 69. New Array Configurations for Hat Creek , M.C.H. Wright, Aug 1998 available as: PostScript This memo presents suggested new antenna configurations for the 10-antenna array at Hat Creek. We use 4 antenna configurations with angular resolutions 18'', 6'', 2'', and 0.4'' at 100 Ghz. We have re-designed the 6'' and 2'' arrays to have more uniform uv-coverage and lower sidelobes. The new arrays have better instantaneous uv-coverage. Two antennas do not need to be moved when re-configuring the array. 68. Atmospheric phase conditions for the Hat Creek 1997-98 observing season , R. L. Akeson, 29 June 1998 available as: PostScript This memo analyzes data collected at Hat Creek on the atmospheric phase conditions during the 1997-98 observing season. The data come from observations of quasars during normal observing and from the dedicated phase monitor. Percentiles of rms phase are calculated as a function of time of year and time of day. The length of time over which the phase remains below a given threshold is measured and discussed with regards to dynamic scheduling. The effect of phase noise on observations in different arrays is briefly discussed. 67. Phase correction for the BIMA array: atmospherical model calculations for the design of a prototype correlation radiometer, J. Staguhn, A.I. Harris, R.L. Plambeck, and W.J. Welch, 10 June 1998 available as: PostScript We present results from atmospheric model calculations for the design of an atmospheric phase correlation radiometer for the Berkeley-Illinois-Maryland Millimeter Array (BIMA). The radiometer will monitor the atmospheric path delay by observing the fluctuations in the emission from tropospheric water vapor which causes de-correlation of astronomical signals which are observed along different lines of sight. We discuss the applicability of monitoring the optically thick 183 GHz water line and the optically thin 22 GHz water line. We conclude that for the BIMA site, which is not extremely dry, optical depth effects make observations of the 183 GHz line unfavorable. We discuss possible observing schemes and conclude that a multi-channel radiometer provides the highest achievable accuracy, as it provides the possibility to fit a line shape model to the observed channels. Systematic errors due to the unknown altitude distribution of water vapor and optical depth effects can be significantly reduced by this scheme, compared to methods which only monitor the change of the peak brightness temperature of the water vapor line. 66. Comments on Array Configurations M.C.H. Wright, May 1998 available as: PostScript This memo briefly compares radial, circular and irregular antenna configurations for aperture synthesis arrays. The best array configuration depends on the source structure. Combining arrays with different antenna sizes is discussed. 65. Wideband 3 mm mixers operated with partial saturation G. Engargiola and R. L. Plambeck, 3 Mar 1998 available as: PostScript We built fixed-tuned SIS mixers for use between 70 and 115 GHz on the Berkeley-Illinois-Maryland Association (BIMA) array. The mixers are similar to 215 GHz SIS devices designed by R. Blundell et al. Our 1.4 X 1.4 micron Nb/Al-Al_2O_3/Nb junctions have extremely sharp I-V characteristics, with R_{sg}/R_{n} ~ 40, and \omega RC ~ 2. Heterodyne tests were made with a temperature-regulated vane mounted in the input waveguide to the mixer block. With vane temperatures of 20 and 35 K, we measure double sideband receiver temperatures T_rcvr of 17 K from 80 to 115 GHz. Some gain compression occurs for input load temperatures greater than 50 K. Hence, with vane temperatures of 85 K and 280 K we measure T_rcvr ~ 35 K. Similar noise temperatures are measured with 77 and 295 K loads mounted outside the dewar. Because the mixers are partially saturated, we bias the junctions approximately 0.1 mV above or below the gain maximum, where the receiver response is a nearly linear function of the input signal level. This yields T_rcvr of 35-40 K and makes it possible to calibrate the receivers on the telescopes using the standard chopper wheel method. With no LO power applied to the mixer, we observe a small IF signal which is proportional to the square of the input load temperature; we believe this is due to self-mixing of blackbody photons in the junction. 64. Ray-tracing the BIMA Reflectors J.A. Hudson, 2 Feb 1998 available as: Micro$oft Postscript

A study has been made of the BIMA Cassegrain optics to assess alignment tolerances, depth of focus, and aberrations, and the present degree of correction of the system. This has been done both by ray-tracing the design specification for the Cassegrain system, and also by investigating holograms made by correlating one antenna with another. The surface accuracy was assessed by RMS and efficiency computations. Residual aberrations are expressed as coefficients of Zernike polynomials, and these can be used to characterize the system.

63. Proposal for a Wide-Band Digital Correlator W.L.Urry, 5 Jan 1998
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The goal of the wide-band digital proposal is to build a system using readily available, off the shelf parts. There are four elements to the wide-band digital correlator proposal. First, a sampler of unique design is proposed to digitize an 8 GHz bandwidth signal. Second, an 80 channel digital correlator is proposed. Third, a digital delay line is proposed to replace the existing analog delay. And finally, a finite impulse response filter is proposed to replace the existing single-sideband mixer and third LO.

62. A Performance Study of the BIMA Archiving System Raymond L. Plante nov-97
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I present results of a study of the BIMA archiving system in an effort to measure the typical rates at which data is transferred from Hat Creek to NCSA for archiving. This was done by altering the archiving software at Hat Creek to measure data transfer times as the data is sent to the archive server. The average archiving rate over a two week period was found to be about 350 kilobits per second with large variations between 200 and 700 kilobits per second. Internet traffic was found to be the primary limiting factor to the archiving rate over software overhead and competition for local system resources. The results are used to make recommendations for limits to be placed on the data production rate to ensure that the archiving system can keep up.

61. The BIMA Data Archive: the Architecture and Implementation of a Real-time Telescope Archiving System. Raymond L. Plante and Richard M. Crutcher nov-97
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We describe the design model and implementation of the data archiving system used by the Berkeley--Illinois--Maryland Association (BIMA) Millimeter Array Telescope. This system transmits data in real-time from the BIMA telescope at the Hat Creek Observatory in northern California via the Internet to an archive server at the National Center for Supercomputing Applications (NCSA) in Urbana, Illinois. Once at NCSA, the data undergo minor processing to make it available to astronomers via an HTML interface: (1) the data are checked for successful transmission, (2) metadata are extracted from the data and inserted into a searchable database, and (3) the data are sent to the NCSA Mass Storage System. When necessary, the system can carry out rollback operations which allow it to easily recover from errors, particularly those associated with the often unstable Internet. We also comment on some ways in which the system can be improved and expanded to adapt to changing observing strategies.

60. DYNAMIC SCHEDULING: Implementation at Hat Creek M.C.H. Wright aug-97
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We plan to use the atmospheric path RMS as a parameter for a dynamic scheduling program at Hat Creek. The RMS path can be measured by a satellite phase monitor or we can use a short observation of a quasar to measure the atmospheric phase RMS every few hours. A power law fit to the quasar observation is used to estimate the RMS path on the longest baseline. The scheduling program can be script driven, providing a convenient way to accommodate the specific observing environment at Hat Creek. We have extended the LST time format used in observing scripts at Hat Creek to allow dynamic scheduling over multiple days.

59. Imaging with Heterogeneous Arrays . M.C.H. Wright aug-97
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This memo briefly reviews some considerations for imaging with aperture synthesis arrays containing mixed antenna sizes. At millimeter wavelengths, where mosaicing observations are important, the effect of the mixed primary beams is significant. For mosaicing observations, the sensitivity depends on the number and diameter of antennas with each primary beam type. The array configuration can be optimized to use the different antenna sizes, and depends quite strongly on the source structure. Data sampling rates are set by both the largest and smallest antenna diameter, and are larger than for an equivalent homogeneous array. The negative primary beam from mixed antenna baselines may be a problem in the mosaicing algorithms. Conceivably it might also serve to knit together the mosaic by providing different weightings of the overall image. The dynamic range of mosaiced observations is often limited by pointing errors. Pointing is more of a problem for larger antennas, but the mixed primary beam patterns may make it easier to implement a pointing self-calibration algorithm. Mosaicing with mixed antenna baselines needs detailed study.

58. Dynamic Scheduling: Selection criteria and evaluation. M.C.H. Wright and R.I. Akeson. 19-jun-97
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We argue that a direct measurement of the atmospheric phase rms provides the best weather parameter for scheduling interferometers for high frequency observations. We evaluate the performance of a dynamic scheduling program under varying atmospheric phase conditions, and compare the simulated schedule with the results from actual observations.

57. Autocorrelation Observations with the BIMA Array Marc Pound and Tamara T. Helfer 2-may-97
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We report on the current state of autocorrelation capabilities at BIMA. Spectral line observations with window bandwidths <= 50 MHz are reliable, but observations with 100 MHz windows and continuum measurements are not currently feasible. The relative calibration among the individual antennas is typically good to about 20%, and often the agreement among all but one or two of the antennas is better than 10%. The line shapes and strengths of BIMA observations of CO and HCN in IRC+10216 agree to within the noise with spectra from the Bell Labs 7-m and the NRAO 12-m telescopes; a 500" x 350" BIMA map of C18-O from NGC 2024 agrees well in structure with an NRAO 12-m map from the same region, though the emission measured at BIMA is about 20% weaker. Sensitive observations of a weak CS line in MBM12 show that there is no noise floor at the level of at least 16 mK per 24 kHz channel width. The measured rms noise in all the antennas is consistently about 8-15% lower than the theoretical noise; we are unsure of the explanation for this discrepancy. We describe new MIRIAD routines to process autocorrelation spectra and give template observation and data reduction scripts.

56. A MIRIAD routine for analyzing structures in Spectral Line Data Cubes Jonathan Williams, and Peter Teuben, 23-apr-97
Available as:
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We describe the use of an automated clump-finding routine for analyzing spectral line data cubes that has been recently installed in the MIRIAD package. There are three programs: the main analysis program, clfind, that reads in the data cube and outputs a cube of the same dimensions with the clump assignments; a statistics program, clstats, that calculates clump positions, sizes, linewidths and masses; and a clump plotting program, clplot, based on velplot, that allows the user to isolate and produce maps of a user input list of clumps.

55. Future mm-VLBI Arrays , M.C.H. Wright, and G.C.Bower, 18-Apr-97.
available as:
PostScript

VLBI observations at short millimeter wavelengths are required to resolve the compact radio components associated with active galactic nuclei. These components are optically thick at longer wavelengths, or obscured by scattering, as in the case of SgrA*. Large new telescopes planned for completion in the next 10 years provide a powerful new platform for millimeter and submillimeter VLBI. The atmospheric phase correction required for successful operation of millimeter arrays on baselines of several km also enable phase coherent VLBI observation with $20 \mu {\rm arcsec}$ resolution. This is sufficient to resolve structure on the scale of an accretion disk around a massive black hole in the nearest quasars. The new telescopes can be outfitted for VLBI at modest cost and should be planned with this capability in mind.

54. Minimum Redundancy Linear Arrays, T.T.Helfer and W.J.Welch, 20-feb-97
available as:
PostScript

The idea of using a minimum redundancy linar array for radio interferometers is familiar to radio astronomers. For East-West linear arrays the minimum redundancy array provides essentially an exact optimum solution for the choice of antenna separations for a 12 hour earth rotation sysnthesis. For two dimenional arrays, there is no corresponding exact solution. The authors discuss the application of the minimum redundancy concept to the 2-d "T-configuration" array.

53. Dynamic Scheduling for the BIMA Array M.C.H. Wright,
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In order to make the best use of varying weather conditions we propose to to implement dynamic scheduling for BIMA array observations. The schedule is determined in real time from a prioritized list of observations and the system status including the weather conditions. We briefly discuss the relative merits and problems of flexible scheduling.

52. 3 mm VLBI at Hat Creek - 1995 - 1997 Geoffrey C. Bower, Donald C. Backer, Dick Plambeck, Melvyn Wright, 25-feb-97
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This memo describes the current approach for using the Hat Creek array as an element in a VLBI array. Theoretical and practical questions are addressed. The principal elements of a VLBI station are the antenna, a phase stable receiver and IF system, a stable frequency standard, usually a hydrogen maser, and a backend for recording the IF (Figure 1). We consider each of these in Sections 2 through 5. We also discuss procedures for the setup and execution of a VLBI experiment at Hat Creek in Sections 6 through 8. Finally, we give a wish list for improvements and technical innovations in Section 9.

51. Proposed 10-Antenna Arrays for the Coming Year W.J. Welch and T.T. Helfer, December 1996
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We propose 10-antenna arrays for the coming year, giving some discussion about the strategy of their design and possible alternatives. For the A and B arrays, we propose those that were worked out and described in the BIMA Array paper (1996, PASP, 108, 93). For the C array, we have a new design that better emphasizes sampling of short spacings in the uv-plane. We have also worked out a new H array for low declinations that has better uv-sampling of short spacings.
We discuss alternative strategies for observing low-declination sources at moderate resolution, and we compare the H array with a combination of C and B arrays. We find that sources at low declinations are as well or better observed with C and B arrays than in H array. Given the scheduling inefficiency of the H array, we propose that it be eliminated.

50. Comments on Self Calibration and Fast Switching Melvyn Wright,
available as:
PostScript, LaTeX file

We briefly discuss the relative merits and problems of self-calibration and fast switching calibration for the BIMA a-array. Self-calibration is very successful for strong sources, but can lead to spurious results for small arrays of antennas if the source is weak or too complex. Fast switching between a source and a nearby calibrator can reduce the rms phase on baselines longer than few hundred meters. Switching times of a few seconds are desirable, but are inefficient.

49. Correlator Normalization With Power Level Change, W. L. Urry, 05-Sept-96
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PostScript

The sampler used in the BIMA correlator quantizes the signal into four levels. This coarse quantization causes the correlation function to be distorted. The distortion may be removed by first normalizing the correlation function and then scaling each measured correlation coefficient by a correction function. The proper function to use and the normalization depends upon the input power level relative to the sampler clipping level. The normalization to use is the topic of this memo.

48. Calibrating Polarization Switched Data, Melvyn Wright, 01-Aug-96
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This memo defines the algebra and outlines the steps used for calibrating polarization switched data, and making polarization images for Stokes I, Q, U and V.

47. Optimizing the Length of Observing Tracks with BIMA, Michael W. Regan and Richard J. Rand, 29-May-96
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Presently the length of observing tracks at BIMA are based on several different criteria: user requests, arbitrary cutoff elevations, or standard track lengths. All of these methods can lead to the continuation of tracks for only minimal gains in signal-to-noise ratio (SNR) or UV coverage. In addition, some tracks are cut short when they are still productively adding to their UV coverage and SNR. To obtain the maximum productivity for the array it is important to determine the optimum track length and the optimum array configuration for each declination. The memo proposes a method of determining the optimum track length by looking at the SNR of a track compared to a horizon to horizon track weighting by the system temperature. We derive a table of minimum and maximum track lengths for different source declinations.

46. DC Bias in the One-Bit BIMA Samplers, Jerome A. Hudson, 23-Apr-96

This is a quick look at how a slight voltage bias affects the BIMA samplers. For brevity, only the one-bit case has been considered, but it is probably reasonable to assume the same general conclusions hold for the two-bit case. The effect of moderate DC bias on wide-band noise is a slight offset of the spectrum, with mild distortion. The DC bias can be approximately removed using a simple algorithm, with residuals in the spectrum for Nyquist-sampled lags of about 0.5%.

45. Bima Array Response to Extended Structure, Melvyn Wright, 23-Apr-96
Available as:
PostScript

The BIMA 9-antenna array provides resolutions 5'', 2'', and 0.5'' in three standard array configurations at 100 GHz. Source structures more extended than a few times the synthesised beam width are attenuated and distorted in the synthesised images; structures larger than about 10 times the beam are not detected. We propose to add a more compact D array configuration with a resolution 17'' and better sensitivity to larger structures. Mosaicing observations using this D array configuration are discussed.

44. A Recipe for Phase Prediction from Total Power Measurements, Melvyn Wright, 11-MAR-96
Available as:
PostScript

We have analysed data obtained with the Hat Creek 9-antenna array with antenna spacings between 80 and 1045m. In clear weather we find a correlation between the atmospheric phase fluctuations and the total power measurements. This memo provides a recipe for correcting the phase, and an analysis of some recent data.

43. Polarization Switching for the BIMA Array, M.C.H.Wright, 15-Dec-95
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The BIMA array receivers each have a single linear polarization. Polarization Switching is required in order to obtain polarization observations. This memo explores efficient switching cycles.

42. Some Comments on Sampling, Antenna Spacings, and Uniform Illumination, W.J.Welch, Aug-95
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41. Gain and phase stability of some components in the BIMA array, B.Frye, R.Forster, J.Lugten, L.Mundy, R.Plambeck, D.Thornton, J.Welch, Aug-95

40. A Cookbook for Reducing BIMA Data using MIRIAD, Mike Regan, 15-Apr-95

This memo describes how to reduce observations from the expanded BIMA array using MIRIAD. It explains the reasons for the various calibration steps assuming the reader has a basic understanding of MIRIAD and interferometry. The two basic methods of antenna based gain calibration are presented and the advantages and disadvantages of each method are discussed. All the steps in the data reduction and calibration process are explained through the making of basic channel and moment maps.

Available as:
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39. Optical Design and Performance of the BIMA Interferometer, John B. Lugten, 2-Mar-95
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The Berkeley-Illinois-Maryland Association millimeter wave interferometer has a very clean and simple optical design. Each antenna holds one dewar which houses mixers for 4 frequency bands; the feed horns for the 4 frequency bands are located slightly off the telescope axis and look directly at the subreflector without any intervening transfer optics. To change frequency bands, the telescope pointing is offset slightly. The feed horn to antenna coupling is frequency independent; aperture efficiency is measured to be 75% at 100 GHz and is expected to be greater than 71% up to 270 GHz. Main beam efficiency is about 83% at 100 GHz. The total antenna surface error is 30 mu-m rms, corresponding to lambda 100 at 100 GHz.

By extrapolating a tipping curve to zero airmass, the residual telescope noise temperature is found to be 3 to 5 K, in good agreement with calculations. Teflon (PTFE) lenses, serving also as windows in 50 K and 10 K radiation shields, provide excellent blockage of infrared radiation but very low loss at millimeter wavelengths.

(paper presented at the European Workshop on Low-noise Quasi-optics MPIfR, Bonn, Germany, 12--13 September 1994)

38. Optical Design Parameters of BIMA's TIW Antennas, John B. Lugten, 2-Mar-95
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This memo describes the optical design chosen for the new BIMA antennas (those manufactured by TIW). The coma due to the off-axis feed locations is calculated and the shape of the primary beam is calculated.

37. Improved Seal for a 4 K Gifford-McMahon Cryocooler, R. L. Plambeck, 28-Jun-94
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PostScript

Gifford-McMahon refrigerators operating at ~4 K are currently used to cool SIS mixers on radio telescopes at the Berkeley-Illinois-Maryland array. The refrigerators are constructed by adding third stages onto standard 2-stage GM cryocoolers; Er3Ni spheres are used as the low temperature regenerators. The refrigeration capacity is 50 mW at 3.5 K.

Two refrigerators have been installed on telescopes thus far. The first has operated for 10000 hours. Its temperature fluctuates between 3.5 and 5.7 K, sometimes on time scales of minutes, because helium leaks intermittently past the seal on the third stage displacer. The temperature stability of the second refrigerator is much better. It uses a spring-energized lip seal (Bal Seal) which is mounted on the third stage cylinder, rather than on the displacer. This refrigerator has now operated on a telescope for 3000 hours. It maintains the SIS mixer at 3.0 K, stable to +-0.07 K over periods of days, even as it is tipped over an elevation angle range of 150 degrees.

The improved reproducibility afforded by the Bal Seal allows one to more reliably compare different regenerator materials. The refrigerator performance is almost identical with Er3Ni or neodymium spheres.

(paper presented at 8th International Cryocooler Conference, Vail, Colorado, 28-30 June 1994)

36. LO, Delay, and Correlator Control for the BIMA Array, W. J. Welch, 16-May-94

The RF circuit on each antenna consists of a double conversion super-heterodyne receiver with first and second local oscillators, amplifiers, and filters which define the pass-bands. Both side-bands of the first local oscillator are detected. The natural fringe rates are rotated to zero by offsetting of the local oscillators, and the sidebands are separated by phase switching of the first local oscillator through pi/4. The second local oscillator is switched by pi/2 to eliminate DC drifts. The switching of both oscillators employs the orthogonal Walsh Function set in order to minimize cross-talk between the different channels. At the correlator output, the correlation functions for the various spectral windows of the IF are stored in pairs of registers in synchronism with the Walsh Functions. A complex, linear combination of each of these pairs transforms (by Fourier Transformation) into complex visibility spectra.

35. A Test of the Homogeneous Array Concept Using the Hat Creek Array, David Wilner, 30-Dec-93
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In this memo, we review the three basic schemes of obtaining low spatial frequency data for millimeter array observations: (1) use of a large single dish, (2) interferometry with a multi-size array, and (3) scanning with a homogeneous array. We describe an application of the homogeneous array concept using the three antenna Hat Creek array. In an appendix, we analyze the effects of random, uncorrelated pointing errors on the effective interferometer primary beam shape, generalizing an analysis originally due to Rachel Padman and since lost.

34. The Sign of Stokes V, R. J. Sault, 14-Dec-93
Available as:
dvi file, PostScript

Here we review the sign conventions used by the ATCA, for data within an RPFITS file and within the MIRIAD system.

33. Passband Correction with the BIMA Array, Melvyn Wright, 12-Jan-94
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This memo describes the current state of the on-line passband calibration, and gives some examples of the astronomical passband calibration for data from the BIMA array. Polynomial fits to each interferometer baseline and correlator window currently provide the most reliable passband calibration.

32. Strategies for Multiple Pointing Observations, W.J.Welch, Nov-93
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Multiple pointing observations may be done either to get more uniform sensitivity over the primary beam for a source that nearly fills the beam, or it may be used to map a source that is larger than the primary beam. In either case, it is necessary to carry out a complete set of all the pointings while still within each uv cell of the source visibility function, in order that the source visibility be properly sampled. This may be difficult if the source is large compared to the beam, requiring many pointings, or if the desired angular resolution is high. This is because the observation may pass completely through the basic uv cell before it is possible to complete all the different pointings. It is always possible to carry out the observations by using tracks on successive days, but here we discuss the limitations to obtaining the complete sampling in one single track.

31. B Configuration, W.J.Welch, Nov-93
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This memo describes a B array which, when combined with the present C array, improves the resolution, by about a factor of three, to 5". Unlike earlier B arrays, this one has only a small range of shortest to longest spacings and gives reasonable sampling of that range for a seven hour track.

30. Imaging with the Bima Array, Melvyn Wright, 29-Jul-93
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This memo provides an easy recipe for calibrating and imaging data from the BIMA array. Images can be made automatically, although at present the data must be edited to obtain the best dynamic range.

29. Long Baselines at Hat Creek, Mundy, Erickson, and Blitz, 03-Sep-93
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The purpose of this memo is to consider two options for implementation of long baselines on the BIMA Array: coaxial cable and fiber optics.

28. A 4 K Gifford-McMahon Refrigerator for Radio Astronomy, R. Plambeck, N. Thatte, and P. Sykes, 23-Nov-92
(paper presented at the 7th International Cryocooler Conference, Santa Fe, NM, November 1992)

We have built a 3-stage Gifford-McMahon refrigerator which is designed to cool a superconducting junction to 4 K on a radio telescope. Using 50 grams of Er3Ni spheres as the third stage regenerator, we measure no-load temperatures as low as 3.8 K at a cycle frequency of 72 rpm, and 2.2 K at 30 rpm. The third stage temperature can be maintained at 3.5 K with heat loads of 10 W on the first stage, 1 W on the second stage, and 50 mW on the third stage. At 72 rpm the refrigerator performance is impaired by the substantial pressure drop through the first and second stage regenerators; using a pressure transducer connected through a capillary tube to the third stage, we find that the helium pressure swing in the third stage expansion volume increases by 50 percent as the cycle frequency is decreased from 72 to 30 rpm. Our prototype refrigerators exhibit occasional 0.5 to 1 K temperature excursions which occur over periods of a few minutes to a few hours. Tests suggest that this erratic behavior is primarily attributable to leakage of warm gas past the third stage seal.

27. The IF/LO Subsystem, W.J.Welch, 12-Jan-92

26. The Switchyard Subsystem, WJW-DDT-WTL, 28-Jun-92

25. The BIMA Directional Couplers, Jan Tauber, Jun-92

24. A Proposal for a Digital Delay Line System at Hat Creek, W.C.Erickson, 06-Jul-92

23. Effect of antenna tilts and alignment errors on pointing and phase, Melvy Wright, 14-May-92
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We obtained optical pointing data for antenna No. 5 on 11-12 May 1992 using the unix operating system. An rms residual of 6 arcsec was obtained with a sin(2Az) residual pattern in elevation. We investigate the effects of large tilts and collimation errors on the pointing equations. An antenna tilt, or an elevation axis mis-alignment of 1', or a collimation error of 5' produce errors of ~1" rms in the pointing.

22. Mapping BIMA Data with MIRIAD; A Cookbook, Eugene de Geus, 10-Nov-91
Available as:
dvi file, PostScript

This cookbook is designed to help the new or slightly experienced user make maps from BIMA data using the MIRIAD data reduction package. Different methods are given for determining the success of the calibration procedure, and some detailed guidelines are presented on how to make dirty and clean maps. A familiarity with MIRIAD is assumed, such as how to run MIRIAD programs and how the observations from a single track on an object are organized into the different MIRIAD datasets. Advanced topics such as mint are not covered here.

21. Correction of Panel 4 Errors by Warping, Jerry Hudson, 19-Jul-91

An idealized model of Panel 4, utilizing plate steel elements, was used to attempt to simulate the deflections that have actually been measured. The Algor "ViziCad" finite-element analysis program was used for the computations. Mel Wright found that a quadratic warping function fitted the panel nicely. Likewise, the simulation seems also to fit a quadratic warping function very closely, despite presence of the 3 x 3 crossed ribbing on the back of the panel. The model was supported on two opposite corners, and unit loads placed on the other corners. With this loading, the maximum deflection (at the corner) was 3.9E-5 inch, relative to the panel center. The corresponding difference on panel4-3.05mar91 was 4.7E-3 inch; the multiplicative scale factor to scale the model to panel4-3 data was hence 119.9. That is, a 120 lb. load would be required. The model may turn out to be stiffer than the real panel, and also the stiffness of steel is roughly twice that of aluminum, so the scaling to what we observe looks reasonable.

20. Deformation of Antenna #3 using Structural Analysis, W. L. Urry, 23-Jul-87

Structural analysis of the Hat Creek interferometer third antenna indicates that RMS surface error due to the flexure of the back-up structure under the forces of gravity is only .00061 inch. The considerable stiffness of the structure means that the panel surface alignment to an accuracy of .002 inch at an elevation of 5 degrees should be preserved at all elevation angles. Finite element analysis aided by the use of Zernike circle polynomials makes it possible to separate the RMS contributions due to tilt and focus from other surface deformations. The Zernike circle polynomials also enable us to obtain a complete quantitative description of the deformed surface.

19. Optimized Calibration, Melvyn Wright, 2-Oct-91
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This memo explores the optimum calibration of millimeter array data. The tradeoff between integration time on source and calibration, and the choice of calibrators is discussed. For strong sources, or at short baselines, calibration errors become more important. On-line calibrations of instrumental subsystems made with high SNR reduces the number of parameters needed to define the gain and passband functions. The gain calibration can be improved by fitting antenna gains to both sidebands simultaneously. This facilitates the selfcalibration and analysis of relative positions of images in both sidebands. For strong sources, or line and continuum observations, special care needs to be taken with the passband calibration.

18. Report from First Hat Creek Fiber-Optic Test, Lee G. Mundy, William C. Erickson, Arie W. Grossman, 2-Oct-91
Available as:
dvi file, PostScript

A fiber-optic link for the distribution of LO and IF signals was successfully installed and tested on antenna #3 at Hat Creek. The link performed to specification; there was no measurable degradation of system performance over the standard coax cable system. Appendix A presents and costs a plan for upgrading the entire array to fiber-optic LO and IF distribution.

17. Design and Measurements of the Conical Corrugated Feed Horns for the Bima Array, Xiaolei Zhan, 9-Sep-91

A new conical corrugated feed horn was designed and fabricated in the fall of 1990 as a test model for the upgrading BIMA antennas. The fractional single-mode operation range of this feed horn is near 50%. The improved design of the mode-launching region allows much superior performances to be achieved on the new feed horn compared to the existing feed horns on the Hat Creek antennas, in terms of impedance matching, beam symmetry and crosspolarization characteristics, as well as effective matching to the telescope focal region fields in order to obtain higher aperture and beam efficiencies. Over most of the operation range of 70 - 115 GHz, the return loss level is reduced from the old value of around - 17dB to around - 30dB, crosspolarization level reduced from around - 20dB to around - 35dB. Using a phase corrector dielectric lens in front of the conical horn aperture, the theoretical aperture efficiency of this new optical system (including illumination efficiency and spillover efficiency) is around 78 - 82% over a 30% bandwidth, compared to the value of 72 - 74% for the existing optical design. Introducing a tertiary lens further improves the theoretical aperture efficiency to about 84%. The drastic fall-off of the aperture efficiency at the high frequency end of the band on all three of the present Hat Creek antennas (measured value of 25 - 50% instead of the theoretical ~ 70%), can not be fully accounted for by the surface loss and feed-leg scatterings. Possible reasons for the dropoff of the aperture efficiency on the existing system is also analysed here from the point of view of the old feed horn performances. Measurements on the characteristics of the old Hat Creek horns, holographical measurements of the dish illumination patterns, compared with the calculation results, seem to support the arguments.

16. Calibrating BIMA Data with MIRIAD: A Cookbook, Christine Wilson, 26-Aug-91
Available as:
dvi file, PostScript

This cookbook is designed to help the new or slightly experienced user calibrate BIMA data using the MIRIAD data reduction package. Some detailed guidelines are given for editing the data and estimating the quality of the data. Some familiarity with MIRIAD is assumed, such as how to run MIRIAD programs and how the observations from a single track on an object are organized into the different MIRIAD datasets. Advanced topics such as selfcal and mint are not covered here.

15. The Effect of Light Wind on Antenna 3, Stephen White, 25-Jul-91
Available as:
dvi file, PostScript

It is well known that the drives on antenna 3 are presently unable to maintain pointing in the presence of even a small amount of wind. However the effect this has on data is probably not widely appreciated. This memo uses solar observations to point out that on a typical summer afternoon antenna 3 may be pointing off the source (i.e., the source is outside the primary beam) for up to 50% of the time without the data being flagged bad. These pointing errors must produce high noise levels on the baselines including antenna 3.

14. A-Priori Self-Calibration, Melvyn Wright and Bob Sault, 24-Oct-90

We describe a method for calibrating aperture synthesis data automatically using a self-calibration algorithm with the a-priori positions and flux densities of the calibration sources. The algorithm adjusts the antenna gains to produce the best least squares fit to the theoretical instrumental point-spread function. These gains are then used to calibrate the source data. As an example of the use of the algorithm we present a map of the radio galaxy Cygnus A which was completely calibrated and mapped from a script command file.

13. Optical Pointing on Antenna Number Four, Melvyn Wright, 15-Aug-91

We obtained optical pointing data for antenna No. 4 on 9-11 Aug 1991. The data from the three nights is consistent with an rms residual of 2.5 arcsec in azimuth and 7.1 arcsec in elevation. Some time dependence is observed in the elevation pointing. This may be due to thermal drifts since the antenna is not currently temperature regulated. Plots of pointing residuals are attached.

12. Fiber-Optic Work at NRAO Green Bank, Bill Erickson, Lee Mundy, and Arie Grossman, 10-Jul-91

The authors traveled to NRAO Green Bank on July 1-2, 1991, to conduct fiber optics tests. Our objectives were to test methods of fusion splicing (especially with the new phase-stable fiber developed by Sumitomo Inc.), to determine the characteristics of fiber optic connectors from Radiall and JDS, to test an ORTEL laser transmitter and receiver, to examine the properties of phase stable fibers, and to become more familiar with fiber-optic techniques.

11. Tests of Sparc and Disk Performance on Standard MIRIAD Tasks, A. Rudolph and P. Teuben, 8-Jul-91
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Tests were run on three of the Maryland SPARC workstations (a SPARC2, an IPC, and a SPARC1), running three standard MIRIAD tasks, INVERT, CLEAN, and RESTORE. These tests were run both using each machine's local disk and using disks on the other machines, in order to test the relative performance of the machines, as well as the importance of working on a local disk. Also, running time versus map size was measured for the three tasks, and a test was run with another user on the machine. The results of these tests is presented.

10. Antenna Shadowing in C-Array, Melvyn Wright, 11-Jun-91
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hypertext

Antenna shadowing occurs if the projected baseline is less than the antenna diameter. This may effect the choice of calibrators in compact array configurations. Here is a script to generate a list of shadowed data. Lists of shadowed data for the c-array are appended.

9. Synthesised Beams and Interferometer Array Response to Extended Structure, Melvyn Wright, 1-May-91

Large scale structure is poorly represented in images made from interferometer observations because of missing low spatial frequencies. In this memo we plot the BIMA array beam patterns for the standard arrays, and tabulate the response as a function of the source size. We briefly review the methods used to estimate the integrated flux density for an extended source.

8. Systematic errors in pointing Determination sing the CROSS Program, Arie Grossman, 16-Apr-91
Available as:
dvi file, PostScript, hypertext

It is shown that pointing offsets determined by program CROSS exhibit systematic errors as large as 70%. These errors are due to improperly fitting a Gaussian to the offset measurements in subroutine RCGAUS. The results of fitting model Gaussians with and without additive noise show that a non-linear fitting method based on the Levenberg-Marquardt is superior to the the cross-correlation method of RCGAUS.

7. A Study of Outrigger Antenna Configurations, Arie Grossman, 15-Apr-91
Available as:
dvi file, PostScript, hypertext

This memo analyzes the effectiveness of using one or two outrigger antennas placed along the lab road combined with 6 or 5 antennas in the conventional "T" configuration. Each configuration is evaluated for three source models at declinations of -20, +15, and +50 degrees. UV tracks are generated for each model, and inverted with uniform weighting to yield a beam. Plots of the UV coverage and beam shape are presented for each potential configuration. Maximum negative sidelobe level and beam size are used to evaluate the quality of the configuration. The optimal location for a single outrigger antenna appears to be at a distance 1570N (feet) and 340E. The optimal location for a second outrigger appears to be at a distance of 2800N. If this location is not practical, a suitable alternate location is at 2440N 220W. These configurations yield sub-arcsecond resolutions with beam sidelobes levels approaching the 10% level for a full track.

6. Measurement of Panels for BIMA Antennas, Melvyn Wright, 05-Apr-91

The panels for the BIMA array are aluminum castings. There are 4 rows of panels with all panels less than 1 m. in size. The panels are machined leaving a scalloped surface equivalent to a 0.8 micron rms surface accuracy. Four test panels have been machined and independently measured. The measured fit to the a-priori parabolic surface varied from 6 microns rms for the inner panel to 41 microns for the outer panel. The increased rms in the outer panel is attributed to the rib structure which becomes increasingly parallel in the outer panels and allow the panel to be easily warped about its diagonal. The warp is clearly evident in the residual errors. Since the panel is mounted on the antenna backup structure by four mounting points at the corners of each panel, this warp will be removed when the panel is mounted on the antenna. Fitting a hyperbolic warp at 45 degrees to the panel axis reduces the rms residual to less than 6 microns for all four panels.

5. Tilt, Pointing and Baseline variations at Hat Creek, J.R.Forster, 02-Feb-91

This note discusses the large tilts observed with the Hat Creek telescopes during the winter of 1990-91, and describes an attempt to determine whether the cause is in the ground or the antenna structure. Theodolite measurements made during settled weather in February showed that tilts on the ground vary by ~15 arcsec, comparable to tilts measured at the azimuth axis. However, no good correlation exists between the ground and azimuth axis tilts so it can only be concluded that both the ground and the telescope mounts tilt by weeks in February show that normal tilts are correlated with ambient temperature and not sun direction. Antenna moves are characterized by a "settling-in" tilt of order 30 arcsec which occurs within 24 hours after shimming, and is followed by temperature-dependent variations of order 15 arcsec. These time-variable tilts can account for the 0.5 arcmin pointing and 1-2 radian baseline instabilities present in Hat Creek interferometer data. The form of the pointing and baseline error produced by an antenna tilt is derived. The pointing error can be determined from tiltmeter measurements; however the current pointing corrections at Hat Creek do not contain the appropriate terms in azimuth. Tiltmeter measurements are not sufficient to determine the baseline error, although the baseline error can be estimated using certain assumptions. It is shown that the baseline error is reduced by correcting the elevation pointing term, and that pointing errors also produce substantial baseline errors.

4. Dispersion in the Coaxial Cables, W.J.Welch, 31-dec-90

The coaxial cables that we use for signal transmission everywhere exhibit some dispersion. It amounts to a few degrees of phase shift in excess of what an ideal cable would have. For most of the cables, this non-linear frequency dependent phase shift is indistinguishable from what is produced in the various other components, and we remove it in our normal bandpass calibration. However, it is more of a nuisance in the variable delay line, because the delay steps change during the course of an integration, and the associated phase steps produce fringe phase errors which are not removed by the current calibration process. The following memo shows that the dispersion is due to the inductive part of the skin effect in the cable walls. It can be approximately predicted by simple theory. Its frequency dependence is quite simple, and its effects can be obtained with a precision of the order of one degree by a table look-up.

3. Phase Prediction from Total Power Measurements, Melvyn Wright, 14-dec-90
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We present a model analysis of interferometer data corrupted by receiver noise and gain instabilities, and atmospheric phase fluctuations which are correlated with total power fluctuations. If the atmospheric fluctuations are larger than the fluctuations due to gain instability, ground radiation etc., then we can estimate the phase errors and correct the data. We have developed software tools to estimate the correlation between atmospheric phase and total power fluctuations, and correct the data. A receiver gain stability of about 10E-4 is required, and the variations in total power from ground radiation must be smaller than about 0.2 K on the time scale of the atmospheric fluctuations.

2. Hat Creek Interferometer Pointing Fitting, Melvyn Wright, 03-Jan-90
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Pointing data for 3 configurations Nov-Dec 1989 is analysed. The effect of antenna structure and solar heating on the pointing is also discussed. The parameters derived from optical pointing are reproducible after an antenna move. There is a large dispersion in the parameters derived from radio pointing, even with large data sets. The pointing parameters should not be changed based on small data sets. The radio pointing can be based on the parameters derived from optical pointing without significantly degrading the rms. Using the refraction computed from temperature and relative humidity does not significantly change the rms.

1. Antenna 3 Efficiency Measurements, Dick Plambeck, 31-Mar-86

The panels on antenna 3 were adjusted to reduce the surface rms error to 50 microns. Observations of Jupiter and Venus were then used to measure the antenna aperture efficiency at 80, 95, and 110 Ghz, with and without Jerry Hudson's dielectric lens installed in front of the feed horn. The aperture efficiency was roughly 55-60% without the lens, 65-70% with it; the lens appears to add less than 2 K to the system temperature. This memo describes the panel adjustment procedure and the aperture efficiency measurements.

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