NRAO is a facility of the National Science Foundation,
operated under cooperative agreement by Associated Universities, Inc.

1 January 1997 -------------------- No. 70



The three photographs on the following page present a picture both of the progress being made at the Green Bank Telescope (GBT) construction site and the magnitude of the assembly work being carried out at the site.

The upper left photo, taken in early December, shows the status of the erection of the tipping structure. The structure has been moved from the 66 degrees position to 78 degrees, which is the telescope access position. The 55x25x31 foot (approx.) dual boxes of the horizontal feed arm are reaching out. Four of the 12 boxes which will make up the horizontal feed arm are in place; two additional boxes are ready for installation. Once the remaining eight boxes are installed,assembly of the vertical portion of the feed arm will begin.

The upper right photo, taken in November, illustrates the progress made on the upper 60 feet of the upper vertical feed arm. The subreflector panels have been installed on the subreflector backup structure, and the subreflector assembly is mounted in position on the feed arm tip. The prime focus feed positioner mechanism may be seen in a partially retracted position. The access and stairways are also completed, which will help make antenna maintenance like a walk in the park (about 430 feet in the air!). As previously reported, the feed arm tip assembly, as shown, will be used for setting the subreflector surface and testing and calibrating all the mechanical elements on the upper feed arm.

The lower photo shows the growth of the main reflector backup structure (BUS), although the photo, taken in October, is already dated. By Christmas, the BUS was over 60 percent trial erected. From Hoop 15 to Hoop 33 all trusses are in place. Trusses R0, R1L, R1R, R2L, R2R, R3L and R3R between Hoops 33 and 45 are also in place. All but 30 pieces of the BUS have been delivered to the site. The site crew reports that progress on the BUS trial erection is in "high gear" and completion is scheduled for the summer. Once the trial erection is complete, the BUS will be disassembled into 22 modules for lifting onto the antenna.

Approximately 1,400 of the 2,200 main reflector surface panels have been fabricated and are awaiting final inspection, painting, and shipment to the site.

W.H. Porter


The Green Bank Interferometer (GBI) has been activated after resting dormant since April 1996. A consortium of NASA X-ray and Gamma-ray groups have pooled resources to provide funding for a year of operation.

In the past two years, the GBI has been increasingly devoted to observation of X-ray binaries and X-ray active AGNs due to interest in comparisons of radio data with data from recent NASA high-energy orbiting observatories. The usefulness of radio monitoring to high energy astronomers was enough to motivate support for the GBI. We can thank Bob Hjellming of NRAO and Marco Tavani of Columbia for organizing their colleagues at NASA to contribute to this effort.

The GBI presently uses two of the 85-foot dishes as a single-baseline interferometer. The third 85-foot dish operates independently, monitoring pulsars for astronomers at Princeton and Berkeley.

In its new incarnation, the GBI will be open to proposals for sources to be monitored. High priority will be given to sources of interest to high energy projects, but other proposals will be entertained for both long and short-term monitoring. A committee consisting of Frank Ghigo and Bob Hjellming of NRAO, Elizabeth Waltman of NRL, and Marco Tavani of Columbia will consider requests for observing time. The prime sources being observed now include GRS1915+105, GRO J1655-40, GRS1716-249, Cyg X-1, Cyg X-3, SS433, LSI+61 303, and Algol.

More information can be found on the GBI web page at: This gives a more complete description of the program and instructions for proposing new observations. This web page also gives access to recent data on all the sources currently being monitored.

F.D. Ghigo


As noted in the last issue of the Newsletter, the 140 Foot Telescope will gradually be phased out as a user facility in 1997. Starting in January 1998, only a few projects connected with space VLBI observations will be scheduled so that efforts at Green Bank can concentrate on bringing the GBT into operation.

There are already a substantial number of 140 Foot proposals in the queue waiting to be scheduled, so we are setting a cutoff date of 03 March 1997 for receipt of any new proposals to use the Telescope. After that date the only new proposals that will be considered will be for observation of time-critical phenomena such as comets or supernovae.

By May 1997 everyone who has a proposal in the 140 Foot queue will be appraised of their project's status and given likely dates for scheduling. Removal of receivers from the 140 Foot for refurbishing prior to their installation on the GBT will begin a few months later.

F.J. Lockman


Project Phoenix of the SETI Institute began observations with the NRAO 140 Foot Telescope in October of 1996. Two observing sessions were completed in 1996: a one-week session in late October and a two-week session in early December. The observations are designed to detect CW signals in the direction of Sun-like stars. Each candidate star is tracked as long as it is above the local horizon. During this time, the entire passband of the receiver (1.0 - 1.8 GHz at L-band and 1.7 - 3.0 GHz at S-band) is swept in 10 MHz chunks with a spectral resolution of about 1 Hz at the Phoenix spectrometer. Detections are checked using a "two star protocol." If a signal is detected from a candidate star, a reference star is observed at the same frequency. A signal is usually attributed to radio frequency interference if it is also detected with the reference star observation. System integrity is checked with observations of space probes such as Pioneer.

Project Phoenix will eventually employ two telescopes in its observations. A primary telescope is used for signal detection, and a secondary telescope is used for verification of detections in real time. In the project's Australian campaign, the Parkes 64-meter antenna was the primary telescope while the Mopra 22-meter antenna was the secondary. In the US, the 140 Foot is the primary telescope, and a 30-meter class telescope in Woodbury, Georgia will be used as the secondary.

A variety of tasks had to be completed by the Green Bank staff before the Phoenix observations could be made. The control room of the 140 Foot Telescope was reorganized to accommodate Phoenix computers and IF racks. Fiber optic cables were run from the telescope to the Jansky laboratory to establish Phoenix's own local and wide area networks. The Phoenix receiver, which was designed and built by CSIRO, was packaged in an NRAO-standard front end box so that the receiver could be installed on the telescope. The Mobile Research Facility (MRF), a shielded container that houses the Phoenix spectrometer and other detection equipment, was set on a concrete pad to the west of the telescope. Optical fibers were run between the MRF and the control room. Almost everyone on the Green Bank staff had a role in the installation of Phoenix equipment. Personnel in the machine shop, plant maintenance, 140 Foot operations, and electronics were particularly helpful in getting Phoenix operational under a very tight project schedule. The costs of NRAO's support are recovered under the terms of an agreement recently concluded between the NSF and the SETI Institute.

M.M. McKinnon



Launch of VSOP, the Japanese Space VLBI mission, is now scheduled for February 1997. The NRAO Space VLBI Project based at the AOC has essentially completed all the enhancements to the VLBA correlator, and to AIPS, required for VSOP observations. Development of operational and user support is well advanced, and both are expected to be complete by the time the first in-orbit checkout interferometry is attempted, sometime in March or April.

Although the VLBA correlator was designed to support the VLBA's primary mission of ground-based VLBI, future requirements for Space VLBI were a design consideration, and were accommodated when it was possible to do so without additional cost to the ground-based VLBA project. As a result, only the following modifications to the correlator were necessary. Most were completed in the two years since the correlator became operational for ground-based observations.

The wavefront model computation was generalized to include the ephemeris for an orbiting element. Corrections to the recorded time tags for errors measured in the two-way phase transfer loop from the ground-based frequency standard were implemented, accommodating delay errors varying as rapidly as 1 Hz. Model computation was enhanced to support the large and rapidly varying delay and phase rates arising in both these features. The potentially large residual fringe rate window required by imperfect orbit knowledge was addressed by several measures: a baseline-dependent integration interval was introduced to limit the rapid correlator readouts to the ground-space baselines only; software support for the transversal filter in the correlator's back end was completed; and new, high-speed data storage disks and DAT drives were installed to finally reach the correlator's specified sustainable output data rate of 0.5 MB/s. The correlator's Playback Interface was generalized to read the Mark 4 format written by new equipment at some EVN and DSN stations. All these enhancements have now been completed.

A number of fairly comprehensive tests of the new correlator capabilities also have been completed. The orbit ephemeris was demonstrated by a "Kitt Peak in Space" test, in which this mechanism was used to reproduce the "orbit" of one VLBA station around the terrestrial rotation axis, in an otherwise normal correlation of a VLBA observation, yielding results comparable with those obtained using the conventional ground-based model. "Simulated Space VLBI Experiments" use a round-trip link to the Surfsat test satellite to impose realistic delay variations on a frequency standard, which then replaces the normal reference in a VLBI observation at a nearby radio telescope. The delay variations are measured at the tracking station, and then corrected at the correlator. Generally positive but not completely satisfactory results have been obtained in the tests performed so far, at two of the five VSOP tracking stations. An excellent overall demonstration of Space VLBI capabilities was achieved by recorrelating one of the decade-old TDRSS experiments, the first and still the only actual Space VLBI observations ever performed. These tapes were written in the now obsolete wide-track Mark 3 format, and the orbit ephemeris, logs, and time corrections all had to be synthesized, but it was possible to reproduce these historic fringes. Finally, a "dual offset" test, still underway, computes a model for the VSOP orbit, which is then added to the conventional model for all stations in a normal VLBA observation, in order to detect potential flaws which may be revealed if this large and variable offset fails to cancel out completely.

It also has been possible to compare spacecraft orbit models with some of the other VLBI correlators expected to process VSOP observations. The comparison is based on the Interferometer Model Table included in the standard FITS format adopted by all the correlators, and requires only that a simulated VSOP correlation be run using identical spacecraft orbit files, source coordinates, and physical constants. A first comparison, in collaboration with personnel at the S-2 correlator at DRAO, Penticton, has achieved agreement at the level of about 10 picoseconds of delay. Comparison with the VSOP correlator in Mitaka is pending.

AIPS, too, required only a limited set of enhancements to support Space VLBI observations. Since these data are written by the correlator in an extension of the existing FITS interface to AIPS, all the general data-manipulation facilities of AIPS are available. Several new features had to be added to cope with the extreme characteristics of Space VLBI observations­- both the angular resolution and the isolation of the orbiting antenna from the others­and the scientific goals of the VSOP mission. New tasks were written to implement baseline- oriented fringe-fitting and fringe-rate mapping, and a new, interactive source model-fitting program was developed.

These new AIPS facilities have been tested with ground-based VLBA observations, and have proven useful in a number of extreme cases. In addition, a Space VLBI data simulator has been written, which is being used to test and quantify the performance of the new tasks.

Operations support also is fairly well developed. A total of 691 new VLBA-type thin tapes have been procured, accepted, and transferred to self-packing reels. This quantity is intended to accommodate the incremental requirement imposed by VSOP observations on the VLBA and other NRAO instruments, and the entire VSOP requirement at the three DSN 70-m antennas, and at Green Bank and the three DSN tracking stations. Formats for the numerous schedule, log, and calibration files that must be exchanged have been adopted by all VSOP mission elements, and a project-wide agreement on the locations of these files and deadlines for their production is well advanced.

Our user-support staff already is in place, and have been contributing substantially to much of the testing mentioned above. Procurement of high-performance computing hardware to support visiting Space VLBI observers is expected to be completed in January 1997.

J.D. Romney


Proposals for VLBI Global Network observing are handled by the NRAO. Global network sessions currently planned are:


Bands Proposals Due
12 Feb to 05 Mar 1997 0.7 cm, 6 cm, 18 cm 01 Oct 1996
21 May to 15 Jun 1997 1.3 cm, 6 cm, 18 cm* 01 Oct 1996
08 Sep to 01 Oct 1997 1.3 cm, 6 cm, 18 cm, 3.6/13 cm 03 Feb 1997
03 Nov to 27 Nov 1997 1.3 cm, 6 cm, 18 cm, 3.6/13 cm 02 Jun 1997

*On the EVN, this also includes 5 cm line observations

It is expected that European VLBI observing in 1997 will be dominated by observations with the VSOP satellite. Further information about Global Network VLBI observing may be transmitted on the VLBI e-mail exploder (send subscription requests to

It is recommended that proposers use a standard cover sheet for their VLBI proposals. Fill-in-the-blanks TeX files are avail-able by anonymous ftp from, directory proposal or via the VLBA web page. Printed forms, for filling in by typewriter, are available on request from Betty Trujillo, AOC, Socorro.

Any proposal requesting NRAO antennas and antennas from two or more institutions in the European VLBI network constitutes a Global proposal. Global proposals MUST reach BOTH Network's Schedulers on or before the proposal deadline date; allow sufficient time for mailing. In general, fax submissions of Global proposals will not be accepted. Proposals requesting use of the Socorro correlator must be sent to NRAO even if they do not request the use of NRAO antennas; proposals for the use of the Bonn correlator must be sent to the MPIfR if they do not request the use of any EVN antennas. For Global proposals, or those to the EVN alone, send proposals to:

	R. Schwartz
	Max Planck Institut fur Radioastronomie
	Auf dem Hugel 69
	D 53121 Bonn

For proposals to the VLBA, or Global Network proposals, send proposals to:

	National Radio Astronomy Observatory
	520 Edgemont Road
	Charlottesville, VA 22903-2475

B.G. Clark



Recently completed installation of /2 Walsh function phase switching of the 200 MHz reference frequency used in the first LO has reduced image responses near 1600 MHz by more than 25 dB (VLA Elec. Memo 224). This has nearly eliminated the 1665-1667 MHz image problem for main line OH observations caused by 1533-1535 MHz signals from Inmarsat satellites as reported in the 01 October 1996 Newsletter. It also has considerably reduced spurious signals below 1500 MHz which are caused by Forest Service transmitters emitting above 1700 MHz. The 200 MHz phase switching combined with suppression of harmonics of the 200 MHz phase detector in the X band LO have reduced the 1400 MHz internal RFI by more than 30 dB (VLA Test Memo 200). This should allow continuum observations with narrow bandwidths containing 1400 MHz, and also allow spectral line observations around 1400 MHz by excising the channel containing the residual 1400 MHz RFI.

W.D. Brundage and D.S. Bagri


Configuration	Starting Date	Ending Date	Proposal

BnA		24 Jan 1997	10 Feb 1997	01 Oct 1996
B		14 Feb 1997	27 May 1997	01 Oct 1996
CnB		06 Jun 1997	23 Jun 1997	03 Feb 1997 5pm EST
C  		27 Jun 1997 	15 Sep 1997 	03 Feb 1997 5pm EST
DnC		26 Sep 1997	13 Oct 1997	02 Jun 1997 5pm EDT
D		17 Oct 1997	12 Jan 1998	02 Jun 1997 5pm EDT
A		30 Jan 1998	18 May 1998	01 Oct 1997 5pm EDT

The maximum antenna separations for the four VLA configurations are: A-36 km, B-11 km, C-3 km, D-1 km. The BnA, CnB, and DnC configurations are the hybrid configurations with the long north arm, which produce a round beam for southern sources (south of about -15 degrees declination).

Approximate Long-Term Schedule

Q1 Q2 Q3 Q4
1997 B B,C C D
1998 A A,B B C
1999 D D,A A,B B
2000 C C,D D A
2001 B B,C C,D D

Observers should note that some types of observations are significantly more difficult in daytime than at nighttime. These include observations at 327 MHz (solar and other interference; disturbed ionosphere, especially at dawn), line observations at 18 and 21 cm (solar interference), polarization measurements at L band (uncertainty in ionospheric rotation measure), and observations at 2 cm and shorter wavelengths in B and A configurations (tropospheric phase variations, especially in summer). They should defer such observations for a configuration cycle to avoid such problems. In 1997, the C configuration daytime will be about 08h RA and the D configuration daytime will be about 16h RA.

Time will be allocated for the VLBA on intervals approximately corresponding to the VLA configurations, from those proposals in hand at the corresponding VLA proposal deadline. It is anticipated that the VSOP satellite will be launched early in 1997, and a gradually increasing fraction of VLBA observing, eventually about 30 percent, will be devoted to observations with the spacecraft. VLBA proposals requesting non-NRAO antennas must be sent to the institution operating these antennas. Any proposal requesting NRAO antennas and antennas from two or more institutions affiliated with the European VLBI network is a Global proposal, and must be sent to the EVN scheduler as well as to the NRAO. Coordination of observations with non-NRAO antennas, other than members of the EVN and the DSN, is the responsibility of the proposer.

B.G. Clark


Very poor polarization performance has occasionally been seen in VLA observations at L band. In the NVSS observations taken during the summer of 1996, 11 of 85 days had noticeably greater instrumental polarization, and 2 days were so severely affected that these fields will need to be reobserved. Recent tests have confirmed the suspicion that this poor polarization performance is due to puddles of water in the L band lens. While drainage holes are in place, the off-axis mount of the lens makes them ineffective at some elevations (depending on the precise placement of the drainage holes in the lens of each antenna). Work is in progress to design and test covers for the L band lenses in order to eliminate this problem. In the meantime observers should expect poor polarization performance at L band during, or just after, significant precipitation at the VLA.

G.B. Taylor


The VLA will come to the big screen in theaters across the country when the Warner Brothers film Contact is released this summer. Moviegoers will see many sights familiar to VLA observers, including the VLA's antennas, control building, Visiting Scientist Quarters, and a very realistic replica of the Control Room. In late September, a large movie crew shot numerous scenes for Contact at the VLA site.

The film is based on the 1985 novel by Carl Sagan, in which radio signals from an extraterrestrial civilization are received at the VLA. Actors include Jodie Foster, Matthew McConaughey, James Woods, Tom Skerritt, Angela Bassett, and John Hurt. The director is Robert Zemeckis, an Academy Award winner whose previous movies include Forrest Gumpand Back to the Future. Sagan (prior to his recent death) and his wife Ann Druyan served as co-producers of the film, with Zemeckis and Steve Starkey.

The filming at the VLA was a major production, the biggest filming effort in our facility's history. Prior to filming, crew members from Contact made numerous visits to Socorro to negotiate with NRAO and to plan their operation. In August, a studio crew erected a temporary platform along the West Arm to house the replica control room.

During the filming week, a movie crew of more than 200 descended upon the VLA, along with a myriad of trailers, trucks, cranes, tents and other vehicles. Their equipment included cameras, audio and video systems, generating plants, and rainmaking apparatus. A pair of U.S. Army Blackhawk helicopters came for one scene, and the film production involved numerous area residents working as extras, as well as members of the NRAO staff.

NRAO provided a modest amount of array time on the VLA to the film crew, pointing antennas to enhance the visual appeal of movie scenes. Except for this, normal observing continued during the week of filming. In return for the array time, Warner Brothers made a significant donation for NRAO's public-education efforts. In addition, the studio paid for all NRAO expenses associated with the filming, such as labor, materials, use of Observatory vehicles, etc.

Any Hollywood film portraying radio astronomy will take some liberties with technical details, but the Contact crew consulted often with NRAO staff members to minimize this, within the bounds of their story line and creative judgement. We expect that this major film will enhance public interest in radio astronomy and in NRAO's instruments. We hope to use this heightened visibility as an opportunity to promote interest in science among young people and to explain some astronomy to them and to adults as well.

D.G. Finley


Over the last year the VLA calibrator manual has continued to grow. A major improvement has been the addition of Q band entries for 276 calibrators. Note however, that most of the calibrators are flat spectrum and rapidly variable, so the flux densities reported may not reflect the current level for a given source. Positions for 475 sources were refined on Oct. 30 1996 using the USNO list of Eubanks 1995-1. These positions are determined through many hours of dedicated VLBI geodetic observations and have typical positional errors less than 1 milliarcsec (see Johnston et al 1995, AJ 110, 880). This database also is the primary source of positions for the VLBA correlator. This update places the VLA and VLBA telescopes on the same reference frame. We stress that all users who desire accurate positions should use the J2000 coordinates, and use calibrators with a position code of "A" or "B." About 30 compact sources in the calibrator manual still do not have good positions (those with a position code of "T"). Work is in progress during this A configuration to improve the positions of these sources.

The calibrator manual is available on the WWW at A search engine for the calibrator database accessible via the WWW is available and has seen frequent use. The calibrator database is also accessible via the VLA Information System (vlais) under the VLA specific menu. The revised calibrator database is regularly incorporated into OBSERVE. For those who require a plain ascii text version of the calibrator database, it is available at Printed copies may be obtained by contacting Theresa McBride ( at the AOC. Comments or questions regarding this manual should be addressed to Greg Taylor ( at the AOC.

G.B. Taylor


The VLA Archive Project is progressing well. Apart from the period 1985 - 1987, all data from the very beginning (1976) to present have been converted and stored on Exabyte tape. Currently antenna files are being retrieved from the older tapes; due to a software problem (which has been fixed in the meantime) this was not done during the original data extraction. As soon as this intermezzo is over, we will return to the last leg: data from the period 1985 - 1987. The whole project is expected to be concluded some time in 1998. By then it is our intention to move all data from Exabyte to CD-ROM or similar technology.

Starting in October, Kiowa (an IBM 580 machine) was made available for general use. Previous to this, it had been reserved for VLBA correlator tests. In addition, we plan to upgrade all IBMs to Sparc Ultra-2 class dual-processor machines, which - per processor - are capable of an AIPS benchmark speed three times that of the fastest IBMs. After this upgrade we believe we are in a good position to handle very large VLBA and some orbiting VLBI projects. We currently are starting a test to study the suitability of a Pentium Pro 200 machine for the reduction of intermediate sized VLA and VLBA data sets. AIPS DDT tests have shown that an AIPS benchmark speed close to that of the IBMs is possible. The aim of the current test is to assess the suitability of PCs for AIPS data reduction by using them for a prolonged time for actual data reduction. If these tests are successful, PCs will be an attractive and moderately priced alternative to various intermediate class workstations.

At the AOC, we are in the process of implementing our planned network upgrade. These plans call for a switched network, consisting of a very fast backbone, and several supplementary switches designed to considerably lower the load on any part of the network. On top of this, the NRAO-wide "Intra-net" came in operation. This Intra-net should result in a much better connectivity between NRAO sites; this is especially useful for the various VLBA sites, but is also beneficial to all other NRAO operations. New 28.8K modems were purchased for the AOC to replace the 14.4K ones we have been using for the last couple of years.

We have dropped maintenance of the Microvax at the AOC. We plan to leave it running until something serious breaks. When that happens, this will mark the end of VAX/VMS at NRAO/New Mexico after 18 years of continuous presence! Also, the line printer, which relies on the Microvax, will disappear when the Microvax does.

G.A. van Moorsel


The most current version of observe is 4.0.1, dated 1996.12.31. This version of Observe supports offset cards (fast switching, mosaic, and tipping). It is available for SUN Solaris 2.4 and higher, SunOS 4.1.x, IBM AIX 3.x, and Linux. The updated calibrator list is included in the distribution.

The latest version maybe obtained via anonymous ftp from or from the WWW at

W. K. Young


We encourage visitors planning a visit to the NRAO in Socorro, NM, to contact the reservations office through one of the methods listed below.

1. WWW: The Visitor Reservation form can be found at The form can be completed interactively and sent directly to the reservations office. The interactive form is the preferred means of communication, and will usually produce the fastest response to questions.

2. E-mail:

3. Voice: (505) 835-7357.

4. Fax: (505) 835-7027.

Other useful information for visitors is found on the NRAO web page under Socorro.

- the schedule of NRAO holidays

- the Calendar of Events, published weekly

The holiday schedule is useful for planning your visit around days when NRAO staff are available to provide technical assistance.

M.T. Romero



Several significant milestones have been reached on the construction of the 3 mm array receiver for the 12 Meter Telescope. This array receiver will have eight channels arranged in four pairs of orthogonally-polarized channels. A dewar and radiation shield have been fabricated by the GB machine shop, and the dewar has been nickel plated, assembled, and vacuum tested in Tucson. The dewar has been outfitted with a JT-refrigerator, and heat straps have been made to the first pair of receiver inserts. The inserts are mounted in the receiver and will be cooled for the first time in the first week of 1997. This will be mainly a cryogenic test, as the inserts have not had the active microwave components mounted or wired yet. The cold-crossed grid will also be cooled in the first test of the cryogenic integrity of that assembly.

Almost all of the receiver parts have been made in the last year in Charlottesville at the CDL. New bias-T's and waveguide

couplers were designed, and all of the mixers and mixer backshort assemblies were built. In addition, the CDL machine shop fabricated all of the detailed components of the thermal assemblies for two inserts, and electroformed eight feed horns. All receiver insert parts for the first pair are ready, and assembly and wiring will begin soon.

We are aiming to do a test of the receiver on the telescope in the early part of 1997. Construction of the receiver frame has not yet begun, and will be a major focus of our work as we attempt to fit a lot of receiver electronics into a small slot on the telescope. The initial test will be only two channels of the eventual eight, and all of the tuning will be manual.

W.P. Shillue


The Tucson electronics group has been engaged in some experimental work for the electronic systems for the MMA. In particular, over the past three months, work has been ongoing on a local oscillator system that may offer major advantages for the MMA. To equip 40 antennas with a conventional local oscillator system such as is used today would be very expensive and complex.

A possible alternative is to generate the local oscillator at some central location and distribute it to all antennas. The availability of relatively inexpensive lasers with very high spectral purity makes possible the idea of transmitting the LO as a difference frequency between two lasers over an optical fiber. At each antenna, a high-speed photo detector converts the difference frequency into an electrical signal for use as the local oscillator. At first sight, it would seem very surprising to be able to mix together two optical signals with a frequency of oscillation of 375 THz (that's 375,000 GHz) and phase-lock the beat note to an external source to give the required line width of a fraction of a Hertz.

Over the past couple of months, the Tucson electronics group has been doing experimental work to determine the feasibility of such an approach. Using two rented lasers and a good deal of equipment on loan from the laser metrology system in Green Bank, we have been able to demonstrate that the phase locking of the two lasers to an external source is actually quite simple. During the course of this initial work, we found out that other groups were attempting to build the same kind of laser-based LO system and, in fact, prototype systems operating at 650 GHz have already been used to pump SIS mixers.

This whole new field of "Microwave Photonics" holds the promise of exciting applications to radio astronomy. Apart from the application mentioned above, the ready availability of such optical devices as spatial light modulators makes the prospects of array receivers at both centimeter wavelengths and millimeter wavelengths potentially much simpler than previously employed methods.

J.M. Payne


The 12 Meter User's Manual is now available on the NRAO Tucson home page ( Since this is an update to the 1990 manual, current and prospective observers are encouraged to consult this new manual for information on the 12 Meter Telescope system. Questions or comments about this manual are appreciated.

J.G. Mangum



In the past quarter efforts have focused on planning for Phase I, Design and Development, of the MMA that we are hoping to start in FY98. A number of interacting tasks have to be accomplished nearly simultaneously including completion of the conceptual antenna design, layout of the receiver packaging, and cryogenics. Coordination of these tasks will be done in 1997, again by means of the MMA Development Consortium (MDC) working groups. Reports of these groups can be found on the MMA web pages as they are issued.

Speaking of the web pages, a good way to stay current with the status of the MMA project and its recent progress is to examine periodically the headings under What's New with the MMA ( On these pages we will keep announcements of all new memos released, workshops, reports, and so forth. Rather than repeat this information here let me emphasize two recent MMA Memos that prospective users of the MMA will want to read.

Memo 164, a report of the MMA computing working group, may be of special interest because it outlines software goals for the MMA that are considerably more inclusive than astronomers are accustomed to with software at array telescopes. Understanding that what is laid out in this memo is a suggested framework for the MMA software capabilities, it is not a set of software decisions. I hope that astronomers will consider it carefully in view of their needs and experiences and help refine the ideas presented there.

Memo 162 may also be of general interest. It addresses some of the physiological effects of work at high elevation based on the experiences of other high altitude observatories and on large-scale mining operations at altitudes above 12000 feet. Although the operational goal for the MMA does not require the astronomer to be present at the high altitude array site itself in order to use the MMA, and hence one need not be exposed to that environment, the question may still be of interest and this memo outlines various areas of concern.

Finally, let me remind you of the Atacama Array Workshop which is March 16-19 in Tokyo. Information about the workshop can be found on the MMA web pages. I will also be glad to answer specific questions.

R.L. Brown


The first meeting of the Scientific and Technical Advisory Group was held in Socorro, NM, November 4-6, 1996. Presentations were made to the group by members of the AIPS++ Project. The group, chaired by Robert Braun, expeditiously produced a report, which is available from The advice given by the group was generally very useful, but particularly so in helping decide development strategies for the next few years, and has already been taken into account in revising our plans for the release strategy. Following their advice, we plan to make two major releases, one in 1997 and one in 1998. The first is designated as a "Limited Public Release" since it will contain only a subset of the functionality expected to be present eventually. The second release, in 1998, is expected to have a more rounded set of capabilities. The limited public release is preceded by an explicit "beta" release. The goals for the various releases are as follows:

Beta Release Due Early 1997

Targeted to consortium sites and a few friendly astronomers;         
Contains basic environment, tools, synthesis applications (including 
  polarization self-calibration and Briggs NNLS deconvolution algorithm);
Will get early exposure for AIPS++.

Limited Public Release Due Mid 1997

Interfaces should be fixed at this point;
Open release, targeted to both astronomers and programmers;
Contains basic environment, tools, synthesis  applications;       
GUI frontends to major DOs and applications;
Will provide environment for code development.

AIPS++ V2 Due Mid 1998

Synthesis package (including some functionality for VLBI);
Single dish package;
Image Analysis package;
General toolkit;
Visualization toolkit and applications;
Contributed code.

The code development environment scheduled for the LPR may be released in a separate, intermediate release if necessary.

The project is very active in a number of different areas, both short-term for the beta release (such as alpha testing of the synthesis code) and longer-term (such as development of an image display library). The beta release is on schedule for late January. For more detailed information on the current and planned capabilities of the AIPS++ system, see the latest project report, covering the period 1 July - 30 November, 1996, at

T.J. Cornwell


As announced in the previous NRAO Newsletter (No. 69, 1 October 1996), the NRAO proposed to install a frame relay intra-net for its internal computer communications.

By the end of 1996, the installation of the intra-net was almost complete. The first circuit implemented was to support Project Phoenix between Green Bank and their facility in Woodbury, Georgia. This was finished in November. Shortly thereafter, the NRAO traffic at both Charlottesville and Green Bank were converted to use the new frame relay service. This provided increased performance of access between Green Bank and the Internet. In December, Socorro and Tucson plus five VLBA stations were added.

By and large the conversion was uneventful. Some work still remains­fine tuning of the circuits and access plus operational procedures­but the increased network connectivity between the NRAO sites is very welcome. NRAO users who access Observatory computers from outside should notice a greatly improved response to Green Bank. Access to other sites may be slightly improved, since the external access no longer competes with internal NRAO traffic.

G.C. Hunt


NRAO is acquiring a number of advanced workstations to augment and replace the Observatory's aging public workstations. The primary function of these new machines will be to support data reduction by visitors to NRAO sites. The new hardware being purchased includes ten Sun Microsystems Sparc Ultra 2's and one Digital Alpha work-station. Two of the machines will go to Tucson and the 12 Meter, one to Green Bank, two (including the Alpha) to Charlottesville, and six to Socorro. The Sun machines have dual processors with a clock speed of 167 MHz; the AIPSMark93 for a single AIPS process on the Sun machines is above 8. The Dec Alpha has a single 400 MHz processor, and is estimated to have an AIPSMark93 of about 11. Each of the machines will have at least 384 MBytes memory and at least 24 GBytes of disk space. These 10 machines represent a very substantial increase in the computing facilities for visitors at the Observatory, and should help ease access to computing facilities for users and visitors. Shipment is expected in January 1997; the new hardware is likely to become available for visitors starting in January or February. Procedures for reserving workstations will remain the same at each NRAO site. Details of the AIPS DDT test suite and benchmarking information on various computers may be found in

Disclaimer: Please note that neither NRAO's procurement decisions nor the published test results constitute an endorsement of any particular vendor by NRAO or Associated Universities, Inc. (NRAO's parent organization).

R.S. Simon


The 1997 meeting of the NRAO Users Committee will be held 05-06 June 1997 in Tucson, Arizona. The Committee Chair this year is Roger Foster (NRL). If you have issues you would like the committee to discuss, or areas of concern, please communicate them to Roger or any of the committee members.

R.L. Brown


Until recently, the NRAO had provided PV-WAVE for general purpose mathematical and image manipulation. We have now decided to replace this with IDL, which has long been standard in some areas of the astronomical community. This change is in recognition of the widening gap between the two products. Recent astronomical data reduction packages available in IDL are not immediately portable to PV-WAVE, plus users familiar with IDL find the conversion to use PV-WAVE inconvenient.

The NRAO has a limited number of floating licenses. If you wish to use IDL during a visit to the NRAO, please contact the local computer system manager for details.

G.C. Hunt


The NRAO offers a summer research experience for undergraduates program, with positions at all of our four major sites (Charlottesville, VA; Green Bank; WV, Tucson; AZ, and Socorro, NM). Research positions are available for those with interests in astronomy, computer science, or electronics. Students obtain an opportunity to perform original research resulting in publications, to observe with mentors at NRAO and other observatories, to build state-of-the-art electronic devices, and to build computer codes for telescope operation and data reduction.

Information and application forms have been mailed soliciting applications for research assistantships next summer. The majority of the assistantships will be offered to undergraduate students who are currently enrolled in U.S. undergraduate institutions and who will not receive their degrees before or during the summer of 1997. A limited number of assistant-ships may be available for graduate students or students from non-U.S. institutions.

For more information about the NRAO, visit our web page at You will also find a report on what the 1996 summer students did here (

The deadline for receipt of application materials will be 20 January 1997; notice of decisions will be sent by 01 March 1997. Forms are available from the WWW, from Department Heads, or by writing to:

	Summer Student Program Director
	National Radio Astronomy Observatory
	520 Edgemont Road
	Charlottesville, VA 22903-2475

H.A. Wootten


The NRAO Newsletter has been available on the WWW since 1 October 1993. It can be accessed from the NRAO home page ( or more directly at We believe that many Newsletter readers enjoy the convenience of the electronic version and the fact that it is available well before the typical arrival of the printed version. Accordingly, we are asking individuals currently on our mailing list to resubscribe if they wish to continue receiving a printed copy.

You may do so by sending e-mail to, or by returning the form below.

	J. Nance
	National Radio Astronomy Observatory
	520 Edgemont Road
	Charlottesville, VA  22903-2475.

We will continue to mail printed copies to institutional subscribers, such as libraries, and they need not resubscribe. The April 1997 issue will be the last printed version mailed to any individual who does not resubscribe.

The Editor

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