Dr. Martha P. Haynes, a distinguished Professor of Astronomy
at Cornell University, on April 9th became the Interim President
of Associated Universities, Inc. (AUI). She was elected by the
Board of Trustees of this independent non-profit organization,
founded by nine northeastern universities, which operates the
National Radio Astronomy Observatory (NRAO) under a
cooperative agreement with the National Science Foundation.
Dr. Haynes, a Trustee for the past four years, will lead AUI
while it seeks a successor to Dr. Lyle H. Schwartz, whose
resignation became effective April 8, 1998.
In tendering his resignation, Dr. Schwartz explained, "For many
months, the Board has known of my plan to move on soon after
management of Brookhaven National Laboratory (BNL) passed
to Brookhaven Science Associates. For the foreseeable future,
AUI will be better served by a President from the astronomy
community, while I can contribute more in a position where my
background in materials science and engineering is more
"I shall always treasure my warm relationship with AUI and the
friends I made at BNL and NRAO," he observed, "and sadly
recall the strange sequence of events that made my hopes and
plans for BNL moot. Scientists everywhere deeply respect the
wisdom of the AUI Board and its commitment to strengthening
and encouraging the best possible science under conditions in
which all can take pride."
Dr. Paul Martin, Chairman of the Board of AUI and a professor
and dean at Harvard University, extended the Board's gratitude
to Dr. Schwartz for his remarkably effective performance
during a year of transition. "It comes as no surprise," Martin
noted, "given his success as a Laboratory Director at the
National Institute of Standards and Technology and at
Northwestern University. We wish him every success in his
"As AUI looks to NRAO and the future," continued Dr. Martin,
"we thank the stars that Dr. Haynes can step in. With her
experience as a researcher at many major optical and radio
astronomy observing facilities worldwide; her long association
with NRAO including service as Director of the NRAO facility
in Green Bank, West Virginia; and her thorough understanding
of the strengths of AUI management as an active member of the
AUI Executive Committee, she's the ideal person to take the
reins as we search for a permanent new President, restructure,
and forge closer ties with universities and the astronomical
In accepting the responsibilities of Interim President, Dr. Haynes commented, "This is an exceptionally exciting time for NRAO, within sight of the completion of the Green Bank Telescope and the anticipated initiation of the Millimeter Array project. The radio astronomy community has long held AUI in high regard, particularly because of the prominence, breadth and personal involvement of its Board membership. My acceptance of this interim position is a direct reflection of the AUI commitment to maintaining NRAO's preeminence in radio astronomy as we enter the 21st century."
P.C. Martin (AUI)
Effective March 5, 1998, Richard Simon joined the Millimeter
Array project management team with responsibility for
developing and maintaining the schedules, cost estimates, and
other planning and reporting requirements of this major new
Observatory project. I want to take this opportunity to thank him
for his service during the past five years as Assistant Director for
Rather than appoint a new Assistant Director for Computing, we
have instituted a new approach to the management of
Observatory-wide computing. An Observatory Computing
Council has been appointed to formulate and recommend
strategy and policy for NRAO computing. The members of the
Computing Council are: A. Beasley, A. Bridle, T. Cornwell,
J. Mangum, P. Murphy, G. van Moorsel, G. Hunt. Alan Bridle
will chair the Council. Ruth Milner has been appointed Assistant
to the Director and will serve as the executive officer of the
Council. Ruth will continue to be based in Socorro. The
membership of the Council is intended to be representative of
major software projects, operating sites, and the scientific staff.
Input from the user community is welcome at any time and
should be directed to Alan Bridle or any Council member.
Finally, since the beginning of the year Gareth Hunt has had responsibility for computing in Green Bank generally, in addition to his duties as head of computing for the Green Bank Telescope. Ruth Milner has been filling in for Gareth, as acting head of Charlottesville computing. With her departure for Socorro, Patrick Murphy has been appointed to this position.
P.A. Vanden Bout
In October 1997, a Tektronix Phaser 560 color laser printer was purchased for testing at NRAO Charlottesville. While the quality of the output is perhaps not as high as could conceivably be achieved, it is clear that it cannot be improved upon without spending three or four times as much money. In light of this fact, and that the 560 performed as well as, or better than, other models in its price range, in December we purchased identical printers for the other three main NRAO sites (Green Bank, Socorro, and Tucson). Advantages of the new printers include higher resolution, faster throughput, better color representation, and much lower cost per page, especially for hard copies. They will provide significantly better color printing facilities than were previously available.
Also in late 1997, NRAO purchased two DLT7000 tape drives.
This type of drive has considerably higher capacity and transfer
speed than the most commonly-used tape media (8 mm and
DAT), and may be useful for storing and transporting large
datasets. The two drives, located in Socorro and Charlottesville,
have successfully been used for loading and backing up large
datasets in AIPS. Unfortunately, one of the two drives
malfunctioned in February and had to be returned to the vendor
for repairs. Concerns about the robustness of the DLT
hardware and its media mean that we should also evaluate
other high-speed tape technologies, such as Exabyte Mammoth,
as the budget permits. However, the price of this equipment
currently prevents wide deployment within NRAO.
During 1997, a total of approximately 65 Sun SPARCstation 1s
and IPCs were traded in for Ultra 1/170s. This represents
substantial progress in the replacement of NRAO's aging
population of desktop workstations; however, there are still
roughly 110 Suns across the observatory which are at least five
years old. We are hopeful that this effort will continue in 1998.
One of the factors in the choice of upgrade path is system
performance as illustrated by the AIPS DDT benchmark.
Recent efforts in this area have yielded significant
improvements on Intel PCs running Linux. A detailed article on
this subject is included in the April 15, 1998 issue of the
Significant progress has also been made in other areas of
PC-based computing. In particular, NT servers have been
purchased in Socorro and Green Bank; one is also planned for
Charlottesville in 1998. Support staff at the three sites are
working together to create an NRAO-wide NT domain, to
facilitate central authentication, co-ordination of user services,
and filesharing between PC users at all NRAO locations.
Computing security is an ongoing concern. Recent
improvements in this area include the availability of "ssh" at
three of the four major sites, increasing use of "tcpwrappers" to
log outside connections so that unusual activity can be
identified, and a plan to purchase a site license for PC anti-virus
When the Year 2000 (Y2K) arrives, the potential exists for mgany computer systems, software, and 'smart' hardware containing embedded microprocessors to malfunction, if not updated or replaced by then. The convention of using two digits for the year instead of four has created a serious problem for date-aware software and hardware.
NRAO's project to identify and resolve Y2K related issues is well underway; all possible areas of risk, including use of date-aware software, communications and networking infra-structure, computing hardware, safety and environmental systems, embedded microprocessors, use of customized routines for financial tracking and reporting, and so on, were considered. No major surprises turned up. Since the Observatory's critical functions are not dependent upon large amounts of internally written, date-sensitive software, and since the Observatory has considerable hardware expertise to deal with the minor hardware problems associated with the Year 2000 transition, no major difficulties associated with Y2K are foreseen. All critical systems and functions at the NRAO, including AIPS, AIPS++, and the major telescopes, have either been tested already, or are scheduled for testing during 1998 to identify and correct any remaining small problems associated with Y2K. Contingency plans will be formulated in the event that critical outside vendors experience Y2K-related difficulties.
The AIPS++ Project now has an on-line newsletter, edited by
Bob Hjellming and Kate Weatherall. This is published monthly
and may be found from our home page off the NRAO home
page or directly from: http://aips2.nrao.edu/aips++/docs/
The AIPS++ newsletter will contain news on the project, accounts of various uses of AIPS++ by different groups, background information on the various capabilities of AIPS++ (such as Glish and the Measures system), and a Programmer's Corner (currently demonstrating the capabilities of Glish for convolution).
At the end of 1997, the rapidly aging IBM RS/6000 system
"polaris," which was for over six years the main general
purpose UNIX system and file server in Charlottesville, was
replaced by two SPARC Ultra 1/170's. One of these is now the
"new"' polaris and serves login accounts and general purpose
computing; the other is a dedicated NFS (file) server.
Significant consolidation of ftp service is planned for the
coming quarter. The separate aips.nrao.edu and ftp.cv.nrao.edu
servers will be merged (though the names will not change) and
served from a 4-processor Sparc-10 system. This should
improve the performance for anyone downloading AIPS or
other general files from the Charlottesville server.
Several Charlottesville Computing staff members are involved in an effort to make it easy for other NRAO sites (and the general Astronomical Community) to install Linux and AIPS, including plans to release the next version of AIPS on CDrom with "live" binaries.
Among the many functions of the NRAO library is that of being an archive for the PhD thesis research done by students on the NRAO telescopes. If you, or one of your students, made observations at the NRAO that became part of a PhD thesis, we would very much like to have that thesis included in the NRAO library holdings. Please send two copies (if possible), bound or unbound, to:
Librarian NRAO 520 Edgemont Road Charlottesville, VA 22903
While we would like to have all recent theses, we are also very
much interested in having a complete collection of theses: even
if your work was done long ago we would appreciate having a
copy of it. The more complete the archive of theses is the more
will be its value in the future.
I would appreciate your help in making this request known to others.
Shortly after the first of the year, Associated Universities, Inc.,
submitted to the NSF the Program Plan for the initial three-year
Design and Development Phase of the MMA. The Program
Plan outlines the tasks to be accomplished in the D&D program
and it illustrates the schedule for those tasks; staffing and
budgetary estimates are included. The MMA D&D project
goals include the following:
Definition and implementation of an organizational and management structure that will serve the entire MMA project.
The MMA Program Plan was reviewed by a NSF Millimeter
Array Oversight Committee (MOC) that met for this purpose in
late February. The report of the MOC is expected at the end of
March. The report will be used as input for a decision to be
made by the National Science Board in May regarding
authorization to expend the funds appropriated by Congress in
1998 for the MMA.
Discussions with European and Japanese groups interested in
partnering in a common large array project have led to specific
joint studies and actions. Some ideas are still evolving
stimulated recently by a meeting of the U.S. and European
technical working group heads held in Charlottesville in March,
and by a meeting of the U.S., European, and Japanese site
testing groups held in Tucson also in March.
Reports of the joint meetings, and news of all recent developments with the MMA, are being reported to the community via an experimental MMA electronic Newsletter. The MMA Newsletter, that includes hypertext links to the meeting reports and all other recent MMA documents, is being emailed to more than 300 individuals who have been involved with the MMA one way or another recently. If you have not received the initial emailing of the MMA Newsletter, and you are interested in being on the email list, please contact Kate Weatherall (email@example.com). The purpose of the Newsletter is to establish a dialog with the community--your views and opinions are solicited; as the project proceeds we welcome, and indeed rely on, your participation in the MMA decision-making process.
The report of the Millimeter Array Advisory Committee (MAC)
following their meeting of November 15, 1997, appeared in the
January 1, 1998, issue of the NRAO Newsletter. It can also be
found on the NRAO web site. Since that meeting of the MAC,
the Committee's recommendations have been considered by the
Observatory and this article presents the current position of the
NRAO on the recommendations. Let me begin by thanking the
Committee for assembling on rather short notice and for the
time and hard work they invested in the meeting.
The Committee endorsement of the potential merger of the
Millimeter Array (MMA) and the Large Southern Array (LSA)
was welcome. We are continuing to work with the Europeans
to achieve this goal, using the working groups in science,
antenna and systems, receivers and electronics, and
management. We are also, as the Committee recommended,
keeping our options open with respect to the Large Millimeter
Submillimeter Array (LMSA) project of the Japanese. A
renewal of our Memorandum of Understanding governing this
cooperation with the LMSA is being negotiated.
The Committee recommended against a heterogeneous array,
opting instead for a homogeneous array of antennas of common
diameter. That is the current NRAO plan. The Committee also
recommended that the design effort be concentrated on an
antenna diameter of 12 meters and that the "fall-back" issue
should not dominate further discussion of antenna size. Because
the National Science Foundation has required us to submit a
plan for the design and development phase of the MMA that is
specific, complete, and as independent of uncertainties
regarding potential partners as possible, we have adopted as the
baseline, stand-alone plan for the MMA a homogeneous array
of 10 m diameter antennas. This in no way precludes the
modification of the antenna size should partnership
arrangements be realized, nor does it indicate any change in the
interest in such partnerships. Ten meter diameter antennas are
the largest that are consistent with a stand-alone MMA, and,
therefore, the largest step we can take at this time toward
accommodating the wishes of both the Europeans and the MAC
for larger antennas in a merger of the MMA and LSA. Should
sufficiently concrete partnership arrangements be put in place
in the next year or so, and should our studies of a 10 meter
antenna design be encouraging, it will still be possible to place
a contract for 12 meter diameter prototype antennas.
A fully international advisory committee is a necessity when partnerships with the European and/or Japanese projects have been realized. In the meantime, we have decided it would be better to proceed with a separate advisory committee that includes observer-members from the other projects. Accordingly, we will be reorganizing the MAC somewhat to reflect this, also adding Japanese observers. It is also likely that interested members of the Board of Trustees of Associated Universities, Inc., will be invited to attend the MAC meetings.
An international science meeting is under consideration.
Effective management structures for the partnerships we are
trying to achieve are a constant topic of discussion. There is a
joint MMALSA management working group that meets
regularly just for this purpose. The Millimeter Array
Development Consortium (MDC) is playing a very important
role, and we fully intend to continue this arrangement,
independent of partnership arrangements.
The Committee's concerns regarding software design and planning are noted, and it is planned to treat this area as equal to the other elements of the project.
P.A. Vanden Bout
Representatives from the LMSA (Japan), LSA (Europe), and MMA (USA) projects met in Tucson on March 10 and 11 to discuss site testing in the area of Pampa la Bola and Cerro Chajnantor. They were joined by colleagues from Cornell, the Smithsonian SMA, CTIO, and Gemini. There was a review of measurements over the last three years consisting of meteorology, atmospheric transparency, and atmospheric stability, with particular emphasis on recent measurements at submillimeter wavelengths. This was followed by discussion of future, possibly joint, efforts to study the physical structure of the atmosphere, in particular the height distribution of water vapor and turbulence, and to test schemes for radiometric correction of interferometer phase.
Those of you who scan the world wide web sites may have
noticed that the WENSS survey went public in the beginning of
January 1998. This marks an important point in the seven years
since we began observations for the survey in December 1990.
Although the last observations were done in March 1996, the
reduction was only completed in the summer of 1997. It has
taken us a further 6 months to make the survey available on the
web and to begin production of the CD-ROM and Atlas. The
web address where you can find the survey is:
What You Can Find in the Survey -- WENSS covers
declinations from +30 degrees and up and includes the galactic
plane at a resolution of about 1'. The survey frequency was
325 MHz, the bandwidth 4.5 MHz. The images (frames) of
6x6 degrees, at 1024x1024 pixels each, are located at the
central positions of the new POSS II survey plates. The frame
centers are spaced 5 degrees. There is a total of nearly 500
frames with some overlap around the transition to the polar cap.
The region above declination +74 degrees was done with the
broadband continuum backend with 8 bands of 5 MHz. The
central frequency in the polar cap was about 350 MHz. We
have also made images with a resolution of 4.2'; they will be
made available soon.
A description of the survey and catalog is given by Rengelink
et al. A&AS 124, 259 (1997). This description was based on
the reduction of about five percent of the survey, and more
complete information will soon be available on the web and in
What you will not find in the public survey products is
polarization information. We regret this very much since low
frequency polarization can be a very interesting diagnostic. We
have made beautiful polarization images of the galactic
foreground polarization in several nighttime mosaics but we
discovered that ionospheric Faraday rotation in the first two to
three years of the survey was often extremely large and
variable. Rotation of the plane of polarization of well over
180 degrees around dawn or sunset was no exception. What
was worse, however, was the fact that this was not well
predictable on the basis of available ionosonde data. A
polarization self-calibration scheme has been developed but is
rather laborious to implement. We therefore stopped the
reduction of the polarization images until we have better
ionospheric TEC data available and more manpower.
The original WENSS goals also included observations at
610 MHz (49 cm) for a significant part of the northern sky
away from the galactic plane. However, the pressure for time on
the WSRT was too large to achieve this goal and we opted for
completing the 325 MHz survey. Although we have acquired,
and reduced, most of the 49 cm data time (a total area of about
2000 square degrees), manpower restrictions prevent us from
making this data available in a uniform and reliable format.
Perhaps at some future date we will, but don't hold your breath.
Data Quality -- The noise level for the polar cap region was
about 2 mJy against 3-4 mJy in the middle declination zone.
However, the noise level can be considerably higher at places
(near extended 3C sources or in the Galactic plane). The
dynamic range also can vary substantially. Strong point sources,
even when as strong as 56 Jy (3C147), leave no image artifacts
around them. Slightly extended sources, on the other hand,
often do not self-calibrate well. Please note that the WENSS
flux scale, based on the Baars et al. flux of 3C286 (26.93 Jy at
325 MHz) differs from that in use at the VLA by about five
Those of you who have been involved with bulk processing of
aperture synthesis data know that several compromises have to
be struck. Automated procedures for the data reduction
(including calibration, data flagging, self-calibration, imaging,
and CLEANing) of up to 6000 pointings prevent the removal of
many obvious errors. Error patterns for east-west synthesis
arrays are easier to spot than those affecting e.g. VLA images
and the experienced eye can easily spot them. It is obvious that
we cannot guarantee the reality of everything there is to be
found in the survey. Consult an experienced observer in case
you are in doubt about a possibly interesting feature.
Each frame contains data obtained from the primary-beam-weighted combination of up to ten different pointings
(separated by 1.3 degrees). These pointings may occasionally
have been observed up to several years apart! Each pointing is
sensitive to an area of about 3 degrees diameter (the
HPBW=2.7 degrees). Errors can therefore vary substantially,
even within a frame.
Here Are Some General Warnings -- 1) Pointings where the
dominant source is extended at the 10"-60" level (i.e., just about
one synthesized beamwidth) will often produce artifacts close
to the source ("ears," etc.; see e.g., the fields around 3C69 and
3C123). These usually are the result of imperfect self-calibration. In an east-west array source structure and
systematic gain/phase errors can not be easily separated.
2) As is well known, standard self-calibration requires position
invariant complex gain errors. Non-isoplanaticity at low
frequencies violates this assumption. It has occasionally
affected the image quality leading to spiky patterns. Errors
resulting from non-isoplanaticity usually affect all BUT the
strongest source in individual pointings. A good illustration of
the transfer of ionospheric phase fluctuations is the field
containing the sources 3C380 and 3C381. However, sometimes
such spikes are the result of interstellar scintillation (e.g., note
the image around PSR B0329+54)!
3) The flux densities at the lower end of the survey may still
contain a systematic bias at a level of 1-2 mJy. This bias
probably varies from field to field, and even within a field,
depending on variable factors like the source distribution, the
depth of the CLEAN, the number of working telescopes, and
the number of 12h syntheses that went into the image. Because
the thermal and (sidelobe) confusion level is 3-4 mJy at best,
we have not systematically analyzed this bias.
4) Within the Galactic plane there are obvious problems with
missing short spacings, "solar fringes" and the areas around
Cyg A and Cas A.
It is obvious that this project required the work of many people so let me end by naming the people who have contributed to the survey. The project was a joint effort of the Netherlands
Foundation for Research in Astronomy and the Leiden
Observatory. George Miley was instrumental in getting the
survey started, arranging for financed manpower, and providing
stimulating support throughout the project. Yuan Tang, based
in Dwingeloo, tirelessly did most of the data reduction on one,
and later, two HP workstations. In Leiden, Roeland Rengelink,
and Martin Bremer in the first two years of the project, wrote
the software to produce a wonderful catalogue. And Roeland,
of course, wrote a whole thesis on WENSS, which should be
out shortly. What he learned has benefitted the whole project.
Huub Rottgering and Malcolm Bremer joined the project after a few years and have been a big help with many aspects, including the scientific exploitation. Finally, now that the job is almost done, David Fullagar and Ernst Raimond are helping to make the survey available to the worldwide community, via a web interface, CD-ROM, and Atlas. You will make their efforts worthwhile by using and/or buying the survey products.
A.G. de Bruyn (NFRA)
The final deadline for receipt of 140 Foot Telescope proposals was 30 March ,1998. From now on, the only proposals that will be accepted will be for observations of a unique event such as a comet, or for use of small bits of otherwise unassigned time. It is expected that there will be very little time available in the latter category. By mid May we plan to send out a status report to everyone who has a proposal in the 140 Foot queue giving final information about scheduling. At the end of 1998 the 140 Foot Telescope will be closed as a user facility, and certain receivers may be taken out of service even earlier than that if they are needed for GBT outfitting. There may be other limitations on telescope availability in the fourth quarter of 1998 if personnel are needed for the GBT.
In collaboration with Charlottesville and Socorro, we have
investigated the installation of an NRAO domain for Windows
NT. The primary domain controller will be in Charlottesville.
In Green Bank, we will have a Windows NT file server, which
will be used as a backup domain controller. The server will
serve AutoCAD, the Corel suite, and other common software to
all site PC users. In addition, it will provide individual disk
areas that will be routinely backed up. The tape units from the
server will also be available for complete system backups for
individual PCs. Two modern PCs running Windows NT are
available to visitors.
We have exceeded the data capacity of the disks used to monitor equipment being developed for the Green Bank Telescope. So, in preparation for full operation of the GBT, we have already increased the size of the disk for storage of
monitor data to 18 GB. Further, we have a need to track the
component maintenance history for the GBT, so we are actively
tracking the progress of the new product for this being acquired
for the same purpose for the VLA and VLBA in New Mexico.
Several Suns were moved when the GBT mockup was relocated to it's final location in the Jansky Lab Addition. In the process, we upgraded them to Solaris 2.6. We were able to solve the problem of supporting multiple monitors on the operators' computers. The Suns used for the OVLBI ground station were also upgraded. The rest of the computers will be gradually upgraded during the year. The most difficult transition will come when we upgrade the local server (sadira) from SunOS 4.1.2.
The accompanying photograph of the Green Bank Telescope
(GBT), taken in March 1998, shows that many of the basic
elements of the tipping structure are in place atop the alidade.
These include the elevation shaft, box structure, horizontal feed
arm, the elevation wheel, bull gear and drive, and most of the
main reflector backup structure (BUS). The 22 counterweight
boxes on the elevation wheel are completely welded, and ten
have been filled with concrete. The boxes are filled in a precise
order to safely counterweight the BUS as additional modules
are added into the structure. Primary elements of the servo and
electrical systems have been installed on the alidade and the
antenna is rotated frequently to aid in the erection process.
Currently, the electricians are running conduit and cable trays
from the elevation feed wrap out the horizontal feed arm.
The reflector BUS trial erection was completed on the 175 foot
square concrete slab at the telescope in June 1997. The BUS
consists of 7,652 different members and joints, weighing
approximately 2.3 million pounds. During the trial erection, all
joints in the BUS were aligned with a positional accuracy of
± 0.25 inches. When finished, the jacks at the top of the 110
scaffolding towers were retracted, leaving the BUS supported
by the 17 reinforced concrete piers on which it was built. The
deflected shape of the BUS under gravity load was measured to
verify the predicted values of the finite element analysis. Lifting
of its 22 modules onto the box girder began in October 1997.
Currently, 17 modules are on the box and the remaining five are
on the ground in the staging area in position to be lifted and
placed in the BUS.
The Contractor has recently brought in an additional tower
crane (Manitowoc Model 4100) to aid in the lifting and
positioning of the remaining modules. Individual modules will
be sequentially placed at the base of the main derrick, the
surface panel support actuators will be installed, and the
module will then be lifted and placed in the BUS. Modules vary
in weight between 25 tons and 74 tons, the rigging used for
lifting weighs an additional 40 tons, making the heaviest lift
114 tons. As the remaining modules are placed on the structure,
they will be held aloft by the derrick cranes while the
interconnecting beams between the modules are reinstalled for
both stability and accurate positioning of the neighboring units.
Completion of the reflector BUS and its permanent supports is
scheduled for early December 1998.
The upper 60-foot portion of the feed arm was trial erected at
the site, including the deployable prime focus boom, the prime
focus positioning mechanism, the subreflector, and the
subreflector positioning mechanism. The feed/receiver room,
which is located directly below the upper feed arm, has been
located nearby with the secondary focus feed turret in its roof.
The feed arm servo, which controls all of the above-mentioned
equipment, has been installed and tested along with some of the
NRAO monitor and control hardware. The first
photogrammetric setting of the subreflector surface and
calibration of the six subreflector "Stewart platform" actuators
has been accomplished to within 0.007" RMS. Additional
photogrammetric measurements are anticipated to allow final
setting of the subreflector to a tolerance somewhat better than
The 200-foot dual tower section of the vertical feed arm was
trial erected at the Contractor's fabrication plant in Mexia,
Texas. It was disassembled and shipped to Green Bank where
final assembly is now underway. All steel is on site, and the
first elements of the vertical feed arm have been assembled and
on the ground.
The 2,004 main reflector panels are now in production at the
Contractor's plant. Approximately 1,400 panels have been
manufactured. Nine of 44 tiers have been accepted by NRAO,
and painting has begun. Several of the larger panels have been
sent to the site where they were positioned on the structure as
a comprehensive fit and alignment check for the structure,
actuators, panels and cabling. Subsequent return of the panels
to the Contractor demonstrates that the shipping method is
acceptable and will not degrade the accuracy of the precision
The schedule for completion of the telescope is now mid-November 1999. Most of the delay of nearly one year is due to
problems in the permanent supports of the back-up structure.
The rest can be accounted for by allowance for bad weather and
schedule contingency. Close inspection of the 16 permanent
supports has revealed that most need to be rebuilt. This is due
to errors in design and/or faulty fabrication, specifically, poor
quality welding of the interior stiffness. Because these supports
are critical to the structural integrity of the GBT, they must be
rebuilt in spite of the consequences for the schedule.
On March 17, 1998 Comsat announced the sale of its subsidiary Comsat-RSI, the contractor for the GBT. The buyer, a Dutch company known as TBG, does not want the obligation of the GBT contract, and Comsat itself will finish the telescope. To do so, Comsat has retained key members of the current GBT project management team at CRSI and placed them under the leadership of John Evans, Comsat Chief Technical Officer. John is well known to many radio astronomers from his years with Lincoln Labs and as director of Haystack Observatory prior to becoming head of Comsat Labs. The sale is waiting for government approval.
R.D. Hall and W.H. Porter
Progress continues in observing and correlation of VSOP
experiments with the HALCA spacecraft. The observing load
on the VLBA now has reached about the maximum sustainable
for the L and C band proposals available, subject to the current
constraints on sun angle and maneuvering.
As of the end of March, a total of 75 VSOP experiments had been observed on the VLBA, including 53 scheduled from General Observing Time (GOT), spanning 516 hours, or more than 25 percent of NRAO's commitment of 2000 hours to the first Announcement of Opportunity. Ten of the GOT observations are to be processed at other correlators, while almost as many, nine observations, with non-VLBA ground arrays were or will be correlated here. Another 13 observations were abandoned due to known or unknown failures on the HALCA spacecraft or at tracking stations. Thus, at the end of March, the VLBA correlator had released 58 VSOP observations, spanning 583 hours and 6320 HALCA baseline-hours, with three experiments awaiting tapes or reconstructed orbits, a backlog of about ten days.
The transfer of responsibility for VSOP observations from the NRAO Space VLBI Project to the AOC Operations Division is essentially complete. Project personnel now have more time for careful analysis of the observations. One recent development of this type was an extensive analysis of the coherence on baselines to HALCA, using data from more than ten VSOP observations. When the spacecraft is farther than about an hour from perigee, coherence times are about 600 seconds at L band, and 300 to 500 seconds at C band -- in accord with pre-launch predictions. Closer to perigee, the coherence times are shorter by factors of two to three, apparently due to variations not modelled correctly in the orbit reconstruction. These results verify that all relevant components of the VSOP observing system, including the two-way phase-transfer links from the tracking stations and the application of the resulting time corrections at the VLBA correlator, are performing properly.
Work has commenced on the effort to produce a real-time
connection between the Pie Town VLBA antenna (PT) and the
VLA, which would double the effective resolution of the VLA
in its A configuration. We are fortunate to have Ron Beresford
working at the AOC, on leave from the Australia Telescope,
where he played a key role in the local implementation of fiber
optics. He is serving as the system engineer for the link
development. Information on the proposal and the goals of the
link were described previously in Newsletter No. 73. The
near-term goal is to produce first fringes over the link, using
only one intermediate frequency (IF) channel and an interim
system, near the end of 1998, or in early 1999.
A system-level block diagram for the VLA-PT link has been
completed, making extensive use of spare VLA modules at Pie
Town. Fringe rotation will be performed at Pie Town, where the
VLBA intermediate-frequency (IF) data also will be converted
to VLA-compatible frequencies before the analog data are
transmitted (by laser) over commercial fiber-optic lines to the
VLA. After the IF data are demultiplexed from the transmission
line, they will be fed into an existing VLA D-rack in much the
same way as the signals from any other VLA antenna.
Use of PT with the VLA requires a much larger delay capability
in the VLA correlator, due to both the extra geometric delay
and the 500 microseconds of delay for the optical-fiber
transmission between Pie Town and the VLA control building.
Modifications of the correlator delay cards that will
accommodate delays as large as 800 microseconds (vs. a
maximum required delay of 690 microseconds for the link)
have been designed, and prototypes are being produced. In
addition, the impact of the large additional delay on the
1.6-millisecond data-invalid cycle when the control building
"talks" to the VLA antennas is being studied. Modifications of
the sequencing of various tasks have been tentatively identified
in order to prevent invalid data from appearing at the correlator
when valid data are expected.
Several possible options have been identified for scheduling of the VLA-PT link, including modifications to either the SCHED or the OBSERV programs that are normally used to schedule the VLBA and the VLA, respectively. It is anticipated that there will be an intermediate stage of testing during which the most efficient means of scheduling will be explored, before a final decision is made about the scheduling interface that will be recommended for observers.
VLBA Amplitude Errors -- Significant amplitude errors
(typically 5-10%) have recently been identified in the raw data
produced by the VLBA correlator, mainly for
narrow-bandwidth [ 4 MHz] observations using oversampling
(sampling at integral multiples of the Nyquist rate). The
corrections required depend on the correlator FFT size, the
recording oversampling factor and fan-out used, and are
constant (and predictable) factors for most observations. These
errors result from FFT cycles being dropped from integrations
due to timing considerations in the correlator, and have been
present since November 1995 (VLBA correlator software
version 4.14) when the playback weight normalizations were
adjusted for these losses without a corresponding adjustment in
The AIPS task ACCOR (available since 1995) will remove
these amplitude errors. Most narrow-bandwidth observers also
use 2-bit sampling to increase sensitivity, and therefore already
run ACCOR by default. NRAO recommends that all observers
using narrow bandwidths and/or over sampling run ACCOR on
VLBA correlator data as part of amplitude calibration. If the
observing mode (e.g., oversampling factor) changes during an
observation, the solution interval for ACCOR must be
shortened sufficiently so that time-variable corrections can be
A planned restructuring of the FITS output of the VLBA correlator in late 1998 will address this problem, and we apologize for any inconvenience. If you have any further questions on this issue, please direct them to firstname.lastname@example.org.
Proposals for VLBI Global Network observing are handled by the NRAO. Global network sessions currently planned are:
|27 May to 10 Jun 1998||6 cm, 18 cm, 3.6 cm||01 Oct 1997|
|11 Nov to 02 Dec 1998||6 cm, 18 cm, 3.6 cm||01 Jun 1998|
Five centimeter spectral line observations also will be available
in November on the EVN. The September EVN session will be
devoted entirely to observations with the HALCA satellite.
It is recommended that proposers use a standard cover sheet for
their VLBI proposals. Fill-in-the-blanks TeX files are available
by anonymous ftp from ftp.cv.nrao.edu, directory proposal or
via the VLBA home page on the WWW. Printed forms, for
filling in by typewriter, are available on request from Betty
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
Networks 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 even 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 Germany
For proposals to the VLBA, or Global network proposals, send proposals to:
Director National Radio Astronomy Observatory 520 Edgemont Road Charlottesville, VA 22903-2475 USA.
Proposals may also be submitted electronically, in Adobe Postscript format, to email@example.com or firstname.lastname@example.org, respectively. Care should be taken to ensure that the Postscript files request the proper paper size.
The VLA has been fully equipped at 74 MHz (4-band) since
late January. In response to a general call for proposals made in
anticipation of this completed system, about 20 proposals,
totaling over 200 hours observing, were accepted for scheduling
during the current A configuration. This observing season will
end in late March, and the dipole feeds removed.
Preliminary results of the data are very encouraging. The
system sensitivity, although not spectacular by the standards set
at higher frequency, are nevertheless far better than ever
achieved at this frequency before. Figure 1 shows an image of
the center of the Coma cluster with 25 arcseconds resolution
and 20 mJy rms noise. This image is the result of approximately
7 hours' integration, with 26 antennas, one IF pair, and an IF
bandwidth of about 1 MHz.
Image fidelity, as measured on very strong objects, is similar to
that found at 327 MHz. Figure 2 shows the nearby radio galaxy
Virgo A, with a dynamic range of well over 1000:1. This image
was made with about 2 hours' data.
The problems associated with RFI have proven to be minor in
this configuration. Nearly all interfering signals are locally
generated, and are located at precise multiples of 100 kHz,
making them easy to remove from the data.
Calibration has proven fairly straightforward, as the ionosphere
has been in a relatively calm state. We find that phase
calibration is reliable for objects which are relatively
unresolved and unconfused and whose flux density exceeds
20 Jy. The angular coherence scale for phase transfer certainly
exceeds 2 degrees, and may be as large as 5 degrees for the
shorter baselines. Specific observing to measure this crucial
quantity has been made.
For weak sources where ordinary self calibration cannot work, and which are too far from a phase calibrator, phase transfer calibration, using a 327 MHz image to estimate the required phase corrections, is expected to suffice. Tests of this method are now beginning.
The sensitivity of the L band receivers below 1250 MHz is
currently being investigated to determine the feasibility of
observing the HI 21 cm line at redshifts up to z=0.30,
corresponding to a frequency of about 1090 MHz. This is a
brief report on work in progress of which the first results are
very promising. Figure 1 shows the RFI environment at the
VLA site between 1080 and 1250 MHz. There is an 80 MHz
wide window centered on 1190 MHz in which the RFI
environment is quite benign. These frequencies are used by
transponders carried by aircraft. Below 1150 MHz, the
spectrum is polluted by air traffic radars and the spikes at
1230-1240 MHz are GPS signals.
We probed this spectral range with the VLA-D to A array at
1217, 1187, 1167, 1137, 1117, and 1087 MHz. A reference
observation was made at 1387 MHz to provide the nominal
noise level. A 12.5 MHz IF filter was employed, and the total
bandwidth was 6.25 MHz with 63 spectral channels. Two
polarizations were received. At each frequency setting we
integrated for three minutes on 3C286 and for ten minutes on a
secondary calibrator of 0.7 Jy. Continuum subtracted
data-cubes of the secondary calibrator were constructed and the
noise was measured in the channel images.
The noise levels at 1217 and 1187 MHz were only about
50 percent above the noise at 1387 MHz. At 1167 MHz, the
noise is roughly a factor of two higher than at 1387 MHz, and
at 1137 MHz a factor of six higher. At 1117 and 1087 MHz
most of the data were flagged by the on-line system due to
excessive RFI and LO's lock problems. It should be noted that
the measured noise levels were normalized by correcting for the
missing flagged visibilities, the weighting of the UV data, and
the different system temperatures due to the varying elevations
of the observations. Also, seven antennas had poorly behaved
bandpasses at the lowest frequencies and had to be flagged.
To demonstrate the performance of the VLA at these
frequencies, we observed the known HI 21 cm absorption line
toward the brightest "Einstein ring" radio source PKS1830-211
at 1191 MHz, corresponding to z=0.193. The result is shown in
Figure 2. The detection is in accord with previous observations
at Parkes and the Australia Telescope, and the noise in this
spectrum is only 50 percent higher than would be expected at
the center of the VLA L band.
Further investigation is underway to delineate the sensitivity below 1200 MHz with finer frequency coverage, and to understand the bandpass shape at low frequencies. From these initial tests it is clear that useful observations can be made with the VLA down to 1150 MHz, and proposals for use of the low frequency part of the L band are welcome.
M.A. Verheijen and C.L. Carilli
The VLA site testing interferometer (STIfR) has been operating
since the fall of 1997. The STIfR is a device that measures the
tropospheric contribution to the interferometric phase using an
interferometer comprised of two 1.5 m dishes separated by 300
m observing an 11 GHz beacon from a geostationary satellite
(Radford, Reiland, and Shillue 1996, PASP, 108, 441). The
initial months of operation involved debugging problems with
the stability of the electronics due to thermal variations and
problems with the general STIfR infrastructure. These problems
have been solved, and the interferometer has been working
reliably since January 1998.
The data from the STIfR are meant to be used for real-time
decision making by the user when observing with the VLA at
high frequency. The data can be accessed via the NRAO home
page, under: http://www.nrao.edu/vla/html/PhaseMonitor/phasemon.html
The software requires an X-windows system for display of the
current data. Data consist of the measured phase time series,
from which is derived the root phase structure function, and a
power-law fit is made to the structure function to quantify the
seeing conditions. Also displayed are data from the VLA
weather station. The web page has a series of help files
providing equations for converting the STIfR standard data
products into VLA observing requirements.
As an example, values for the standard data products from the
STIfR on the cold, clear night of February 2, 1998, were: corner
time = 30 second (the time for a parcel of atmosphere to cross
the 300 m baseline), saturation rms phase = 4 degrees (the rms
of phase fluctuations on time scales > 30 s), and power-law
index of 0.7 (the phase fluctuation spectrum is a power law
with power increasing towards longer time scales, up to the 30 s
One can use equation 4 under the "How to Interpret Data from
the STIfR" help-file on the web page to derive the required Fast
Switching phase calibration cycle time for a particular
experiment. For instance, for a 43 GHz observation in A array
under the conditions cited above, a Fast Switching phase
calibration cycle time of 130 second would result in residual
rms phase variations of 20 degrees after calibration.
As a second example, during the day of February 19, 1998, the
power-law index and corner time were the same as above, but
the saturation rms phase was 15 degrees. In this case the
required cycle time in order to reach 20 degrees rms phase
residuals is 23 second. This cycle time is shorter than allowed
by the current capabilities of the VLA (absolute minimum cycle
time = 40 sec), and it would be recommended to switch to a
back-up project at lower frequency, unless an rms phase noise
larger than 20 degrees is acceptable or unless phase
self-calibration is possible.
In a future revision, the residual phase errors to be expected for various calibration cycle times will be calculated from the measured seeing conditions and made available in real time, along with improved help, and cosmetic improvements to the displays. Suggestions on improvement of the system are welcome (contact email@example.com or firstname.lastname@example.org).
C. L. Carilli and A. Roy
The Sixth Summer School in Synthesis Imaging will take place
Wednesday, June 17 through Tuesday, June 23, 1998. The
summer school will be hosted by NRAO and New Mexico
Tech, and will be held in the new Workman Center on the Tech
campus in Socorro, New Mexico. In addition to lectures
covering all aspects of radio interferometry, data reduction
tutorials on June 20 at the Array Operations Center (AOC) will
allow attendees to get "hands-on" experience with data
calibration and imaging for both VLA and VLBA data.
The timeline for the school is reproduced here. Further
information, including the complete program can be found at
http://www.nrao.edu/~gtaylor/synth98.html. Participants may
register electronically from this web page before May 15.
|15 September 1997||First Announcement|
|1 February 1998||Early Registration Due|
|15 February 1998||Second Announcement|
|15 May 1998||Deadline for Registration|
|1 June 1998||Contributions Due from Lecturers|
|17 June 1998||First Day of School|
|20 June 1998||Data Reduction Tutorial at AOC|
|21 June 1998||VLA Tour|
|23 June 1998||Last Day of School|
G.B. Taylor and C.L. Carilli
|Configuration||Starting Date||Ending Date||Proposal Deadline|
|A||20 Feb 1998||01 Jun 1998||01 Oct 1997|
|BnA||12 Jun 1998||29 Jun 1998||02 Feb 1998|
|B||02 Jul 1998||19 Oct 1998||02 Feb 1998|
|CnB||30 Oct 1998||16 Nov 1998||01 Jun 1998|
|C||20 Nov 1998||01 Feb 1999||01 Jun 1998|
|DnC||12 Feb 1999||01 Mar 1999||01 Oct 1998|
|D||05 Mar 1999||01 Jun 1999||01 Oct 1998|
The above schedule takes into account the increased length of
time for the B array to accommodate the extension of the
FIRST survey. To keep to a 16 month cycle through the
configurations, C, D and A configurations will be shortened by
one week below their normal durations.
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) and extreme northern sources (north of about 80 degrees declination).
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 1998, the B
configuration daytime will be about 8h RA and the
C 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. The VLBA spends about half of available observing
time in coordinated observations with other networks, with the
scheduling dictated by those networks. In decreasing order of
the time devoted to the observations, these are: HALCA space
VLBI, Combined Millimeter VLBI Array, Global astronomical
VLBI with the EVN, and geodetic arrays coordinated by GSFC.
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. VLBA proposals requesting only one EVN antenna, or requesting unaffiliated antennas, are handled on a bilateral basis; the proposal should be sent both to NRAO and to the operating institution of the other antenna requested. Coordination of observations with non-NRAO antennas, other than members of the EVN and the DSN, is the responsibility of the proposer.
Preparation of the Upgrade Design Document continues, with completion expected in June. This document will summarize all the proposed modifications to the VLA, along with estimates of the cost and implementation time scales, presuming funding availability. As many of the desired upgrade improvements are intimately linked to MMA development, many of these estimates will necessarily be approximations based on our current understanding of MMA developments.
Completion of the Design Document is tied to the design review meeting scheduled for June 29 and 30 in Socorro. This is an important meeting whose purpose is to permit the astronomical community to comment on our plans for the upgrade. All interested parties are invited to attend this meeting. If you wish to attend, please contact Terry Romero (email@example.com) to register.
R.A. Perley and R.A. Sramek
During the fall of 1997, equipment was purchased to fully
connect the outlying buildings at the VLA site (technical
services building, warehouse, maintenance building, antenna
barn and visitors' center) to our computer network. Previously,
employees in these buildings have had to rely solely on dial-up
access; this is no longer acceptable for their work. All of the
inside wiring is now complete. Burying of the optical fiber
between VLA buildings is currently in progress, and we expect
the network to be fully functional by mid May.
It was discovered that for older (pre-1988) data and certain correlator modes (line data only), visibilities in the VLA archive have zero amplitudes and phases. About 20 percent of line data are affected. The bug in the conversion program that caused this has been fixed. The effects of this should be transparent to the user community: when faulty data are requested we will re-run the corrected conversion program on the old nine-track tapes before sending the data out. After finishing the re-archiving process later this year, we intend to redo all affected data and reinsert them in the existing archive. We are still in the process of assessing the time required to do this.
We have decided to replace the software that is presently being
used to track VLA and VLBA module maintenance ("MAINT")
with a commercial PC-based product. An RFP for such a
system was sent to three vendors in early March. The bids have
been received; we will invite the top vendor(s) to give
presentations at the AOC during April and May. We expect
final selection of the software to follow soon after. The
Green Bank Telescope is also interested in using the new
system to track their component maintenance.
In February 1998, we introduced a new way to run Windows applications on a UNIX workstation. While being displayed on the local UNIX workstations in an X-window, the Windows applications run natively on the remote hardware of a Windows NT 3.51/WinDD server with dual 200-MHz Pentium Pro CPUs and 128 MB of RAM. Using five concurrent user licenses, a number of Windows applications is available, such as MS Office 97 and Corel Suite 7.
G.A. van Moorsel
In the early 1990s NRAO developed a simple menu-based information system for ASCII files covering Socorro, VLA, and VLBA topics. Portions of this information system, commonly known as VLAIS, have slowly been migrating to the Socorro, VLA, and VLBA home pages. In March, this migration was completed and telnet access to VLAIS from the home pages was disabled.
J.M. Wrobel, S.W. Witz, and M.P. Rupen
The telephone numbers at the Very Large Array Telescope Site
are scheduled to change on April 24, 1998.
The construction on the new correlator for the 12 Meter
Telescope continues. The 12 Meter correlator is identical to the
Green Bank Telescope correlator in many respects and the
experience gained in working on the GBT correlator is directly
transferable. The main difference is in the samplers. The
Tucson correlator will have only eight broadband samplers that
make up two quadrants versus the four quadrants + narrowband
samplers for the GBT correlator.
The following is a list of the modes that we are planning to support in the Tucson correlator:
|Total Bandwidth||Usable BW/Chan||Lags/IF||Freq. Res/IF|
|-8 Active Samplers|
|6.4 GHz||600 MHz||1024||0.781 MHz|
|3.2 GHz||300 MHz||2048||0.195 MHz|
|1.6 GHz||150 MHz||4096||48.83 kHz|
|0.8 GHz (.75)||75 MHz||8192||12.20 kHz|
|-4 Active Samplers|
|3.2 GHz||600 MHz||2048||0.390 MHz|
|1.6 GHz||300 MHz||4096||0.097 MHz|
|0.8 GHz||150 MHz||8192||24.41 kHz|
|400. MHz||75 MHz||16384||6.10 kHz|
|-2 Active Samplers|
|1.6 MHz||600 MHz||4096||0.195 MHz|
|0.8 GHz||300 MHz||8192||48.83 kHz|
|400. MHz||150 MHz||16384||12.20 kHz|
|200. MHz||75 MHz||32768||3.05 kHz|
The software for the new spectrometer, developed by Jeff Hagen in Tucson, will be similar to that developed for the GBT correlator. We anticipate installation of this new system on the telescope during the 1998 summer shutdown period.
M. Waddel for the Tucson Electronics Group
Note to 12 Meter Observers -- In order to make the best use of the telescope, prospective and scheduled observers are strongly encouraged to contact the Friend of the Telescope, Jeff Mangum, for advice regarding their proposed experiments. This is of particular importance for observers conducting 1mm Array and on-the-fly experiments given the complexity of these observing modes. Send questions via email to firstname.lastname@example.org or by voice to (520) 882-8250, ext.113.
J.G. Mangum and D.T. Emerson
The 8-beam 1 mm array receiver has been in routine use on the telescope since late January. The 4x2 beam cluster can map in both (RA,Dec) and (lII,bII) coordinates while tracking parallactic angle with an arbitrary position angle offset. The current performance of the receiver represents a substantial gain over the existing dual-beam system for making maps of extended regions. During this past summer and fall work on this receiver concentrated on solving problems with servo stability and mixer system temperature uniformity. These efforts by the Tucson electronics group have resulted in vastly improved total power stability and good mixer system temperature uniformity across the eight receiver channels. We anticipate demand for this improved instrument to increase for the coming fall and winter quarters.