In late June the last of the twenty-two modules which comprise
the backup structure of the GBT was raised into place. The figure
shows the completed BUS mounted on the box structure. With
this milestone achieved, one of the most complex and
time-consuming portions of the construction program is nearly
finished. There remains the completion of the welding of the
beams joining the modules together. Approximately 97 percent
of this "intermodule welding" is complete.
The primary effort now being expended at the site is aimed at
correcting the poor quality welds discovered in the interior
stiffeners in the permanent supports of the backup structure. The
rework of the permanent supports and the connecting of the
supports to the box will continue over the next several months.
After the permanent supports are installed the load of the backup
structure will be transferred to them, and the tipping structure can
then be rotated.
Additional work on the structure includes the preparation for the
installation of the surface actuator cables, the checking of the
shape of the backup structure on the temporary supports,
painting, and installation of electrical gear. The assembly on the
ground of portions of the vertical feed arm has been completed,
and it is planned to lift the first of the lower modules onto the
structure in late summer.
W.H. Porter and D.E. Hogg
140 Foot Telescope to Operate in 1999
The 140 Foot Telescope, which was previously scheduled to
close at the end of 1998, will continue in operation through the
first half of 1999. The regular suite of prime focus receivers
covering frequencies up to 5 GHz will be available, as will the
GBT receivers at the Cassegrain focus that cover 4-5 GHz (for
VLBI only), 8-10 GHz (cannot be used for VLBI) and
18-26 GHz. The available back ends include the MKIV
autocorrelator, the Spectral Processor, the Digital Continuum
Receiver, and the VLBA and S2 systems for VLBI.
In view of the extended period of operations of the telescope,
new proposals will be accepted. Proposals for time in late 1998
and the first quarter of 1999 must be received in Charlottesville
by October 1, 1998. Proposals to use the instrument in the
second quarter of 1999 should be received by January 4, 1999.
Further information on 140 Foot Instrumentation and
instructions on proposal preparation are available on the
Green Bank web site.
Status of the Millimeter Array
With the approval of the National Science Board, the three-year
Design and Development phase of the Millimeter Array has
begun. On June 1, 1998, much of the NRAO staff who are
assigned to the MMA Project were transferred formally onto it
so this provides an easily remembered start date for the formal
project at the NRAO. We are enormously pleased and very
grateful for the efforts of everyone at the NRAO and in the U.S.
community who worked to bring this about.
The Design and Development phase of the MMA is meant to
provide us with an opportunity to prototype representative
modules of some of the technically most demanding
instrumentation. A prototype antenna is the most visible
example of the effort but many of the other prototyping efforts
are equally challenging: these include prototyping SIS mixers
that are in fact MIC. SIS circuits on a single chip that
incorporate balanced image separation; 4K cryogenics that is
reliable and economical even when running at an ambient
pressure 70 percent of that at sea level; an optical fiber signal
transmission system with 16 GHz bandwidth; and a computing
environment that is designed from the outset to be developed,
used, and maintained remotely. A primary goal of the MMA
D&D effort is to allow us to establish a firm cost basis for the
construction phase of the project.
Approximately 20 people at the NRAO are now working on the
MMA project, about a third of these people are at each of the
Tucson, Socorro, and Charlottesville sites. By the end of the
year the number of staff should grow to more than 30 all
together; their efforts will be augmented by the design and
research efforts of another six people working at the OVRO and
BIMA arrays. Advertisements for open positions with the MMA
project can be found on the NRAO homepage
Personnel assignments to the MMA project at the NRAO have
brought about some changes that users of the Observatory will
notice. In particular, Brian Glendenning has transferred to the
MMA Project from AIPS++ where he worked for many years
directing the technical work on that project; Brian is now the
Division Head for all of MMA computing. Al Wootten has
assumed the role of MMA Project Scientist replacing Frazer
Owen. Frazer was the inspiration for much of what is now the
MMA, and we are very grateful to him for his seminal insights
The burden of organizing initial D&D work, assembling the staff
and making the facilities ready for the staff to work effectively,
has prevented much progress from being made with potential
international partners in the MMA or in that larger, combined
array that will subsume the MMA. However, in the weeks ahead
discussions and negotiations on possible partnership
arrangements will resume with a realistic expectation that a
mutually satisfactory, and beneficial, international partnership
can indeed be forged.
R. L. Brown
MMA Activities in Tucson
The impact of MMA development activities in Tucson is
growing and is welcomed by all. The site testing interferometers
in operation in Chile, designed and built by Tucson engineers
and scientists, continue to be operated and maintained from
Tucson. A submillimeter tipper is now operational at this test
site. Earlier this year we hosted an international gathering of
European, Japanese, Chilean, and U.S. groups to discuss existing
and future millimeter and submillimeter site testing activities.
MMA receiver development and production are scheduled to
take place in Tucson, using mixer blocks supplied from the
Central Development Lab (CDL). In collaboration with the
CDL, an orthomode transducer for the MMA has been built in
Tucson; this prototype is designed for 90 GHz, but the goal is to
extend the principle up to 300 GHz. MMA cryogenics design
and development are also centered in Tucson under the guidance
of Larry D'Addario. The MMA antenna group is based in
Tucson, headed by Peter Napier in Socorro. An experimental
laser local oscillator system is being developed in Tucson, and
if successful, potentially provides a simpler, cheaper, and more
reliable local oscillator system for the MMA; we will report on
the progress of this project in later Newsletters.
The additional staff in Tucson now supporting the MMA has already strained the available office and lab space; more MMA employees will be added by the end of this year. To accommodate this expansion, we are looking at various options of increasing the space available to NRAO at the University of Arizona.
NRAO-News: Introducing An NRAO Electronic News Bulletin
Since June 1981, the NRAO has disseminated news of activities
important to users via this Newsletter, which appears every four
months. The Newsletter is both sent as hard copy and appears
on the NRAO home page. We now plan to introduce a
complementary vehicle called NRAO-NEWS, which will be sent
via an e-mail exploder to a selected list of recipients initially
numbering ~1000. The format will be quite different from that
of the Newsletter: it will appear more frequently, on an
as-needed basis, and the items will be much more cryptic,
consisting of typically a couple of sentences and a URL path to
more complete information on the WWW. The URL may be
accessed by simply clicking on a link provided for each item.
There are several reasons for the new e-mail news vehicle.
(i) Since the number of people receiving the hard copy
Newsletter was diminished a year or so ago as people elected to
get the Newsletter directly from the WWW, we believe that use
of the WWW has not been as extensive as we had hoped; (ii) at
three-month intervals, the Newsletter is not always timely, and
is sometimes out of phase with important news; (iii) technical
matters in electronics or computing that can have an important
effect on observers (e.g., discovery of a calibration error at a
given telescope) may not be efficiently disseminated by the
Newsletter. In the latter case, such matters are described on a
regularly updated basis on the web sites of the individual NRAO
sites, but attention is currently not otherwise directed towards
users, who often miss them. NRAO-NEWS will deliver this
information in an active and timely fashion.
It is difficult to guess how often NRAO-NEWS will appear, but
we anticipate on average every six weeks. We are presently
preparing the lists of recipients and constructing the exploder.
We expect the first issue sometime before the end of the
summer. If you have not received the NRAO-NEWS Bulletin e-mail by the first of September and you are interest in being
on the subscription list please subscribe via
http://www.cv.nrao.edu/nrao.news.html or send e-mail to:
firstname.lastname@example.org. and include in the
body of your message "subscribe NRAO-NEWS." We
encourage comments from recipients.
The NRAO Newsletter will maintain its current format and
purpose; it will continue unchanged.
I am pleased to announce that the Jansky Lecturer for 1998 will
be Bernard F. Burke, the William A. M. Burden Professor of
Astrophysics at the Massachusetts Institute of Technology.
Bernie Burke is one of the most familiar names in radio
astronomy, with a record of research that extends over a large
variety of topics, from his co-discovery of Jupiter radio bursts to
more recent work in the search for gravitational lenses and in
very long baseline interferometry from space. He was for many
years a member of the Board of Trustees of Associated
Universities, Inc., until he joined the National Science Board.
Professor Burke will present a lecture entitled "Radio
Telescopes: Reaching for the Astronomical Frontiers." Tentative
dates are: Green Bank on October 21; Charlottesville on October
20; Tucson on November 6; and Socorro on November 4. The
New Mexico Symposium will be held on the same date as the
Jansky Lecture in Socorro. Astronomical researchers from
NRAO and neighboring universities and laboratories are invited
to present their research in 10 to 15 minute talks at this
P.A. Vanden Bout
An area of software effort which will ultimately affect observers
is the purchase of a PC-based computerized maintenance
management system, described in the Socorro computing
section. This software is required to track repairs, scheduled
maintenance, and engineering changes to instruments. It will
replace the existing MAINT program used in Socorro, which
was developed in-house several years ago. It is likely that the
same package will be adopted in Green Bank as well. We view
this software as a long-term investment in the reliability of the
VLA, the VLBA, and the GBT.
In 1997 NRAO was able to upgrade approximately 65 old Sun
workstations to new models. This year we are again going
through this process. With the significant improvements
achieved earlier this year in AIPS performance on Intel x86
platforms, Linux has become an attractive option for many
computing tasks. Accordingly, about three-quarters of the
roughly 50 UNIX upgrades we plan to purchase in the next few
months will be Linux systems; the rest are SPARC Ultra 10s.
After these purchases, there will be approximately 65 Linux
systems NRAO-wide, compared with about three times that
many Suns. The popularity of Linux is growing rapidly at many
academic institutions and among our user community, but
SPARC/Solaris continues to be the most common platform for
AIPS use by a considerable margin. Due to the limited
availability of some of the commercial software that NRAO
requires, PCs running Linux cannot completely replace our
Suns; nor can PCs compete at the higher end of the workstation
market. We therefore anticipate that both environments will be
used in-house for the foreseeable future.
Charlottesville Computing Developments
A simple CD-ROM recorder was purchased during the quarter;
this device is now used both for generating the current release of
AIPS for the astronomical community, and for providing a
reliable long-term backup medium for staff and visitors. Interest
in AIPS on CDROM (with "live" binaries for Linux and Solaris)
has been quite strong and has kept this new machine busy.
In conjunction with the Observatory-wide initiative to upgrade
our oldest Sun workstations, the Charlottesville computing group
has worked hard to ensure that the Linux/Intel platform can be
widely deployed with a minimum of administrative overhead.
This includes generation of an "NRAO-tuned" Linux system that
can be distributed to any NRAO site that requests it.
AIPS++ Newsletter for June 1998
We are pleased to announce the publication of the June 1998
edition of the AIPS++ Newsletter at URL:
Older copies of the newsletter are available at:
To alert those interested to each new issue of the AIPS++ Newsletter, we have inaugurated an AIPS++ newsletter mailing list. When a new edition is published, we will send an announcement to this mailing list. To subscribe to the mailing
list, send e-mail to email@example.com with the following command in the body of your e-mail message:
If you ever want to remove yourself from this mailing list, you
can send e-mail to firstname.lastname@example.org with
the following command in the body of your e-mail message:
If you ever need to get in contact with the owner of this mailing
list, have trouble unsubscribing, or questions about the list
itself, send e-mail to
The Users Committee met June 4-5, 1998, in Socorro. The
meeting was chaired by Donald C. Backer, UC Berkeley.
Many thanks to the Director, Assistant Directors, and NRAO
staff for an informative meeting. What follows is a brief
summary of issues and concerns; our full report may be found on
the NRAO home page at the following URL:
GENERAL ISSUES - The Committee would like a brief report
at the beginning of each meeting from the Director on specific
questions and issues raised in the previous year's report. In
general the presentations at the meeting need a sharper focus on
user issues following general points of information. The user
community's independent voice in issues of funding, spectrum
management, and international development is weak. We suggest
that delegates from U.S. national observatory user committees
could form an AAS committee to make themselves heard.
VERY LONG BASELINE ARRAY (VLBA) - The VLBA is
coming of age. We recommend development and deployment of
a "first look" imaging procedure for simple experiments to
encourage wider use. This would necessarily include some
phase referencing experiments as these are increasingly popular
ASTRONOMICAL IMAGE PROCESSING SOFTWARE
(AIPS) - The continuity and modest improvement of AIPS is
essential for synthesis imaging with the VLA and VLBA. The
current "simple-image" development in AIPS++ is an important
building block toward a useable system. A release of AIPS++
in early 1999 is a critical milestone to achieve. Small AIPS++
workshops around the country may be useful when sufficient
general purpose imaging capability is available.
SPECTRUM MANAGEMENT AND RFI - The large effort in
battling with Iridium is important on the one hand, yet
frightening on the other hand owing to it being just the tip of an
iceberg. The millimeter spectrum is the new frontier and early
protection of the Chile site is warranted. The Green Bank staff
have set a model for minimization of on-site RFI. How can
radio astronomers be more effective in preserving our capability
to observe in the future?
GREEN BANK - We encourage the NRAO to do all it can to
keep the Green Bank Telescope (GBT) construction and the
commissioning on track and to introduce the GBT monitor &
control interface to external users as soon as, and as widely as,
possible. Delay of the GBT leads to the need for continued use
of the 140 Foot Telescope in 1999. Use of the GBT
spectrometer on the 140 Foot for a limited set of observations
would provide both important scientific results and early input
on its GBT use.
VERY LARGE ARRAY (VLA) - The C-Short configuration
needs further assessment of its impact and consultation with past
users before being made a permanent substitute for the
C configuration. Continued advertising of the status of the
C configuration in newsletters is important. The looming
Modcomp problem needs prompt attention. We enthusiastically
support the VLA Upgrade Plan. We urge NRAO to continue
consulting with the community about instrumentation projects
that are proceeding through MRI channels as these are driven by
funding partners and their goals, not by widely-discussed
TUCSON - Use of the 12 Meter Telescope for 3 mm and 1 mm
VLBI and MMA development are reasonable requests.
MILLIMETER ARRAY (MMA) - We are pleased to hear that
a project scientist has been appointed for the MMA Project and
look forward to the opening of communication channels with
users regarding design issues. A number of "user questions"
that will feed into the Project were raised and one or more of the
advisory committees need to appoint members who will look out
for the general user (i.e., those not associated with NRAO,
OVRO, or BIMA).
Recent funding leads to the need for a "full steam ahead"
approach in the U.S., coupled with a need to settle issues about
foreign partners and their concerns.
OTHER - The resources of the Central Development Laboratory
have been stretched lately by the competition between the needs
of NRAO telescopes and the development of amplifiers for
outside users. No one is completely satisfied, and therefore
careful attention to future demands--current telescopes, MMA,
and outsider users--is needed.
The role of NRAO staff at the various sites in the area of public
outreach was raised in 1997 and needs to be revisited in 1999.
Documentation of past and planned efforts is requested for our
D. C. Backer, Chair
University of California, Berkeley
Major Summer Shutdown Projects at the 12 Meter Telescope:
The 12 Meter Telescope will be shutdown for approximately
five weeks starting on July 6. We will follow this with an
approximately three week period of system tests before resuming
regular observing. During these periods we will complete a
number of repair, maintenance, and upgrade tasks which include:
(1) Installation of a new digital correlator.
(2) Installation of a new prime focus control system.
(3) Dome inspection and repair.
(4) Maintenance on the central cold load calibration system.
(5) Improvements to the receiver and IF system to increase continuum sensitivity.
J.G. Mangum and D.T. Emerson
On-The-Fly Analysis at NRAO Tucson
In order for us to provide assistance with the analysis of OTF
data, visiting observers can make arrangements to use the NRAO
Tucson downtown computing services for their OTF analysis.
Observers who might benefit particularly from the use of the
NRAO-Tucson computing system are those with limited OTF
analysis experience, or those whose home computing resources
are strained in handling the quantity and processing needs of
OTF data. In particular, on-site assistance from the 12 Meter
scientific staff in the analysis of OTF data may prove helpful.
If you are interested in visiting Tucson to analyze 12 Meter OTF
data, contact Jeff Mangum (email@example.com, 520-882-8250
ARISE Science Workshop
A two-day workshop focused on the science to be done with
the proposed ARISE future space VLBI mission will be held in
Green Bank on August 19 and 20, 1998 (Wednesday and
Thursday). ARISE is included in the long-range roadmap for
the Structure and Evolution of the Universe theme of NASA.
The purposes of the workshop are to increase community
involvement in determining the science to be done with the
mission, and to develop a more detailed scientific case for
ARISE for the upcoming astronomy decade committee. There
will be a few presentations summarizing the mission and its
technical status, but the remainder of the workshop will be
devoted to the scientific goals of ARISE rather than to technical
details of the mission. The workshop will be organized around
several key science areas to be addressed by ARISE, such as
gamma ray blazars, high-frequency polarization VLBI of AGN,
and water megamasers. It is anticipated that there will be
sessions on each of these topics, including one or two invited
talks of about 30 minutes in length, plus some shorter
contributed papers and considerable time for discussion. These
talks would present the scientific rationale for particular
observations, and any specific technical requirements on the
mission that would enable those observations. About one and
a half days will be devoted to the science sessions.
The remaining time will be used for tours of the GBT
facilities, and of the Green Bank Earth Station that is presently
used for VSOP tracking. For more information about the
workshop, including the tentative schedule and a registration
form, please see
http://www.nrao.edu/~julvesta/arise_gb.html. You may
also contact the meeting organizer, Jim Ulvestad, at
firstname.lastname@example.org. Contributed papers should be proposed on
the registration form or by contacting the meeting organizer.
For reservations and accommodations, please contact Becky
Warner in Green Bank at email@example.com.
Fringes at K-band between the VSOP mission's HALCA
spacecraft and the VLBA were detected on April 9, in an
observation recorded on March 22, downlinked from HALCA
via the Goldstone tracking station. This test observation
exploited the appearance of an unusually bright, bursting water
maser in Orion-KL. The successful detection occurred during an
11-minute interval near perigee, during which projected
baselines varied from 400 to 2700 km. Previous estimates of the
K-band system's performance were confirmed and refined:
system temperature of about 1500 K and effective aperture of
0.1 square meter. This was the first operation of the overall
VSOP system at K-band, and a variety of subsystems such as the
tracking station's phase transfer loop and the correlator's orbit
implementation all appeared to work satisfactorily at that
frequency. Further information on this result is available on the
NRAO Space VLBI Project page of the NRAO web site.
Data from VSOP observations must often be distributed to
multiple users, as a result of the mergers and data sharing
decided by the Scientific Review Committee. In many cases,
some users receive a subset--by array, time, or polarization--of
the full observation. It has taken some time to develop the
software necessary to extract these subsets, and for the mission
Science Operations Group to specify completely the
distributions to be done. In April, it became possible to start
processing this backlog, and by the end of May it had been
brought up to date. It is expected that future observations will
be distributed immediately in as many copies and/or subsets as
necessary. This process makes the mission more productive
scientifically, and in any case is less burdensome operationally
than the alternative of making and correlating multiple, partially
J. D. Romney
VLBI Network Call for Proposals
Proposals for VLBI Global Network observing are handled by
the NRAO. There are usually four Global Network sessions per
year, with up to three weeks allowed per session. The Global
Network session currently planned is:
11 Nov to 01 Dec 1998
6 cm, 18 cm, 3.6 cm
01 Jun 1998
Five centimeter spectral line observations will also be available
in November on the European VLBI Network (EVN). The
September EVN session will be devoted entirely to observations
with the HALCA satellite.
Proposals will be due October 1 for the first two 1999 Global
It is recommended that proposers use a standard cover sheet for
their VLBI proposals. Fill-in-the-blanks TeX files are available
via 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 Lori Appel,
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 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 firstname.lastname@example.org or
email@example.com respectively. Care should be taken to ensure
that the Postscript files request the proper paper size.
B. G. Clark
Summarized below are some major VLBA developments during
the first half of 1998 of potential interest to array and correlator
users. For further information, please contact the undersigned or
those identified with specific topics. Other major VLBA
developments are described in accompanying articles.
TAPE CONTROL: Automatic tape allocation was exercised
extensively by AOC staff in January and February, then adopted
for all recordings at the VLBA and VLA starting in March. This
was an important step toward multiple projects per tape at the
VLBA and VLA, which is targeted to commence in July for all
projects to be correlated in Socorro. At that time, tape control
will become fully automated. Tape changes will no longer be
synchronous across the VLBA, but will tend to change during
normal working hours, and thus at different times depending on
time zone. Observers will no longer be able to predict where on
the tape their data will be written. This step will enable us to
schedule short observations efficiently and will provide greater
flexibility for scheduling Targets of Opportunity. (In the original
VLBA concept, much of the time on the array was to be reserved
for allocation on short timescales, in queue-based or dynamic
scheduling mode; we expect to start doing some of this next
"CALIBRATION" TRANSFER: The VLBA correlator currently
distributes visibility data to observers, but does not attach to
those data the ancillary tables needed for calibration and editing.
The process of building such tables and attaching them to the
visibility data is known as "calibration" transfer. The initial tests
of calibration transfer by AOC staff revealed serious problems
applying transferred tables. The basic difficulty is that current
table formats used by the VLBA correlator have diverged from
those specified in VLBA Memo #108. The revised table formats
will be defined and documented, for use in Socorro and at other
correlators. The AIPS group intends to mend calibration
transfer, including various FITLD and general table problems
uncovered, for the October 15, 1998 release. (Contact - C.
ASPEN SERVER CHANGES: VLBA operations now
consolidates and archives all information for each observing
project in one location on the aspen file server in Socorro. This
scheme makes it easier (a) for anyone, including the observer, to
access the complete history of a project by stepping through files
related to the scheduling, observing, correlation, and quality
control stages; (b) for the project's AOC contact to assist with
quality control and to diagnose any problems revealed; and (c)
for all ground antennas (radio or tracking) to obtain performance
feedback from the VLBA correlator. (Contact - M. Stanley)
1000TH PROJECT: The VLBA correlator released its 1000th
scientific project April 28, 1998 not quite four years after its first
release on May 4, 1994 and six years after the correlator detected
its first fringes on May 6, 1992. For these 1000 projects, a total
length of 2.0 Gm of tape was recorded at the ten VLBA stations
and played back in Socorro. This length is 50 times the
equatorial circumference of the Earth, five times the mean
distance to the Moon, and half again the diameter of the Sun.
(Contact - J. Romney)
DOCUMENTATION: "The VLBA Observational Status
Summary" and "The SCHED User Manual" were updated.
These VLBA documents can be accessed from the NRAO home
IN PROGRESS: Efforts continue on 512 Mbps and the rewrite
of the operations software.
A. J. Beasley and J. M. Wrobel
VLBA 3 mm Status
Prototype VLBA 3 mm dual-polarization receivers, covering
nominally 80-90 GHz, are now in use at Pie Town and
Los Alamos. System equivalent flux density (also known as Tsys
in Janskys) is about 8000 Jy. NRAO participation is routine in
the Coordinated Millimeter VLB Array, scheduled by Haystack
Two more antennas, Fort Davis, TX, and Mauna Kea, HI, are
expected to receive prototype receivers by year's end.
First VLBA-only fringes were reported in late April, and can be
viewed at: http://www.nrao.edu/vlba/html/3MM/3mm.html.
A Major Research Initiative proposal was submitted by NRAO
to the National Science Foundation earlier this year, which, if
approved, would permit completion of VLBA receivers for the
3 mm band.
More discussion of the current status, calibration, and proposed
frequency coverage can be found in VLBA Scientific Memo #17,
reachable from the NRAO home page at:
Contact: Vivek Dhawan, firstname.lastname@example.org.
The VLBA Correlator Pulsar Gate
Pulsar gating is now available on the VLBA Correlator. The gate
is operational for nearly all combinations of observing and
correlating modes. The gate has a pulse phase resolution of 1024
points with the pulsar phase updated at 16 microsecond intervals.
The delays in the pulse arrival times due to dispersion are tracked
across the individual VLBA bands and between bands separated
in frequency. Observers may set the gate open and close phases.
Gating operation has been tested using a number of VLBA pulsar
observations including polarization runs. For a complete
description of the pulsar gate, please see the VLBA Pulsar Gate
Observer's Guide on the VLBA web page,
For the past year, NRAO has offered to provide astrometric
quality source positions in response to requests from users. The
positions are measured during the joint NASA/USNO/NRAO
18-station geodetic/astrometric observations that occur about
every two months and that mainly serve other purposes. A
modest number of requests have been received and, so far, all
were for sources that have adequate strength and for which no
previous astrometric positions have been accepted. Of these,
positions for nine sources, with formal errors between about 0.1
and 3 mas, have been returned. Several more are still in the
With this note, we remind the community of the availability of
this service. It has proven especially useful in cases where the
calibrator source to be used for an upcoming VLBI phase
referencing observation has a poorly known position. But,
please do not wait until too close to the time the position is
needed to make a request. It takes a minimum of two to four
months to obtain a position because of the wait between
observing runs and the amount of time it takes to schedule,
observe, fringe search, correlate, release, and reduce these large
runs. Also, do not request too many sources; only a small
number can be added to each run.
If you have a source for which you need a position, first check
that it really qualifies. We have received a number of requests
for sources that were too weak, or for which good positions were
already available. A source should have at least 0.1 Jy at 2.3 and
8.4 GHz and there should be some indication, either from past
observations or from a flat spectrum, that it can be seen with
VLBI. A good place to check for known source positions is at
the USNO Web site
http://www.usno.navy.mil/neos_vlbi.html. Also, VLA
positions for a large number of sources, good to 1255 mas, are
available by anonymous ftp at ftp.aoc.nrao.edu in file
pub/sources.jvas (the JVAS survey--see the article by Joan
Wrobel in this Newsletter).
Requests for positions may be made by e-mail to
email@example.com. Please include the following information:
1. A short scientific justification.
2. Source names.
3. Source coordinates good to at least 0.5 arcseconds.
4. The expected flux densities at 2.3 and 8.4 GHz.
5. Evidence that there will be compact structure.
Requests for positions of more than ten sources will not be
considered. The requested sources will be compared with objects
in recent and proposed VLA and VLBA proposals so that
conflicts are avoided. Accepted sources will be observed as
soon as possible and positions will be returned by e-mail. The
results will be public domain immediately, and source positions,
and in many cases even images, will appear on the geodetic web
R. C. Walker
Radio Emission from Galactic and Extragalactic Compact Sources
The printed proceedings from IAU Colloquium 164, held during
April, 1997 in Socorro, New Mexico, are now in press:
1998, IAU Colloquium 164: Radio Emission from Galactic and
Extragalactic Compact Sources, Astronomical Society of the
Pacific Conference Series, Volume 144, eds. J. A. Zensus, G. B.
Taylor, & J. M. Wrobel.
Registered participants of the meeting will receive the
proceedings from the editors this summer, as each registration
fee included the cost of one copy. Others can order the
proceedings directly from the ASP via their web site
Everyone is welcome to consult the electronic preprints from the proceedings at:http://www.cv.nrao.edu/iau164 or http://www.mpifr-bonn.mpg.de/zensus/iau164 or via the NRAO home page at http://www.nrao.edu/, under "Meetings & Workshops," then select "IAU Colloquium 164."
The page numbers in the electronic preprints are identical to
those for the printed proceedings.
J. M. Wrobel, G. B. Taylor, & J. A. Zensus (MPIfR)
VLA Configuration Schedule
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).
Proposal Deadline B
03 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 A
19 Jun 1999
27 Sep 1999
01 Feb 1999 BnA
08 Oct 1999
25 Oct 1999
01 Jun 1999
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 C
configuration daytime will be about 18h RA and in 1999 the D
configuration daytime will be about 2h 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.
B. G. Clark
Nearly 150 people attended a symposium in Socorro entitled
"Interferometry and Imaging: Barry Clark at 60," (his birthday
was actually March 5, 1998), on June 25 and 26. Participants
included scientists from across North America as well as from
Europe, India and Australia. The sessions featured talks
outlining the development of high-resolution radio astronomy, its
impact on our understanding of the universe, and future prospects
Speakers laced their talks with personal reminiscences and stories from Barry Clark's nearly 40 years of involvement in interferometry. At an evening banquet, Barry was honored by numerous colleagues, and received 60th birthday gifts from Associated Universities, Inc., Mexico's UNAM, India's Raman Research Institute, and individuals.
D. G. Finley
With the July 1, 1998 update to the VLA online software, we
plan a change to the correlator continuum signal processing that
is incompatible with continuum gated correlator observations.
Gated spectral line observations will not be affected. In the short
term, a version of the continuum processing software that does
work for gated observations will be secreted away and can be
made available if requested. Requests should be made to the
undersigned, at least two weeks prior to observing. In the longer
term, we intend to unify the continuum processing systems as the
After the July 1 update, all ungated continuum observations will
benefit from "complex correlation" or will (ultimately) benefit
from "widebanding" efforts:
COMPLEX CORRELATION: In continuum mode the VLA
correlator produces twice as many products as are needed to
generate all polarization products for each baseline with a
bandwidth of 50 MHz. Historically we have only used half of
these products. D. Bagri (1998, VLA Test Memo 210) has
shown that combining the used and unused correlator products
increases the SNR by about eight percent for a bandwidth of
50 MHz and also leads to a measurable decrease in the average
WIDEBANDING: Alternatively, using all the products will
allow the VLA correlator to produce data for 100 MHz input
signals if the samplers can work effectively over that range.
D. Bagri (1998, VLA Test Memo 211) modified three antennas
to transmit bandwidths of 75 MHz and, with proper signal
processing, showed that the sensitivity increase was as expected
for the bandwidth increase. The widebanding of more antennas
will progress as funds become available.
K. Sowinski and J.M. Wrobel
The VLA re-archiving project finished reformatting and re-archiving data from 1986, and has now progressed well into
1987, its last year. In another couple of months, we will have
re-archived all previous VLA data from 1976 to the present. As
reported in the last newsletter, a bug was discovered in the
conversion program which, for pre-1988 data, caused zero
amplitudes and phases for certain observing parameters. In the
meantime, we have compiled a full list of affected observations.
After completing 1987, we will spend a number of months
reformatting all observations in this list using the corrected
Good progress was made with the installation of fiber optic links
at the VLA site. All the fiber is in place; we are currently busy
with the precise and time consuming task of terminating the
fibers. If this is done incorrectly, unacceptable loss of signal will
result. The link connecting the VLA Control Building and the
Technical Services Building is operational; connections to the
other main buildings is planned in the near future. We expect to
complete this project by the end of June.
After receiving bids from several vendors, and based on local
demos of their products, we selected a commercial PC-based
maintenance system to replace our locally developed product.
Primary use is at the AOC, the VLA, and the VLBA sites, but it
is not unlikely that Green Bank will adopt this system as well.
We expect to receive the product and the dedicated server it will
run on early this summer. We are currently developing an
implementation plan in order to make the transition from the old
to the new system as smooth as possible.
We have noticed that in the current Exabyte drive market there
is a tendency to move away from full support for low-density
tapes. We predict that in the near future we will have no choice
but to buy drives that can only write at high density (though
reading low density tapes may still be supported). This means
that the possibility for AOC visitors to write tapes at low density
is expected to gradually decline in the future. Scientists who
tend to use Exabyte tapes originating at the AOC are advised to
ascertain that reading tapes at high density is supported at their
An important personnel change took place. Ruth Milner, for many years Manager of Computer Systems, moved to a position related to NRAO Observatory-wide computing. It is hard to replace Ruth, but we are fortunate to have hired in her place James Robnett, who previously held a comparable position at New Mexico Tech.
G. A. van Moorsel
The Sixth Summer School in Synthesis Imaging took place from
June 17 through June 23 of 1998 in Socorro, NM. Attendance at
this school was a record-breaking 139 people from 13 countries.
The participants heard 28 lectures on interferometry, spent one
day in a data reduction tutorial and enjoyed an in-depth tour of
the VLA. A book containing the lectures will be edited by Greg
Taylor, Chris Carilli and Rick Perley and published by the
Astronomical Society of the Pacific (ASP). For periodic updates
on the status of the book visit the school web page at
G. B. Taylor
The full-time activation of the Iridium constellation of communication satellites in September 1998 will have a significant impact on VLA observations of the 1612 MHz transition of OH. Besides occasional interference when a satellite flies through the near sidelobes, the 90 millisecond uplink/downlink cycle of the Iridium constellation will continuously beat against the 102 millisecond calibration cycle of the VLA. When any Iridium satellite is above the horizon and transmitting (100 percent of the time after September 1998) this beating is sufficiently strong to corrupt the on-line system temperature corrections applied to the data and to cause all data to be flagged due to wild system temperature variations. For this reason, the normal calibration cycle must be turned off during all 1612 MHz observations (or any other observation that includes the frequencies between 1620-1630 MHz in the 50 MHz front-end used for measuring the system temperature). This will result in some degradation in the accuracy of the absolute flux density calibration. To turn off the calibration cycle a request should be made to the array operator to set columns 64 and 65 blank in a special Front End Parameters file (better known as the SYSLROT file). For an example of such an instruction, or those planning a 1612 MHz observation are encouraged to contact the undersigned (firstname.lastname@example.org).
G. B. Taylor
A Catalog of 2118 Compact Radio Sources in the Northern Sky
A catalog of 2118 compact radio sources was derived from the
Jodrell Bank - VLA Astrometric Survey (JVAS) of flat-spectrum
sources (Patnaik et al. 1992, MNRAS, 254, 655; Browne et al.
1998, MNRAS, 293, 257; Wilkinson et al., MNRAS, submitted).
Each compact VLA source (a) has a peak flux density at 8.4 GHz
50 mJy at a resolution of 200 milliarcsecond; (b) contains
80 percent or more of the total source flux density; and (c) has a
position known to a rms accuracy of 12-55 milliarcsecond. The
2118 sources are uniformly distributed in the northern sky at
galactic latitudes |b| 2.5 degrees.
Although these sources are primarily intended for use as phase
calibrators for the Jodrell Bank MERLIN, they will also be
suitable as phase calibrators for the NRAO VLA and can be
considered as candidate phase calibrators for VLBI arrays
(Peck & Beasley 1998, IAU Colloquium 164, 155) and the
NRAO MMA (Holdaway, Owen & Rupen 1994, MMA Memo
No. 123). Furthermore, compact radio sources close to the
galactic plane can be used to probe the interstellar medium,
through studies of scintillation, angular broadening, Faraday
rotation, and both molecular and atomic absorption. Compact
radio sources also are useful as navigation aids for spacecraft
missions to solar system bodies. Finally, masing conditions in
cometary comas can be examined by observing compact radio
sources during occultation events.
The catalog of 2118 compact radio sources is available via
anonymous ftp from host ftp.aoc.nrao.edu; cd to directory /pub
and get file sources.jvas. The file can be read by the NRAO
VLBI scheduling program SCHED.
J. M. Wrobel for the JVAS Team