GBT News Update
5 December 2001
This autumn has seen the successful resolution of most of the remaining mechanical issues with the antenna and considerable progress in scientific
commissioning. Good progress has been made toward introducing general spectral line observing capability, and the first spectral line
Early Science programs should be underway by mid-December.
Early this autumn, it was determined that the antenna braking system applied peak torque more rapidly than was
desirable. Consequently, a hard stop at maximum slew rate could overstress the structure. To protect the
structure until a solution could be implemented, the maximum slew rate was reduced to one-quarter of its design specification. This made antenna
movement slow, and impeded commissioning progress. To solve this problem, NRAO engineers, in cooperation with the contractor, Lockheed-Martin,
developed a new braking system. Most of the brakes were replaced with a newly-available Stearns brake that applies torque gradually over a
three-second interval. These new brakes serve as stopping and holding brakes. About one-fourth of the old, rapid-actuating brakes were left in
place, and serve as slowing brakes. This brake replacement was completed in mid-November, and was a
success. Slew rates in both azimuth and elevation have now been returned to their design specifications of 40
deg/min and 20 deg/min, respectively.
The stability of the GBT's azimuth track has been a major concern over the past year.
A reworking of the azimuth track was carried out by Lockheed-Martin last May and June in
which the track hold-down bolts were increased in length and diameter, and tripled in number. This work appears to be very successful -- track
wear-plate slippage was reduced by a factor of ~50 and there have been no bolt failures since the reworking. To assess the long-term viability of
this solution, NRAO let a consulting contract with VertexRSI for detailed modeling and analysis of the reworked track.
The consultant concluded that the solution is effective and the track should not require further work in the short term.
However, we will continue to monitor the track as a precautionary measure.
A new Antenna Coordination Group has been formed in Green Bank with overview responsibility for all aspects of the antenna structure and
electro-mechanical systems. This group is led by Tim Weadon, and has members from the GBT Computing, Electronics, and Telescope Operations
Divisions, as well as the NRAO structural engineering staff. This group has already been successful in addressing a number of issues with
the GBT.
As an interim measure until mechanical issues were fully resolved, control of the GBT has been carried out from the Servo Room located on the lower
level of the alidade structure. Both the operator and observer have worked from this room, which is rather noisy and inconvenient. As
mechanical, remote control, and monitoring issues are now being resolved, control of the GBT is scheduled to move to the main Jansky Lab control
room in December. The working environment is much better in this room, and assistance from the operational and technical staff is more readily
available.
Commissioning effort over the past few months has focused on the spectral line capability. A number of components are required for spectral line
observing, including local oscillator control and Doppler tracking, operation of the new GBT spectrometer, the synthesis of data files from
the modular control system, and a rather complete implementation of the new AIPS++/DISH reduction package. In September, we brought a "tiger
team" together from all the working groups to complete the implementation and testing of this capability. The team has worked very hard and been
quite focused in their efforts. We are now taking test spectral line data routinely with both the GBT Spectrometer and the Spectral Processor. By
mid-December, basic spectral line capability in selected observing modes and configurations should be in place and verified, and the first spectral
line Early Science observations will be initiated. Further work will continue during the winter months to implement and test all common
spectral line observing configurations. An HI spectrum of a galaxy, taken
in the 50 MHz bandwidth mode of the GBT Spectrometer appears below.
Commissioning and development activities needed for high frequency operation of the GBT are also being addressed. High frequency operation
requires the implementation of the Precision Telescope Control System (PTCS) that brings together the metrology system, active surface system,
focus-tracking, and finite-element model of the antenna. The PTCS group achieved an important milestone in November in which they were able to use
production hardware and software to achieve an absolute XYZ trilateration measurement of a position on the GBT structure to a few hundred microns
rms using a set of four laser rangefinders. The group continues to refine their techniques. The holography group also began testing instrumentation
for their measurement campaign. The first holography maps are expected in January.
A number of Early Science pulsar observations were done during the autumn using the Berkley-Caltech Pulsar Machine (BCPM) and the Spectral Processor
in pulsar mode. A VLBI "ties" experiement was done using the GBT and the Green Bank 20 Meter antenna to refine the coordinates of the GBT. Another
bi-static radar observation is scheduled for early December.
P. R. Jewell
A five minute on-off observation of the galaxy UGC8024 taken with the GBT Spectrometer. The observation, taken in the Spectrometer's 50 MHz, 8192 channels mode, was processed by AIPS++ and has no baseline removed. Even though the data were taken during the day, the spectrum is essentially flat. The small dip at 1420.4 MHz is due to the difference in Galactic HI emission between the on- and off-source positions.