GBT Status Update

3 March 2003

Observing and Commissioning News 

The GBT continues to move toward routine observing status.  The percentage of total telescope time scheduled for observing programs was about 30% in the autumn, is up to 46% in March, and should grow through the spring.  During the winter months, commissioning of the K-band (18-26.5 GHz) system was essentially completed.  We are now checking out observing programs at K-band, and expect to schedule a number of these in late winter and spring.  The priority, single-bank modes of the Spectrometer have now been confirmed and are available for use.  Checkout of the multi-bank modes, which allow multiple IF inputs, is underway.  We plan to install the Q-band (40-50 GHz) receiver in early March for initial engineering checkouts. 

Over the past few months, significant telescope time, engineering, and scientific effort have gone into investigating spectral baseline shapes.  The GBT is expected to have inherently good spectral baselines because of the unblocked optics that eliminate many of the usual sources of standing waves and reflections.  The staff is working to ensure that electronic stability and other sources that affect baseline curvature match the performance of the optics over the wide spectral bandwidths available to GBT observers.  The group studying baselines has made excellent progress in isolating many sources of baseline curvature.  Detailed investigations continue, and they are now in the process of documenting their findings, implementing a number of improvements and mitigation schemes, and planning for further work.

Since K-band and Q-band observing depends on good weather, we have implemented a program of primary and backup observing.  In this system, a block of several days is assigned during which high frequency observing and low frequency backup programs are scheduled.  Since statistically, about half the days during the winter months are suitable for K-band observing, the potential K-band observing block is about twice the required time.  Each day, based on the weather, a decision is made on whether to proceed with the high frequency or low frequency program.  Once the required K-band time is satisfied, the remaining time will default to the lower frequency program or another K-band program.  If the K-band observer falls well short of his allocated time at the end of the block, then additional time may be assigned at another date.  This system increases the odds of success for the higher frequency program, while conserving total telescope time through use of approved, lower frequency programs.  This is a manual method of dynamic scheduling.  In the longer term, it is our intention to move to a more automated, queue-based form of dynamic scheduling.

Engineering and Development News

In early February, the electronics staff completed RFI suppression modifications to the feed arm servo system.  The feed arm servo, as originally installed, has been a major source of RFI at longer wavelengths.  In addition, we are refitting the shielded windows of the GBT Control Room.  Poor shielding by the windows, owing to dissimilar metal corrosion, meant that the Control Room was also a major source of RFI.  This project will be completed in March.  Together, these two projects should significantly improve the RFI environment at the GBT at L-band and lower frequencies.  RFI is a serious challenge to radio astronomy, in general.  At the GBT, we now have a group working full-time on RFI and Quiet Zone issues, and have significantly expanded overall resources devoted to this area.

One of the key development projects for the GBT is known as the Precision Telescope Control System (PTCS), which is the integrated system that will allow the GBT to operate at 3 mm wavelengths.  This project is essential to achieve many of the scientific objectives of the GBT, and to support a suite of instrumentation that should be available in about two years.  The project team has developed a proposed architecture for the system that was discussed, and well-received, in an open meeting with Green Bank scientific and technical staff in mid-January.  They plan a formal conceptual design review in early April.

Azimuth Track News

There have been new developments with the GBT azimuth track.  On 16 January, we discovered that one of the track wear plates had developed a significant crack on its end near the splice.  Since then, we have removed and carefully inspected all 48 wear plates.  Thirteen of the plates have cracks on their ends.  Four of these are deemed serious, either because of the size of the crack or its direction of propagation.  Only one plate – the first one detected – has a crack all the way through the plate, however.  The cracks on the other nine plates are mostly hairline and are not serious at present, and can perhaps be repaired.  The GBT remains in astronomical operation, but the cracks are being frequently and carefully monitored.  In addition, we have restricted azimuth wheel movement over the most serious cracks and operations at low ambient temperatures, until the most seriously cracked plates are replaced.

The NRAO engineering staff has taken a number of steps to address these problems.  As part of the process described above, all plates have been inspected for cracks both visually and with a magnetic particle machine.  The lower edge of each plate end has been chamfered to reduce future crack initiation sites.  Each plate has been shimmed to address the fretting wear problems described in previous news updates and, to provide better support at the ends.  We have placed an expedited order for several replacement wear plates that should be received in early March.  The three most seriously cracked plates will be replaced in-kind in March, and we will use some of the other plates for a trial modification of the track.  When the cracked plates are removed, we will submit one to metallurgical analysis that should help us to understand the problem and to mitigate further cracking.

These new developments have increased the urgency of implementing the recommendations of the azimuth track review panel that met last November in Green Bank.   As described in previous reports, the panel recommended a three-part program of analysis, trial retrofits and tests of the existing track, and development of concepts for a new track, should that be ultimately required.   The program of analysis and trial retrofits has been expedited, and we hope to have the first trial retrofit installed by the end of April.  This will consist of a reinforced and welded base plate with a bridging wear plate.  Some of the new wear plates described above will be used for this purpose.  This field trial will be thoroughly characterized and analyzed, and its performance will be closely examined while the telescope is in use over a period of 6-8 months.  We also expect to complete the other elements of the program recommended by the panel by the end of 2003, and be ready for a decision on how to proceed at that time.  In the meantime, the GBT will continue with astronomical operations.

                                                                                                Phil Jewell