Commissioning Status

GBT commissioning has been defined by three phases: operation to 15 GHz (Phase I), to 50 GHz (Phase II), and to 115 GHz (Phase III). We are now approaching the final stages of Phase I and II commissioning that will establish routine operation to 50 GHz. At the present time, we are focusing our commissioning effort on improving spectral baselines over wide bandwidths, and on K-band, Spectrometer, and observing program checkouts. We are also placing considerable emphasis on the Precision Telescope Control System project, which will provide the control facilities necessary for operation at 115 GHz.

The spectral baseline program is designed to improve baseline flatness over bandwidths of up to 800 MHz, which are much broader (by factors of 10-20) than those available at the 140 Foot and previous centimeter-wave telescopes. The offset optics of the GBT greatly reduce spectral standing wave artifacts, but the wide bandwidths expose the effects of small, but cumulative system gain instabilities, mismatches, frequency resonances, etc. Many of the scientific programs proposed for the GBT require very flat spectral baselines over wide bandwidths. To meet this requirement, the baseline project team is systematically examining all constituents of the signal path and is making improvements whenever possible. The team has set a goal of significant progress by March.

K-band commissioning has concentrated on beam offset measurements, focus position determination, active surface performance, and general characterization of both the lower (18-22.4 GHz) and upper (22-26.5 GHz) frequency bands. The system is ready for some K-band observing now, and further program checkouts are planned for January and February.

There has been good progress on both the engineering and software side of the Spectrometer over the past few months. A number of fixes to hardware and firmware problems have been made or are underway, and many modes have passed their engineering checkouts. The reliability of Spectrometer control software has been greatly improved. In addition, support for multi-bank operation of the Spectrometer was provided in the December release of the Monitor and Control system. Multi-bank operation allows use of all four digital quadrants and up to eight sampler inputs. Astronomical checkouts of these modes are underway.

Scientific Operations

Throughout the late summer and early autumn, about 30% of total time was allocated to scientific observations. Observations have included HI spectroscopy, pulsar observing, VLBI, and planetary radar reception. Scheduled observing time in January is smaller owing to the commissioning requirements described above, although we expect to make additional time available to low-frequency backup programs when weather conditions are not suitable for K-band observing. Available observing time will increase again in February and March, and by mid-spring, the focus will shift from commissioning to routine, scientific operation. For about three months in the summer, up to one-third of the time will be needed for structural inspections and painting and other heavy engineering tasks, which will likely be an annual requirement. In the autumn, we will need some additional time to complete Q-band commissioning, improvements in operation of the active surface, and some general commissioning. These activities have not been scheduled in detail, but should continue to leave the majority of time for scientific observations.

GBT Internal Review

On 12 November, an internal review of the GBT was held in Green Bank. Scientists and engineers from Green Bank, Charlottesville, Socorro, and Tucson, including NRAO Director Fred Lo, participated. The review was highly successful in clarifying status and priorities, and broadening participation in GBT activities by staff from around the Observatory. The priorities outlined above -- spectral baseline improvements, Precision Telescope Control, K-band commissioning, and azimuth track work (described below) – were defined at the review. We also took the opportunity of the review to improve our project management system to allow better planning and scheduling of both short-term and intermediate-range goals.

Azimuth Track Review and Plans

As described in previous GBT news updates and in the NRAO Newsletter, the GBT azimuth track has been exhibiting significant, premature wear. The principal problem has been fretting wear on the interior faces of the wear plate and base plates, particularly near the ends of track segments at the splice joints. The fretting is wearing out cavities near the segment ends. In turn, these cavities are causing tilts of the azimuth wheels as they pass over the joints. The bogie flex plates from which the wheels are suspended are designed to accommodate some tilting, but if the cavity wear continues, the tilts may eventually exceed allowable limits and overstress the flex plates. Observatory engineers have been investigating the problem and have taken several steps to slow the wear and extend the life of the track. Nevertheless, it has been clear for some months that more extensive action would be required eventually.

On 31 October and 1 November, the Observatory convened a review panel of engineering experts to examine the track issue and to recommend appropriate, corrective measures. The panel recommended three tasks, to be done concurrently. These were (1) an analytical program to improve the engineering analysis of the wheel assembly and the track joint; (2) a trial retrofit and evaluation program to test methods of stiffening the track joints through braces, partial-penetration welds, and by bridging the joint with a staggered wear plate; (3) a program to develop design concepts for a new azimuth track, should that ultimately prove necessary.

The GBT antenna engineering team is presently developing a work plan to address the recommendations of the panel. We anticipate that this plan, including the evaluation period for modifications, will require a year or more to complete. In the meantime, we are confident that we can shim the wear cavities and slow the rate of fretting so that the GBT can remain in operation. It is likely that a significant modification or replacement of the azimuth track will be required at some time within the next 1-3 years.

P. R. Jewell