5 September 2001
The GBT came back on line on 20 July following azimuth track modifications and wheel bearing replacement. All indications are that the azimuth track modifications have been successful in securing the wear strips to the base plates. Motion of the wear strips has been reduced from ~3.5 mm to ~0.05 mm (~0.140 in to ~0.002 in). The problem of retention bolts breaking has apparently been eliminated by the track modifications. Although azimuth drive operation is smooth, we have commissioned an engineering study to perform a detailed analysis of track performance and to advise on long-term operation.
GBT commissioning resumed in late July with work at 800 MHz with the Prime Focus 1 receiver, and with the Gregorian L band (1.5 GHz) and S band (2 GHz) receivers. Commissioning of the X band receiver (10 GHz) began in mid-August. Considerable progress in commissioning has been made. Pointing accuracy is at about 6 arcsec rms (total rss of both axes) as measured at X band using conventional pointing runs and model fitting. The rms will improve further when a refinement is made to the refraction calculation. Focus tracking curves have been refined, and the "box offsets," which indicate the pointing changes from receiver to receiver on the Gregorian turret, are too small to measure. The aperture efficiency at X band is over 50%, which is consistent with expectations for Phase I surface accuracy. Overall, antenna performance is looking very good.
First fringes between the GBT and four VLBA stations were achieved in observations taken on 15 August. Correlation amplitudes on the GBT to St. Croix baseline were about 5 times higher than on the next shortest baseline, which was consistent with expectations. Commissioning observations with the GBT Spectrometer also began in August, and will proceed through September and October. Development of the metrology system continues. An metrology experiment run in early August showed that all but one of the rangefinder instruments were able to find their targets by dead reckoning, within 3 encoder counts. Early Science observing programs were resumed in mid-August with pulsar observations using the Berkeley-Caltech Pulsar Machine as the backend. Pulsar programs will be run throughout September. Spectral line programs should begin by late October or early November. Early Science programs will continue to be interspersed with commissioning work throughout the remainder of this year and the early part of 2002.
Although commissioning is proceeding apace in many areas, a number of problems that are typical of new telescopes has slowed progress. In late July and early August, telescope systems were, overall, less robust than experienced earlier, resulting in a number of system hang-ups and startup problems. By mid-August, these problems were largely smoothed out, although there are still some bugs to be resolved. Progress with Spectrometer commissioning has been slowed by on-going software development, but some important progress has been made recently in that area. At X band, we are beginning to see the effects of sway in the feedarm (amplitude of order 10" on the sky) induced by sudden starts in antenna tracking at the beginning of a mapping raster, for example. We believe that this effect can be eliminated by refined servo algorithms. These algorithms have already been developed, but require implementation and some debugging.
One significant problem area has been with the antenna braking system. The existing brakes apply very rapidly with large torque and bring the structure to a stop very quickly. When the antenna is driving at slew rate and the brakes are applied, large stresses can be placed on the antenna structure. To prevent damage to any structural members, we have temporarily reduced the maximum slew rates in both axes to one-quarter of their design rates. This protects the structure, but has had a significant impact on commissioning progress since movement to different targets and sky positions is rather slow. Both the contactor and the NRAO have placed high priority on a solution to this problem. It presently appears that a fairly straightforward brake modification will allow the slew rates to be increased to design values, and can probably be implemented within about 6 weeks.
P. R. Jewell