Astronomical commissioning of the GBT began last night (5 Feb. 2001) with pointing and focusing observations using the prime focus receiver PF1 at 800 MHz. Successive observations were taken throughout the night of the continuum source 3C295. A number of system areas were found that need further debugging or refinement, but the pointing and focusing observations were largely successful. These observations will be conducted mostly at night over the next two weeks while contractor and other NRAO engineering activities proceed during daytime hours. We expect to move on to Gregorian focus L-band observations after the prime focus measurements are completed.

Azimuth track measurements were made on 26 January after four base plates were welded together. The movement of the welded base plates relative to the concrete foundation as the wheel trucks rolled over them was reduced from 0.055 inches to 0.003 inches (1.4 mm to 0.076 mm), compared to sections that were not welded. Based on this significant improvement, Lockheed-Martin now plans to weld a succession of 6 base plate sections together so that there are 8 "super-segments" around the circular track. This work will take about a month to complete. Lockheed-Martin and NRAO have agreed that commissioning observations can proceed in the interim.

Static load measurements of the reinforced section of the elevation bullgear showed that the amount of non-elastic (i.e., permanent) shift in the section position had not improved following the reinforcement. The static load applied corresponds to a worst-case condition that would be encountered with a hard, emergency stop under maximum slew velocity with a 50 mph wind blowing in the direction of telescope motion. Resolution of this matter thus remains an open issue. However, maximum slew velocity has been reduced by half, which reduces the forces of a hard stop by a factor of 4. Commissioning observations can proceed under these conditions.

Other news from last week included:

• Lockheed-Martin installed the two remaining surface panels.
• "Reflectometry" tests to look for possible standing waves in the optics were made. Results were generally good, although there was a fairly high level reflection off the subreflector. More investigations will be made into this.
• The S-Band receiver was cooled down. Cryogenic capacity of the GBT compressor systems is holding up well.
• Telephones connected to the site-wide PBX have now been installed in the Servo Room, Actuator Control Room, and Gregorian Receiver Room.
• Servo tests were performed. All drive limits were checked in both the Az and El directions. An unexpected hard stop occurred while trying to stop an auto-stow procedure (a soft stop was expected). This will be investigated this week .
• During actuator tests, some actuators were found to be stuck in position. Water had accumulated in the motor housings of these actuators. This problem will be investigated further.

The commissioning schedule for the week of 5 February 2001 can be found at

www.gb.nrao.edu/~pjewell/comm_sched.txt.

P. R. Jewell

M. M. McKinnon