GBT News Update

6 February 2001

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:

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

P. R. Jewell

M. M. McKinnon