GBT News Update
26 February 2001
Commissioning with the S-band (~2.1 GHz) Gregorian receiver began last Friday night, 23 February. In the words of Ron Maddalena, the Commissioning Project Scientist, "it was a beautiful night of observing." A great deal was accomplished in the first night of S-band commissioning, and the results were very positive, albeit preliminary. The first sidelobes were ~ -30dB down (compared to ~ -20dB on the 140 Foot). Aperture efficiency was ~70%, as expected. The beam shape was circularly symmetric within ~2%. The plate scale agreed reasonably well with theory, and there was no measurable beam squint in circular polarization, which indicates that the beam transformation through the Gregorian subreflector optics is working properly. There were a few system problems found, including some strong RFI at 2300 MHz, that will be worked on.
In other work last week, progress was made in acquiring and tracking retroreflectors with the ground laser rangefinders. The control software performed well and is becoming stable. A systematic program will begin this week to bring all 12 ground laser rangefinders on line and to characterize their pointing. This will allow us to begin examining the GBT structure to see how well it conforms to the finite element model and to look for repeatability in the structure after it is moved and returned to the same, nominal position. A security fence is now being erected just outside the ring of ground rangefinder monuments and will allow the system to be used more regularly.
The L-band receiver was modified in the lab last week to eliminate a noise resonance in the band. It has now been reinstalled on the telescope and is cooling down. Further work on the telescope servo systems was done last week, including tests of board modifications that slow the slew and acceleration rates.
The coming week will be devoted to continued commissioning at S-band (mostly pointing and focusing observations), to laser rangefinder tests, and to mechanical and system work during the daytime.
P. R. Jewell
M. M. McKinnon