LASSI: keeping the Green Bank Telescope in shape
Abstract
The 2008 panels that make up the primary surface of the Green Bank Telescope (GBT) can be adjusted in real time to achieve an rms error ~230 microns, allowing observations up to 116 GHz. At present, deviations in the surface (primarily thermal) are corrected using "out-of-focus" holography, which takes ~30 min, but the results can remain valid for many hours under benign night time conditions. During the day, however, thermal gradients in the antenna backup structure can vary on time scales approaching 1 hour, requiring out-of-focus holography measurements at least that often. This reduces observing efficiency so significantly that observations at the highest frequencies (wavelengths of 3 mm) are rarely made during the day. However, recent advances in laser scanning technology have made it possible to purchase a commercial laser scanner that when mounted near the focal point of the GBT, produces tens of million angle and range measurements of the surface in just a few minutes. When these range measurements are averaged together their accuracy improves, and it becomes possible to detect large scale surface distortions with amplitudes as small as ~20 microns. The distortions can then be corrected using the active surface. The GBO has been funded through the NSF-MSIP program to develop such a scanning system and install it on the GBT, the Laser Active Surface Scanning Instrument (LASSI). We will report on the first year's work, which has confirmed the basic concept and shown that the scanner can make surface measurements with an rms repeatability ~150 microns. The system will be installed permanently in the summer of 2020, and should increase the observing efficiency for all projects above 25 GHz on the GBT, the largest telescope in the world operating at mm wavelengths.
- Publication:
-
American Astronomical Society Meeting Abstracts #235
- Pub Date:
- January 2020
- Bibcode:
- 2020AAS...23517516S