| NSF Org: |
AST Division Of Astronomical Sciences |
| Recipient: |
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| Initial Amendment Date: | September 7, 2018 |
| Latest Amendment Date: | June 21, 2021 |
| Award Number: | 1836009 |
| Award Instrument: | Standard Grant |
| Program Manager: |
Nigel Sharp
nsharp@nsf.gov (703)292-4905 AST Division Of Astronomical Sciences MPS Directorate for Mathematical and Physical Sciences |
| Start Date: | September 15, 2018 |
| End Date: | February 28, 2022 (Estimated) |
| Total Intended Award Amount: | $1,345,903.00 |
| Total Awarded Amount to Date: | $1,345,903.00 |
| Funds Obligated to Date: |
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| History of Investigator: |
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| Recipient Sponsored Research Office: |
2650 PARK TOWER DR STE 700 VIENNA VA US 22180-7300 (202)462-1676 |
| Sponsor Congressional District: |
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| Primary Place of Performance: |
155 Observatory Road, Box 2 Green Bank WV US 24944-9705 |
| Primary Place of
Performance Congressional District: |
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| Unique Entity Identifier (UEI): |
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| Parent UEI: |
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| NSF Program(s): | MID-SCALE INSTRUMENTATION |
| Primary Program Source: |
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| Program Reference Code(s): |
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| Program Element Code(s): |
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| Award Agency Code: | 4900 |
| Fund Agency Code: | 4900 |
| Assistance Listing Number(s): | 47.049 |
ABSTRACT
The Green Bank Telescope (GBT) is the world's largest fully-steerable telescope and a unique asset for the US scientific community. Its surface, which is larger than a football field, must be adjusted very precisely to bring the incoming radio waves into focus, so the surface was designed with more than 2000 individual panels mounted on precise motors. The motors can move the panels to compensate for any deviations from the perfect shape. While this works quite well at night, it can be very difficult to make this adjustment during daylight hours, when sunlight falling on different parts of the dish causes temperature changes and unpredictable distortions that cannot currently be measured. These distortions limit use of the GBT at its highest operating frequencies, where tolerances on surface accuracy are the tightest. The current project will implement a laser ranging measurement system on the GBT that will measure any surface distortions very accurately, allowing the GBT to be focused precisely both day and night. This will increase the available usable time of the telescope at its highest operating frequencies by as much as 1,000 hours every year, with a corresponding increase in the scientific output of the GBT and its utility to the US scientific community for a broad range of investigations. The Green Bank Observatory also supports a wide range of public outreach activities through its visitor center.
Enhancements to the Green Bank Telescope (GBT) metrology system are planned that will enable expanded operation in the 3mm wavelength band during daylight hours. This work will increase the usable telescope time at 3mm by as much as a factor of two, and increase available time on the molecule-rich inner Galactic plane and the Galactic center by up to a factor of four. Recent technological advances in commercial metrology make it possible to purchase a Terrestrial Laser Scanner (TLS) which, when mounted near the prime focus of the GBT, can scan the entire dish in one to two minutes, producing ~10 million data points with individual range accuracies of the order of a few mm. A least-squares fit to these data provides an estimate of the surface with accuracies ~30 microns rms, significantly better than the current method of "out-of-focus holography" and well in excess of the accuracy needed. The measured surface would be compared to the ideal surface to produce commands to the surface panel actuators to compensate for the thermal distortions.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
PROJECT OUTCOMES REPORT
Disclaimer
This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.
Large radio telescopes, like the Green Bank Telescope, are subject to deformations due to gravity and the thermal expansion of its parts. The effects of gravity can be modeled and then corrected, as these are repeatable in time and depend only on the elevation of the telescope. On the other hand, thermal deformations depend in a complex way on the ambient conditions, so it is necessary to monitor the telescope to correct for these effects.
This project developed the Laser Antenna Surface Scanning Instrument (LASSI), which uses a commercial off-the-shelf terrestrial laser scanner (TLS) to measure how the surface of the 330 foot-diameter primary reflector of the Green Bank Telescope changes due to temperature gradients. The TLS is able to acquire 10 million range measurements in less than two minutes. These range measurements, or point cloud, provide a three-dimensional representation of the primary reflector (image 1) and are used to determine how it has deformed. This project funded the purchase and permanent installation of the TLS near the focus of the Green Bank Telescope (image 2).
During the development and testing of the LASSI it was shown that it is possible to use a TLS to measure deformations of a few hundred microns (the width of a handful of human hairs, image 3) over a surface as big as two football fields (1.94 acres). This is the level of accuracy needed to efficiently carry out observations at 3 mm. It was also shown that the accuracy was not impacted by sunlight, so the instrument can be operated during the day or night. To measure such small deformations over such a large area it is very important for the scanner and the object being scanned to remain fixed. In practice this means that the wind must be gentle enough that it does not sway the telescope. This project also investigated additional uses for the TLS, finding that it can identify snow on the primary reflector.
Last Modified: 02/28/2022
Modified by: Felix J Lockman
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