GBT Upgrade to Sharpen Telescope’s Vision

2018-09-12 | Green Bank Observatory
Image of GBT with laser scanning device

Image: The Green Bank Telescope will use a laser scanning system to measure and adjust its surface precisely.
Credit : GBO/AUI


Plans for 1000 Hours of Day and Night High-Frequency Observing Each Year

The National Science Foundation (NSF) has awarded more than $1.3 million to upgrade its Green Bank Telescope (GBT) in West Virginia with an innovative precision laser ranging measurement system. This upgrade will allow the telescope to observe celestial objects, day or night, at radio wavelengths as small as 3 millimeters. This new capability is essential for studying the chemistry and composition of galaxies, stars, and the dense clouds in the regions near the center of our galaxy.

The GBT is the world’s largest fully steerable telescope and the largest single-dish telescope capable of millimeter-wavelength observations. Currently, these high-frequency observations can only be made at night, a limitation radio telescopes normally do not face.

Daytime solar heating of the telescope, however, adds just enough distortion to the dish to make high-frequency, 3-millimeter observing impractical. The GBT’s planned upgrade will overcome this obstacle by providing real-time precision measurements of the dish, giving the telescope’s control systems the data they need to correct for these distortions, even while the telescope is observing.

This daytime capability is essential to study the central regions of our galaxy. In winter, when the atmosphere is most cooperative to high-frequency observing, the center of the Milky Way is visible only in the daytime from Green Bank.

The newly developed laser ranging system will work [with] the telescope’s existing surface-correction system, which controls the dish’s more than 2000 individual panels and mechanically compensates for any deviations from a defined perfect shape for observing.

“While the GBT’s existing systems work well at night, it can be very difficult to make high-precision adjustments during daylight hours, when sunlight falling on different parts of the dish causes temperature changes and unpredictable distortions that cannot currently be measured,” said Jay Lockman, an astronomer at the Green Bank Observatory and principal investigator on the new laser metrology system.

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, precisely correcting the GBT’s focus 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.

A similar system was already tested on the telescope, providing proof-of-concept that the upgrade will perform as expected. Much of the upgrade of the GBT will focus on the software needed to interpret the data from the laser system so the existing actuators can maintain the shape of the dish with the precision necessary for daytime millimeter observations.

“This upgrade will also enable the GBT to work in concert with other millimeter-wavelength telescopes, opening up new observing capabilities for the national and international astronomical community,” said Lockman. Testing for the new system is expected to begin in about two years and it should be fully deployed in three years.

The Green Bank Observatory is a facility of the National Science Foundation and is operated by Associated Universities, Inc.

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