UR #20: Pointing the Green Bank Telescope

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The undergrad research series is where we feature the research that you’re doing. If you’ve missed the previous installments, you can find them under the “Undergraduate Research” category here.

Are you doing an REU this summer? Were you working on an astro research project during this past school year? If you, too, have been working on a project that you want to share, we want to hear from you! Think you’re up to the challenge of describing your research carefully and clearly to a broad audience, in only one paragraph? Then send us a summary of it!

You can share what you’re doing by clicking here and using the form provided to submit a brief (fewer than 200 words) write-up of your work. The target audience is one familiar with astrophysics but not necessarily your specific subfield, so write clearly and try to avoid jargon. Feel free to also include either a visual regarding your research or else a photo of yourself.

We look forward to hearing from you!

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Ellie White

Marshall University

Ellie White is a second-year undergraduate studying Physics at Marshall University, and plans to pursue a career in radio astronomy. She conducted this research as part of an Independent Study course at MU in collaboration with experts at the Green Bank Observatory.

The Green Bank Telescope (GBT) is an engineering marvel. Weighing in at 17 million pounds with a 100-meter by 110-meter dish, it is the largest fully-steerable telescope on the planet. One of the challenges for large, ground-based telescopes like the GBT is achieving pointing accuracy that is good enough for observing at the high end of the telescope’s 0.1 – 116 GHz range. As the frequency at which the telescope is observing increases, the beam size (or “pixel size”) gets smaller, meaning the pointing accuracy must be very high – within just a few arcseconds for the GBT. The pointing performance of the GBT is degraded by factors such as the telescope’s flexure due to gravity; when the telescope tilts to different elevations, it sags due to the Earth’s gravitational pull, which causes the  pointing direction to change slightly. Similarly, thermal expansion and contraction can cause deflections to the telescope’s line of sight, as can tilt and bumpiness in the azimuth track (the circular track that the telescope rotates on with its 16 wheels), as well as small misalignments and offset errors within the structure itself. The GBT’s pointing model corrects for these effects by including terms for structural misalignments, as well as terms incorporating metrology data from the GBT’s structural temperature sensors, and from logs of measurements of the track’s surface.

In our project, we found that when the pointing model is applied with no calibrations, the blind nighttime pointing error RMS (root mean squared, which is a statistical measure obtained by taking the square root of the average squared value of your data points) was a mere 9 arcseconds, which is about 5 thousandths the diameter of the full moon). When calibrations are applied, the RMS pointing error is a fraction of this — observers will see pointing accuracy on the order of 2-3 arcseconds, though further analysis is needed to determine a more exact value. Despite the GBT’s asymmetrical design, which makes it more challenging to correct for pointing errors, our results show that the telescope achieves excellent blind pointing performance.

The Robert C. Byrd Green Bank Telescope (GBT) located in the mountains of West Virginia. Image courtesy GBO / AUI / NSF.

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