Dear friends,
I hope that you are all safe and healthy and managing this difficult period. I am writing to share some good news and the suspenseful story of our recent observations with the Green Bank Telescope (GBT).
On Tuesday, April 21, 2020 at 6:22 am PDT, I received an email informing me that our observations had been scheduled for Wednesday, April 22, 2020 from 1:30 pm to 3:30 pm PDT, giving us just a little over 30 hours to finalize our observing plans. By that point, students in the SETI course had learned enough astronomy that they had identified and proposed targets for our observations. But we had not yet finalized the selection of targets, identified good pairs of targets for our A-B-A-B observing mode, nor worked out an optimal sequence for the observations. We immediately got to work and completed these tasks with sufficient time to cross-check and validate our plans. The following SETI class was scheduled on Wednesday at 1:00 pm PDT, half an hour before the observations were due to start, which provided enough time to describe to the students the remote interface to control the telescope and the observing script. But I started the class with an ominous announcement: the telescope was not working.
About an hour before the observations were due to start, I learned from the GBT operator that there was a problem with the subreflector (also known as the secondary mirror), an essential component of the off-axis Gregorian telescope. The GBT subreflector is an imposing structure that is 8 m (24 ft) in diameter and perched at the very top of the support arm of the telescope. It is composed of 40 aluminum panels machined and assembled to achieve a surface accuracy of 100 microns. Radio waves reflect off the primary reflector towards the subreflector and then reflect off the subreflector towards the receiver room. Because gravitational and thermal deformations of the primary reflector and support arm affect the location of the focal point, the subreflector mount is designed to accommodate five degrees of freedom, three translations and two tilts, in order to chase the focal point around as the telescope moves and deforms. These translational and tilt motions are enabled by electrical actuators that must be carefully controlled to avoid damage to the structure. This control is provided by strategically placed limit switches that signal the limits to the allowable ranges of motion. Our observations were now threatened by a malfunctioning limit switch.
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The GBT subreflector and support arm as seen from the secondary focal point atop the receiver room. Photo credit: Jay Lockman.
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The uncertainties were numerous. Would the cognizant engineers and technicians be available to diagnose the issue? Would the Green Bank Site Director give approval to a crew to go to the top of the support arm? Would it be possible to assemble the required crew given the COVID-19 work-at-home restrictions? Would the GBT have spare parts available if one of the parts was defective? How long would the repair last? Would there be any time left to conduct our observations before the next observing run? As it turned out, the repair lasted until after 3:00 pm PDT, past the end of our two-hour class. Fortunately, I had a prepared lecture and related exercise ready to go, so my teaching assistants Paul and Robert kept the students busy while the GBT was idle. Meanwhile, my adrenaline surged as I was hastily selecting new targets and redesigning our observing sequence to accommodate a delayed start time and the fact that the GBT would be facing a different part of the sky by the time the repair was complete. There was additional anxiety related to a prior GBT directive demanding the use of a different data-taking device called the Versatile GBT Astronomical Spectrometer (VEGAS). The ideal mode for our observations (see our 2016-03-29, 2016-04-19, and 2019-05-26 newsletters) is seldom used by other observers and can fail if not set up properly. Fortunately, this story has a happy ending. The Site Director, Karen O'Neil, and Telescope Scheduler, Toney Minter, were both very gracious. They authorized the repair of the telescope and allowed us to observe for a full two hours immediately after the repair crew was safely off the telescope. Ryan Lynch, our collaborator at the GBT, did a great job setting up VEGAS. Thanks to the great work at the GBT, our observations were a complete success and we observed 16 planetary systems!
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The Spring 2020 UCLA SETI class prior to our GBT observations on April 22, 2020.
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Although the class was over when we started our observations, more than 20 students stayed on to participate remotely. I shared the remote desktop on Zoom so students were able to witness our observations in real time. The Zoom chat window was full of great questions from the students. Ryan Lynch was on the Zoom call as well and answered some of the questions.
It took almost exactly 48 hours to transfer the data we collected to our server at UCLA at an average rate of 20 megabytes per seconds or 160 Mbps. (Please note the typo in my previous newsletter: MBps should have read Mbps.) These data will provide a powerful motivator and the basic ingredients for the second half of our annual SETI course, now in its 5th edition. I can't wait to see what we find!
I am happy to confirm that the Spring 2020 class is indeed progressing at a more rapid pace than in previous years due to a variety of instructor and student adjustments to enhance remote learning. So take that, COVID-19!
Warmest regards,
Jean-Luc Margot
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