As I write this, the DESI Commissioning Instrument run at the Mayall 4-meter telescope at Kitt Peak National Observatory will be nearing completion. The Commissioning Instrument is an array of five digital cameras that view the sky through the telescope’s new optics. Once the Commissioning Instrument comes off, the actual DESI fibers and robot positioners will be assembled at the focal plane. This is a process that’s estimated to take about three months to complete. During that time, I’ll be returning to day shifts at the Mayall telescope, helping with the installation. The DESI fiber “wedges” are starting to arrive and assembly has actually begun on some components down on the telescope’s ground floor.
There is a terrific video describing the DESI project and showing these wedges being populated with the fibers in the lab that you can watch at: https://newscenter.lbl.gov/2018/10/17/dark-energy-project-robots-3d-map-universe/
The DESI fibers are the business end of getting light from distant galaxies where it needs to go to be analyzed. Light traveling for billions of light years will be sent through those fibers to be separated and photographed by spectrographs. Before light gets to the fibers, it has to be collected by the telescope, where it will pass through an optical corrector lens. The corrector makes sure that when the telescope is focused, each fiber will also see an equally focused object. Of course, to do this, the whole instrument has to be aligned well with the primary mirror so we know each target lines up with a fiber.
The goal of the Commissioning Instrument is to give us a simple camera that lets us check that the corrector lens is doing its job. It also allows us to test the alignment and focus mechanism, which we call the hexapod. We want to make sure these critical components work before going to all the work of assembling all those fibers at the top of the telescope. In fact, during the Commissioning Instrument run, we discovered that the corrector was eight millimeters too close to the primary mirror. This was a result of telescope blueprints from 1973 not being updated with as-built specs. Eight millimeters may not sound like much, but it’s enough to keep the fibers from being in focus during the warmest nights of the year! So, the hexapod and corrector assembly were moved, which is much easier to do now than when all the fibers are in place.
I have enjoyed my day shifts at the Mayall this past year. It’s given me a chance to interact with more of the maintenance and engineering crew than I normally do in my nighttime operations. I won’t be working exclusively during the daytime. I will still spend one week a month supporting nighttime observations at the WIYN 3.5-meter telescope. If you would like a behind the scenes look at what it’s like to work at an observatory at night, along with something of a scary story, check out my novel, The Astronomer’s Crypt. You can learn more about the novel at: http://www.davidleesummers.com/Astronomers-Crypt.html