Summer Shutdown 2021

I returned to work on site at Kitt Peak National Observatory in November 2020. Social distancing regulations were put in place along with several other protocols to minimize the risk of COVID-19 infection. In that time, we’ve been making great strides commissioning the DESI spectrograph and starting it’s five-year survey, which is intended to result in the most comprehensive 3D map of the universe yet made. The instrument is already getting results. For those who don’t recall earlier posts about DESI, it has 5000 optical fibers mounted at the prime focus of the Mayall 4-meter telescope. Each fiber can be positioned to align precisely with an object on the sky. The fibers run to a spectrograph where the light is analyzed and redshifts of distant objects such as galaxies and quasars can be measured. The following image shows how much sky DESI gets in one pointing. It shows the nearby Andromeda Galaxy taking up much of the field, but as an example, you see that one fiber has landed on a distant quasar. It’s spectrum is displayed in the inset box. Each of the pizza-slice segments represents the 500 fibers in one petal of the DESI instrument.

The disk of the Andromeda Galaxy (M31), which spans more than 3 degrees across the sky, is targeted by a single DESI pointing, represented by the large circular overlay. The smaller circles within this overlay represent the regions accessible to each of the 5000 DESI robotic fiber positioners. In this sample, the 5000 spectra that were simultaneously collected by DESI include not only stars within the Andromeda Galaxy, but also distant galaxies and quasars. The example DESI spectrum that overlays this image is of a distant quasar that is 11 billion years old. Credit: DESI collaboration/DESI Legacy Imaging Surveys/LBNL/DOE & KPNO/CTIO/NOIRLab/NSF/AURA/unWISE

Summer in Arizona is monsoon season. In short, we get a lot of rain. Clear skies can be few and far between. As a result, this is the time of year engineers often choose to shut down the telescopes to do maintenance and make modifications. The DESI instrument has been performing well, but that doesn’t mean it can’t be improved. The fibers in each of those pizza-slice shapes are aligned by a system called the “Command Action Network” or CAN-Bus for short. It was determined that the CAN-Bus system in DESI could be improved. To do this, each petal has to be removed from the Mayall’s prime focus and placed in an area where it can be worked on. We’re able to do this work this summer because of the availability of COVID vaccines. We do take care to practice social distancing where possible and, especially in the wake of the Delta variant’s rise, we’re staying masked throughout the day. This next photo shows DESI with four of the petals removed.

DESI opened up. The red device in the foreground is used to carefully extract the petals.

The trickiest part of this operation is that the DESI petals are all attached by several yards of fiber optic cable to the spectrographs two stories below. When we remove the petals, we don’t want to torque or strain those cables too much. The petals are lifted down and placed on the floor beside the telescope. Once there, they’re placed into clean tents where they’re worked on. Here we see two members of the DESI team diligently working on the CAN-Bus electronics behind the fiber positioners.

Working on the petals. The fiber optic cables come out of the tent, run along the top and then over the rail to the spectrographs below.

Finally when all the new electronics are installed, the petals have to be tested. Among other things, we need to make sure we didn’t break any of the fibers as we handled the petals. DESI is designed to be able to shine light from the spectrograph up through the fibers. We call these “back illuminators” and a camera mounted just below the telescope’s primary mirror can take an image of the back illuminated fibers to see what position they’re in. Here we see the petal out of the telescope with the back illuminators turned on.

DESI’S fibers glowing a friendly blue, telling us all is well after the work has been completed.

Once the upgrades are completed, the petals are reattached to the telescope. This is a big collaborative effort involving many people from around the country and around the world. Once it’s done, we should have made what was already a powerful machine designed to answer questions about dark energy into an even more powerful machine.

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