David Schlegel, Lawrence Berkeley National Laboratory
May 14, 2022
In its first year of survey operations, DESI has dwarfed all prior redshifts surveys by mapping 12.8 million unique galaxies and quasars. One-quarter of those are at redshifts greater than 1.0.
Exactly one year ago, DESI began its five-year survey with over 100 DESI scientists on Zoom to witness the event. Although the instrument had been demonstrated to deliver science-quality data, this was not yet a smooth operation. At least a few scientists and engineers would join the observing staff each night to trouble-shoot problems with the telescope, the robotic focal plane, the guide cameras, the spectrographs, the CCD detectors, the cooling systems, or the control systems.
As the nights and weeks progressed, problems that would interrupt observations were identified and fixed. A 72-night shutdown starting July 11, 2021, serviced and replaced components of the robotic focal plane. The return-to-survey-operations on September 21 switched to our current nightly staffing model of an on-site Telescope Operator, an on-site Lead Observer, and two remote Support Observing Operators (who split the night). Only rarely are instrument experts still called in the middle of the night to trouble-shoot problems.
The observing efficiency has greatly improved from those first few nights of the survey. We routinely achieve 90% open-shutter time, defined as the time when the spectrograph shutters are open and collecting light from galaxies. The other 10% of the time is spent slewing the telescope to the next field, reading out the CCD detectors and re-configuring the focal plane (with various steps choreographed to occur concurrently). To date, the record open-shutter time was 10 hours 35 minutes on the long winter night of December 7, 2021.
During this first year, survey data has been collected on 242 nights. Of the remaining nights, 72 were for the maintenance shut-down, and only 51 were completely lost to weather or other engineering tasks. 2462 Main Survey DARK tiles and 2073 BRIGHT tiles have been observed. The raw data are transferred to the NERSC supercomputing center as they are collected, and are fully-reduced to calibrated spectra and redshifts by 10 AM the following morning. To date, the galaxy and quasar map consists of 12.8 million unique, reliable redshifts. In addition, 3.6 million unique stars have been observed. The number of redshifts is plotted as a function of time below.
These next two figures show the distribution of observations on the sky. The survey began by observing non-overlapping tiles on the sky. Starting in September 2021, overlapping tiles were being observed that bring the typical number of visits of each patch of the sky to 5 for the DARK survey and 3 for the BRIGHT survey. Currently, prioritization is given to observing near the equator (declination 0), with diversions elsewhere to avoid pointing within 50 degrees of the moon or in the direction of the wind windy nights. At Kitt Peak, the strongest winds are typically from the south, which has driven some observing to declinations > 32 degrees to avoid telescope wind shake. The DARK survey has observed 9500 square degrees (of a 14,000 square degree footprint) with at least one visit, with approximately 1300 square degrees fully completed with multiple visits.