Precision alignment and integration of DESI's focal plane using a laser tracker

William Shourt, Robert Besuner, Joseph Silber, Patrick Dunlop, Matthew Evatt, David Brooks, Peter Doel, Yutong Duan, Kevin Fanning, Enrique Gaztañaga, Paul Martini, Francisco Prada, Michael Schubnell, Gregory Tarle

Research output: Chapter in Book/Report/Conference proceedingConference contribution


The recently commissioned Dark Energy Spectroscopic Instrument (DESI) will measure the expansion history of the Universe using the Baryon Acoustic Oscillation technique. The spectra of 35 million galaxies and quasars over 14000 sq deg will be measured during the life of the experiment. A new prime focus corrector for the KPNO Mayall telescope delivers light to 5000 fiber optic positioners. The fibers in turn feed ten broad-band spectrographs. We describe the use of a Faro Laser Tracker with custom hardware and software tools for alignment during integration of DESI's focal plane. The focal plane is approximately one meter in diameter and consists primarily of ten radially symmetrical focal plane segments ("petals") which were individually installed into the telescope. The nominal clearance between petals is 600 microns, and an alignment accuracy of 100 microns and 0.01 degrees was targeted. Alignment of the petals to their targeted locations on the telescope was accomplished by adjusting a purpose-built alignment structure with 14 degrees of freedom using feedback from the laser tracker, which measured the locations of retroreflectors attached to both the petal and the telescope and whose positions relative to key features were precisely known. These measurements were used to infer the locations of aligning features in both structures, which were in turn used to calculate the adjustments necessary to bring the system into alignment. Once alignment was achieved to within acceptable tolerances, each petal was installed while monitoring building movement due to wind and thermal variations.

Original languageEnglish
Title of host publicationGround-Based and Airborne Telescopes VIII
EditorsHeather K. Marshall, Jason Spyromilio, Tomonori Usuda
PublisherSPIE Press
ISBN (Electronic)9781510636774
Publication statusPublished - 13 Dec 2020
EventGround-Based and Airborne Telescopes VIII 2020 - Virtual, Online, United States
Duration: 14 Dec 202022 Dec 2020

Publication series

NameProceedings of SPIE - The International Society for Optical Engineering
ISSN (Print)0277-786X
ISSN (Electronic)1996-756X


ConferenceGround-Based and Airborne Telescopes VIII 2020
Country/TerritoryUnited States
CityVirtual, Online


  • alignment
  • dark energy
  • DESI
  • hexapod
  • laser tracker
  • Mayall
  • retroreflector
  • Stewart Platform
  • UKRI
  • STFC

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