ProtoDESI: First On-Sky Technology Demonstration for the Dark Energy Spectroscopic Instrument

Abstract

The Dark Energy Spectroscopic Instrument (DESI) is under construction to measure the expansion history of the universe using the baryon acoustic oscillations technique. The spectra of 35 million galaxies and quasars over 14,000 square degrees will be measured during a 5-year survey. A new prime focus corrector for the Mayall telescope at Kitt Peak National Observatory will deliver light to 5,000 individually targeted fiber-fed robotic positioners. The fibers in turn feed ten broadband multi-object spectrographs. We describe the ProtoDESI experiment, that was installed and commissioned on the 4-m Mayall telescope from August 14 to September 30, 2016. ProtoDESI was an on-sky technology demonstration with the goal to reduce technical risks associated with aligning optical fibers with targets using robotic fiber positioners and maintaining the stability required to operate DESI. The ProtoDESI prime focus instrument, consisting of three fiber positioners, illuminated fiducials, and a guide camera, was installed behind the existing Mosaic corrector on the Mayall telescope. A Fiber View Camera was mounted in the Cassegrain cage of the telescope and provided feedback metrology for positioning the fibers. ProtoDESI also provided a platform for early integration of hardware with the DESI Instrument Control System that controls the subsystems, provides communication with the Telescope Control System, and collects instrument telemetry data. Lacking a spectrograph, ProtoDESI monitored the output of the fibers using a Fiber Photometry Camera mounted on the prime focus instrument. ProtoDESI was successful in acquiring targets with the robotically positioned fibers and demonstrated that the DESI guiding requirements can be met.

The commissioning instrument for the dark energy spectroscopic instrument

“…The DESI CI will perform vital commissioning tasks that range from verifying basic telescope function after its major-rebuild to verifying that DESI can accurately determine the focal plane position required to observe a target at a given sky position. The early testing of DESI components was demonstrated to be quite valuable during the ProtoDESI campaign 8 and we expect similar value to come from the DESI CI campaign.…”

“…The benefit of testing DESI systems with on-sky observations was demonstrated previously with the Pro-toDESI 8 experiment, which successfully demonstrated integration between the hardware, the DESI ICS, and the Mayall TCS. ProtoDESI also demonstrated the successful acquisition of targets and that these targets were held in place using the guide camera hardware and software.…”

“…These illuminated fiducials are the same model used for the full DESI Focal Plane System 4 and are produced by Yale University. 8 Their distribution for the CI is shown on Fig. 1.…”

“…This hardware and the software was demonstrated to to work during the ProtoDESI campaign. 8 The observed positions of the pinholes allow for the mapping of the distortion pattern over the full field of view, and thus between X, Y on the focal surface and α, δ positions of targets on the sky. Recently developed methods 11 * http://diffractionlimited.com/product/stxl-6303e/ demonstrate that spin-weighted Zernike polynomials can be used to efficiently model optical distortion patterns, and that at most 16 coefficients will be required for DESI.…”

Installation of the Dark Energy Spectroscopic Instrument at the Mayall 4-meter telescope

“…The system can be operated remotely or on-site. The first elements of the ICS software were integrated with the Mayall Telescope Control System 34 in 2016 (for operation of ProtoDESI, an early technology demonstrator 5 ), with the full production software package and DESI-specific computing hardware (Figure 1, left) installed on the mountain in May 2018.…”