The Dark Energy Spectroscopic Instrument (DESI) on the Mayall 4-m telescope has begun an ambitious survey of spectroscopic measurements of 40 million galaxies and quasars over an area of 14,000 deg2 . DESI is a wide field, multi-object, fiber-fed spectrograph, operating in the wavelength range of 360 - 980 nm. In this paper we present an overview of the DESI instrument focusing on key components including the prime-focus corrector, the focal plane with 5,020 remotely controlled fiber positioners, the procedures to position the fibers on selected targets and the spectrograph system. We then discuss the performance of the instrument during the first year of the DESI survey.
In preparation for the Dark Energy Spectroscopic Instrument (DESI), a new top end was installed on the Mayall 4-meter telescope at Kitt Peak National Observatory. The refurbished telescope and the DESI instrument were successfully commissioned on sky between 2019 October and 2020 March. Here we describe the pointing, tracking and imaging performance of the Mayall telescope equipped with its new DESI prime focus corrector, as measured by six guider cameras sampling the outer edge of DESI’s focal plane. Analyzing ~500,000 guider images acquired during commissioning, we find a median delivered image FWHM of 1.1 arcseconds (in the r-band at 650 nm), with the distribution extending to a best-case value of ~0.6 arcseconds. The point spread function is well characterized by a Moffat profile with a power-law index of β ≈ 3.5 and little dependence of β on FWHM. The shape and size of the PSF delivered by the new corrector at a field angle of 1.57 degrees are very similar to those measured with the old Mayall corrector on axis. We also find that the Mayall achieves excellent pointing accuracy (several arcseconds RMS) and minimal open-loop tracking drift (< 1 milliarcsecond per second), improvements on the telecope’s pre-DESI performance. In the future, employing DESI’s active focus adjustment capabilities will likely further improve the Mayall/DESI delivered image quality.
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