Performance of Kitt Peak's Mayall 4-meter Telescope During DESI Commissioning

TL;DR

This paper reports on the successful commissioning and performance evaluation of the Mayall 4-meter telescope with the new DESI instrument, highlighting improvements in image quality, pointing accuracy, and tracking stability.

Contribution

It provides the first detailed performance metrics of the Mayall telescope with the DESI corrector, demonstrating significant enhancements over previous configurations.

Findings

Median image FWHM of 1.1 arcseconds in the r-band

Pointing accuracy of several arcseconds RMS

Tracking drift less than 1 milliarcsecond per second

Abstract

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 $\beta$ ~ 3.5 and little dependence of $\beta$ 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.