# The Mayall z-band Legacy Survey (MzLS)

The Mayall $z$-band Legacy Survey (MzLS) imaged the Dec ≥ 32° region of the Dark Energy Spectroscopic Instrument (DESI) footprint. Although the focus of the survey was Dec ≥ +32°, a few per cent of the observations included in the Legacy Surveys are at Dec < +32°. Notably, ~2% of these are in equatorial regions (to facilitate studies of imaging in a region where MzLS overlaps with DECaLS).

MzLS used the MOSAIC-3 camera at the prime focus of the 4-meter Mayall telescope at Kitt Peak National Observatory. MzLS was scheduled over 230 nights during semesters 2016A and 2017A through an agreement between the National Science Foundation and the Department of Energy. The imaging camera underwent a major upgrade in 2015 to improve its $z$-band efficiency. The KPNO 4m telescope control system and the imaging camera software were upgraded for improved operational efficiency. NOAO purchased a new $z$-band filter to match the DECam filter bandpass and to thereby minimize any differences between the DECaLs and MOSAIC $z$-band surveys.

The MOSAIC-3 camera is a new version of the prime focus imaging system. This upgrade made use of the dewar from the MOSAIC-2 camera at CTIO and the MOSAIC-1.1 mechanical system and guider from KPNO. Yale University designed and built a new cold plate for the dewar which it populated with four super-thick (00µm-thick) fully depleted 4096² pixel CCDs with the same 15 micron pitch. The readout system consists of four DESI controllers, one for each CCD that simultaneously reads the four quadrants of each device. These controllers were modified to synchronize to a single clock. The dewar was delivered to NOAO in September 2015 where it was integrated with the MOSAIC-1.1 mechanical enclosure, shutter, filter wheel and acquisition and guider system. This upgraded camera, christened MOSAIC-3, saw first light in October 2015 and underwent further on-sky commissioning runs in November and December 2015. The $z$-band efficiency has been measured to be improved by 60% as compared to the MOSAIC-1.1 camera.

The MzLS survey tiles the sky in three passes, similar to the DECaLS survey strategy. At least one of these passes was observed in both photometric conditions and in seeing conditions better than 1.3 arcsec.

MzLS began official survey operations on February 2, 2016, and ended on February 12, 2018. During this period, MzLS used a total of 382.7 nights, 276.8 of which were clear enough to allow observations.

Additional information is available in the DESI Legacy Imaging Surveys Overview Paper.