# The Beijing-Arizona Sky Survey (BASS)

The Beijing-Arizona Sky Survey (BASS) imaged regions at Dec ≥ +32° in the Dark Energy Spectroscopic Instrument (DESI) North Galactic footprint, in the $g$ and $r$ optical bands. Although the focus of the survey was Dec ≥ +32°, about 4% of the observations included in the Legacy Surveys are at Dec < +32°. Notably, ~1% of these are in equatorial regions (to facilitate studies of imaging in a region where BASS overlaps with DECaLS).

BASS used the 90Prime camera at the prime focus of the Bok 2.3-m telescope. The Bok Telescope, owned and operated by the University of Arizona, is located on Kitt Peak, adjacent to the Mayall Telescope. The 90Prime instrument is a prime focus 8k x 8k CCD imager, with four University of Arizona ITL 4k x 4k CCDs that have been thinned and UV optimized with peak QE of 95% at 4000Å. These CCDs were installed in 2009 and operated routinely throughout the course of BASS. 90Prime delivers a 1.12 deg field of view, with 0.45 arcsec pixels, and 94% filling factor. Typical delivered image quality at the telescope is 1.7 arcsec.

The BASS survey tiled 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 1.7 arcsec.

BASS was awarded 56 nights in Spring 2015 and 100 nights in each of Spring 2016 and 2017 (PIs: Zhou Xu and Xiaohui Fan; see http://batc.bao.ac.cn/BASS). The Bok survey targeted about 5500 deg² in the NGC, including about 500 deg² of overlap with the region covered by DECam surveys in order to understand and correct for any systematic biases in DESI target selection. The existing Bok $g$-band filter is well-matched to the DECam $g$-band filter used by DECaLS. The existing Bok $r$-band filter had a significantly different bandpass as compared to the DECam $r$-band filter, therefore a new $r$-band filter from Asahi was acquired for the Bok and delivered in April 2015. This newer $r$-band filter has been used for all BASS observations.

BASS began observations in Spring 2015. Since this time, a number of instrument control software updates, new flexure maps, and new observing tools were implemented that greatly improved the pointing accuracy, focusing of the telescope, and observing efficiency. 15% of the $g$-band and 2% of the $r$-band tiles were observed in Spring 2015 before these critical updates. It was discovered that data from prior to these updates suffered from defective electronics in the read-out system that introduced A/D errors, gain variations and non-linearities. Those electronics were replaced in September 2015 followed by a recommissioning of the system in Fall 2015.

BASS completed 40% of its survey in 100 scheduled nights in the 2016A semester (January-July). BASS was ultimately expected to complete observations in July 2018 with an additional 100 nights in 2017A and 50 nights in 2018A. A few final weeks of observations were conducted in 2019A to complete the survey.

Main BASS website: http://batc.bao.ac.cn/BASS/doku.php