# The Dark Energy Camera Legacy Survey (DECaLS)

The Dark Energy Camera (DECam) on the Blanco 4m telescope, located at the Cerro Tololo Inter-American Observatory, will provide the optical imaging for targeting for 2/3 of the Dark Energy Spectroscopic Instrument (DESI) footprint, covering both the North Galactic Cap region at Dec ≤ 32° and the South Galactic Cap region at Dec ≤ 34°. Due to the combination of large field of view and high sensitivity from 400-1000 nm, DECam is an efficient option for obtaining photometry in the $g$, $r$, and $z$ bands.

DECam can reach the required depths for DESI targets in modest total exposure times of 166, 134 and 200 sec in $g$, $r$, $z$ in median conditions. These data reach required 5σ depths of $g=24.0$, $r=23.4$ and $z=22.5$ for an ELG galaxy with half-light radius of 0.45 arcsec. Because of the incomplete fill factor of DECam, a 3-pass observing strategy has been designed where these depths are met with 2 of the 3 exposures. Accounting for weather loss, DECam is capable of imaging 9000 deg² of the DESI footprint to this depth in 157 scheduled nights.

A public survey, "The DECam Legacy Survey of the SDSS Equatorial Sky" (PI: D. Schlegel and A. Dey), was allocated 64 nights over three years beginning in semester 2014A as part of the NOAO Large Surveys programs. This initial allocation was proposed to only image the existing SDSS-III equatorial footprint. The camera throughputs and sky brightness distribution increased the average exposure times from an expected 50, 50, 100 seconds to 87, 67, 100 seconds per pass. The imaging program was supplemented to a total of 157 scheduled nights to both account for these longer exposure times and enlarge the footprint to the full DESI equatorial footprint. The program completed in March, 2019.

The DECaLS program made use of other DECam data within the DESI footprint. The most significant of these other data sets is from the Dark Energy Survey (DES), which includes a 5000 deg² contiguous area in the South Galactic Cap. DECaLS explicitly did not re-image that area, instead incorporating the DES imaging itself. Data from the early DECam science verification period has a number of problematic features, and is not included in our imaging releases.

Additional information is availalble in the 2013A-0741 and 2014B-0404 proposal abstracts, the full proposal text, the DECam Legacy Survey Principles and the DESI Legacy Imaging Surveys Overview Paper.