The DESI Legacy Imaging Surveys (http://legacysurvey.org/) are a combination of three public projects (the Dark Energy Camera Legacy Survey, the Beijing-Arizona Sky Survey, and the Mayall z-band Legacy Survey) that will jointly image ≈14,000 deg 2 of the extragalactic sky visible from the northern hemisphere in three optical bands (g, r, and z) using telescopes at the Kitt Peak National Observatory and the Cerro Tololo Inter-American Observatory. The combined survey footprint is split into two contiguous areas by the Galactic plane. The optical imaging is conducted using a unique strategy of dynamically adjusting the exposure times and pointing selection during observing that results in a survey of nearly uniform depth. In addition to calibrated images, the project is delivering a catalog, constructed by using a probabilistic inference-based approach to estimate source shapes and brightnesses. The catalog includes photometry from the grz optical bands and from four mid-infrared bands (at 3.4, 4.6, 12, and 22 μm) observed by the Wide-field Infrared Survey Explorer satellite during its full operational lifetime. The project plans two public data releases each year. All the software used to generate the catalogs is also released with the data. This paper provides an overview of the Legacy Surveys project.

We report evidence for excess blue light from the Type Ia supernova (Sn Ia) SN2012cg at 15 and 16 days before maximum B-band brightness. The emission is consistent with predictions for the impact of the supernova on a nondegenerate binary companion. This is the first evidence for emission from a companion to a normal SNIa. Sixteen days before maximum light, the B V -color of SN2012cg is 0.2 mag bluer than for other normal SNIa. At later times, this supernova has a typical SNIa light curve, with extinction-corrected M 19.62 0.02 B = - mag and m B 0.86 0.02 15 ( ) D =  . Our data set is extensive, with photometry in seven filters from five independent sources. Early spectra also show the effects of blue light, and high-velocity features are observed at early times. Near maximum, the spectra are normal with a silicon velocity v Si =−10,500 km s −1 . Comparing the early data with models by Kasen favors a main-sequence companion of about six solar masses. It is possible that many other SN Ia have main-sequence companions that have eluded detection because the emission from the impact is fleeting and faint.

We present extensive optical (UBV RI, ¢ ¢ ¢ ¢ g r i z , and open CCD) and near-infrared (ZY JH) photometry for the very nearby Type IIP SN 2013ej extending from +1 to +461 days after shock breakout, estimated to be MJD 56496.9±0.3. Substantial time series ultraviolet and optical spectroscopy obtained from +8 to +135 days are also presented. Considering well-observed SNe IIP from the literature, we derive UBV RIJHK bolometric calibrations from UBV RI and unfiltered measurements that potentially reach 2% precision with a B−V colordependent correction. We observe moderately strong Si II l6355 as early as +8 days. The photospheric velocity (v ph ) is determined by modeling the spectra in the vicinity of Fe II l5169 whenever observed, and interpolating at photometric epochs based on a semianalytic method. This gives =  v 4500 500 ph km s −1 at +50 days. We also observe spectral homogeneity of ultraviolet spectra at +10-12 days for SNe IIP, while variations are evident a week after explosion. Using the expanding photosphere method, from combined analysis of SN 2013ej and SN 2002ap, we estimate the distance to the host galaxy to be -+ 9.0 0.6 0.4 Mpc, consistent with distance estimates from other methods. Photometric and spectroscopic analysis during the plateau phase, which we estimated to be 94±7 days long, yields an explosion energy of 0.9 0.3 10 51 erg, a final pre-explosion progenitor mass of 15.2±4.2  M and a radius of 250±70  R . We observe a broken exponential profile beyond +120 days, with a break point at +183±16 days. Measurements beyond this break time yield a 56 Ni mass of 0.013±0.001M  .

In 2021 May, the Dark Energy Spectroscopic Instrument (DESI) began a 5 yr survey of approximately 50 million total extragalactic and Galactic targets. The primary DESI dark-time targets are emission line galaxies, luminous red galaxies, and quasars. In bright time, DESI will focus on two surveys known as the Bright Galaxy Survey and the Milky Way Survey. DESI also observes a selection of “secondary” targets for bespoke science goals. This paper gives an overview of the publicly available pipeline (desitarget) used to process targets for DESI observations. Highlights include details of the different DESI survey targeting phases, the targeting ID (TARGETID) used to define unique targets, the bitmasks used to indicate a particular type of target, the data model and structure of DESI targeting files, and examples of how to access and use the desitarget code base. This paper will also describe “supporting” DESI target classes, such as standard stars, sky locations, and random catalogs that mimic the angular selection function of DESI targets. The DESI target-selection pipeline is complex and sizable; this paper attempts to summarize the most salient information required to understand and work with DESI targeting data.

The Dark Energy Spectroscopic Instrument (DESI) is carrying out a five-year survey that aims to measure the redshifts of tens of millions of galaxies and quasars, including 8 million luminous red galaxies (LRGs) in the redshift range 0.4 < z ≲ 1.0. Here we present the selection of the DESI LRG sample and assess its spectroscopic performance using data from Survey Validation (SV) and the first two months of the Main Survey. The DESI LRG sample, selected using g, r, z, and W1 photometry from the DESI Legacy Imaging Surveys, is highly robust against imaging systematics. The sample has a target density of 605 deg−2 and a comoving number density of 5 × 10−4 h 3 Mpc−3 in 0.4 < z < 0.8; this is a significantly higher density than previous LRG surveys (such as SDSS, BOSS, and eBOSS) while also extending to z ∼ 1. After applying a bright star veto mask developed for the sample, 98.9% of the observed LRG targets yield confident redshifts (with a catastrophic failure rate of 0.2% in the confident redshifts), and only 0.5% of the LRG targets are stellar contamination. The LRG redshift efficiency varies with source brightness and effective exposure time, and we present a simple model that accurately characterizes this dependence. In the appendices, we describe the extended LRG samples observed during SV.