We describe the Sloan Digital Sky Survey IV (SDSS-IV), a project encompassing three major spectroscopic programs. The Apache Point Observatory Galactic Evolution Experiment 2 (APOGEE-2) is observing hundreds of thousands of Milky Way stars at high resolution and. high signal-to-noise ratios in the near-infrared. The Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey is obtaining spatially resolved spectroscopy for thousands of nearby galaxies (median z similar to 0.03). The extended Baryon Oscillation Spectroscopic Survey (eBOSS) is mapping the galaxy, quasar, and neutral gas distributions between z similar to 0.6 and 3.5 to constrain cosmology using baryon acoustic oscillations, redshift space distortions, and the shape of the power spectrum. Within eBOSS, we are conducting two major subprograms: the SPectroscopic IDentification of eROSITA Sources (SPIDERS), investigating X-ray AGNs. and galaxies in X-ray clusters, and the Time Domain Spectroscopic Survey (TDSS), obtaining spectra of variable sources. All programs use the 2.5 m Sloan Foundation Telescope at the. Apache Point Observatory; observations there began in Summer 2014. APOGEE-2 also operates a second near-infrared spectrograph at the 2.5 m du Pont Telescope at Las Campanas Observatory, with observations beginning in early 2017. Observations at both facilities are scheduled to continue through 2020. In keeping with previous SDSS policy, SDSS-IV provides regularly scheduled public data releases; the first one, Data Release 13, was made available in 2016 July.

Building on the legacy of the Sloan Digital Sky Survey (SDSS-I and II), SDSS-III is a program of four spectroscopic surveys on three scientific themes: dark energy and cosmological parameters, the history and structure of the Milky Way, and the population of giant planets around other stars. In keeping with SDSS tradition, SDSS-III will provide regular public releases of all its data, beginning with SDSS Data Release 8 (DR8), which was made public in 2011 January and includes SDSS-I and SDSS-II images and spectra reprocessed with the latest pipelines and calibrations produced for the SDSS-III investigations. This paper presents an overview of the four surveys that comprise SDSS-III. The Baryon Oscillation Spectroscopic Survey will measure redshifts of 1.5 million massive galaxies and Ly alpha forest spectra of 150,000 quasars, using the baryon acoustic oscillation feature of large-scale structure to obtain percent-level determinations of the distance scale and Hubble expansion rate at z < 0.7 and at z approximate to 2.5. SEGUE-2, an already completed SDSS-III survey that is the continuation of the SDSS-II Sloan Extension for Galactic Understanding and Exploration (SEGUE), measured medium-resolution (R = lambda/lambda Delta approximate to 1800) optical spectra of 118,000 stars in a variety of target categories, probing chemical evolution, stellar kinematics and substructure, and the mass profile of the dark matter halo from the solar neighborhood to distances of 100 kpc. APOGEE, the Apache Point Observatory Galactic Evolution Experiment, will obtain high-resolution (R approximate to 30,000), high signal-to-noise ratio (S/N >= 100 per resolution element), H-band (1.51 mu m < lambda < 1.70 mu m) spectra of 105 evolved, late-type stars, measuring separate abundances for similar to 15 elements per star and creating the first high-precision spectroscopic survey of all Galactic stellar populations (bulge, bar, disks, halo) with a uniform set of stellar tracers and spectral diagnostics. The Multi-object APO Radial Velocity Exoplanet Large-area Survey (MARVELS) will monitor radial velocities of more than 8000 FGK stars with the sensitivity and cadence (10-40 ms(-1), similar to 24 visits per star) needed to detect giant planets with periods up to two years, providing an unprecedented data set for understanding the formation and dynamical evolution of giant planet systems. As of 2011 January, SDSS-III has obtained spectra of more than 240,000 galaxies, 29,000 z >= 2.2 quasars, and 140,000 stars, including 74,000 velocity measurements of 2580 stars for MARVELS.

The third generation of the Sloan Digital Sky Survey (SDSS-III) took data from 2008 to 2014 using the original SDSS wide-field imager, the original and an upgraded multi-object fiber-fed optical spectrograph, a new near-infrared high-resolution spectrograph, and a novel optical interferometer. All of the data from SDSS-III are now made public. In particular, this paper describes Data Release 11 (DR11) including all data acquired through 2013 July, and Data Release 12 (DR12) adding data acquired through 2014 July (including all data included in previous data releases), marking the end of SDSS-III observing. Relative to our previous public release (DR10), DR12 adds one million new spectra of galaxies and quasars from the Baryon Oscillation Spectroscopic Survey (BOSS) over an additional 3000 deg(2) of sky, more than triples the number of H-band spectra of stars as part of the Apache Point Observatory (APO) Galactic Evolution Experiment (APOGEE), and includes repeated accurate radial velocity measurements of 5500 stars from the Multi-object APO Radial Velocity Exoplanet Large-area Survey (MARVELS). The APOGEE outputs now include the measured abundances of 15 different elements for each star. In total, SDSS-III added 5200 deg(2) of ugriz imaging; 155,520 spectra of 138,099 stars as part of the Sloan Exploration of Galactic Understanding and Evolution 2 (SEGUE-2) survey; 2,497,484 BOSS spectra of 1,372,737 galaxies, 294,512 quasars, and 247,216 stars over 9376 deg(2); 618,080 APOGEE spectra of 156,593 stars; and 197,040 MARVELS spectra of 5513 stars. Since its first light in 1998, SDSS has imaged over 1/3 of the Celestial sphere in five bands and obtained over five million astronomical spectra.

The fourth generation of the Sloan Digital Sky Survey (SDSS-IV) began observations in 2014 July. It pursues three core programs: the Apache Point Observatory Galactic Evolution Experiment 2 (APOGEE-2), Mapping Nearby Galaxies at APO (MaNGA), and the Extended Baryon Oscillation Spectroscopic Survey (eBOSS). As well as its core program, eBOSS contains two major subprograms: the Time Domain Spectroscopic Survey (TDSS) and the SPectroscopic IDentification of ERosita Sources (SPIDERS). This paper describes the first data release from SDSS-IV, Data Release 13 (DR13). DR13 makes publicly available the first 1390 spatially resolved integral field unit observations of nearby galaxies from MaNGA. It includes new observations from eBOSS, completing the Sloan Extended QUasar, Emission-line galaxy, Luminous red galaxy Survey (SEQUELS), which also targeted variability-selected objects and X-ray-selected objects. DR13 includes new reductions of the SDSS-III BOSS data, improving the spectrophotometric calibration and redshift classification, and new reductions of the SDSS-III APOGEE-1 data, improving stellar parameters for dwarf stars and cooler stars. DR13 provides more robust and precise photometric calibrations. Value-added target catalogs relevant for eBOSS, TDSS, and SPIDERS and an updated red-clump catalog for APOGEE are also available. This paper describes the location and format of the data and provides references to important technical papers. The SDSS web site, http://www.sdss.org, provides links to the data, tutorials, examples of data access, and extensive documentation of the reduction and analysis procedures. DR13 is the first of a scheduled set that will contain new data and analyses from the planned similar to 6 yr operations of SDSS-IV.

We use photometric and spectroscopic observations of four detached eclipsing binaries in the globular cluster NGC 3201 to derive masses, radii, and luminosities of the component stars. Spanning across almost three magnitudes in the colour-magnitude diagram, the components offer a unique possibility to test the theory of stellar evolution. Their masses, radii, and luminosities range from 0.66 to 0.84 M-circle dot, 0.68 to 2.46 R-circle dot, and 0.38 to 5.56 L-circle dot, respectively. The distance to the cluster measured from the distance moduli of the component stars amounts to 4.54(-0.14)(+0.11) kpc and agrees with the recent estimate based on Gaia parallaxes. By comparing the M - R and M - L diagrams of the component stars and the colour-magnitude diagram of NGC 3201 to Dartmouth model isochrones, we estimate the most probable age of the cluster to be 11.5 +/- 0.5 Gyr. This estimate is based on three binaries only, as the fourth one seems to evolve along a different path, probably due to non-standard chemical composition and/or history. We confirm the tendency, observed in earlier CASE papers, for the age indicated by the M - R diagram to be younger than that implied by the colour-magnitude diagram.

We develop a machine learning based algorithm using a convolutional neural network (CNN) to identify low H I column density Ly alpha absorption systems (log N-H I/cm(-2) < 17) in the Ly alpha forest, and predict their physical properties, such as their H I column density (log N-H I/cm(-2)), redshift (z(H I)), and Doppler width (b(H I)). Our CNN models are trained using simulated spectra (S/N similar or equal to 10), and we test their performance on high quality spectra of quasars at redshift z similar to 2.5-2.9 observed with the High Resolution Echelle Spectrometer on the Keck I telescope. We find that similar to 78 per cent of the systems identified by our algorithm are listed in the manual Voigt profile fitting catalogue. We demonstrate that the performance of our CNN is stable and consistent for all simulated and observed spectra with S/N greater than or similar to 10. Our model can therefore be consistently used to analyse the enormous number of both low and high S/N data available with current and future facilities. Our CNN provides state-of-the-art predictions within the range 12.5 <= log N-H I/cm(-2) < 15.5 with a mean absolute error of Delta(log N-H (I)/cm(-2) = 0.13, Delta(z(H I)) = 2.7 x 10(-5), and Delta(b(H I)) = 4.1 km s(-1). The CNN prediction costs < 3 min per model per spectrum with a size of 120 000 pixels using a laptop computer. We demonstrate that CNNs can significantly increase the efficiency of analysing Ly alpha forest spectra, and thereby greatly increase the statistics of Ly alpha absorbers.

We present the design of Henrietta, is a wide-band (0.6 - 2.4 mu m) low resolution spectrograph located at the 1-m Swope Telescope in Las Campanas Observatory. Henrietta is designed to routinely suppress instrumental variations in spectrophotometric flux in order to reach the photon noise limit. The primary way Henrietta achieves this is by employing a wide-slit at the telescope focal plane to mitigate time-dependent slit losses; employing a diffusing optical element to broaden the shape of the PSF and mitigate flux variations due to the intra-pixel quantum efficiency variations; a wide field-of-view for access to reference stars with similar brightness and spectral type; and minimizing the number of optical elements to keep throughput high across a wide spectral range. Henrietta is currently in the integration and testing phase and will begin science operations in early 2023.

PHANGS-HST is an ultraviolet-optical imaging survey of 38 spiral galaxies within similar to 20 Mpc. Combined with the PHANGS-ALMA, PHANGS-MUSE surveys and other multiwavelength data, the data set will provide an unprecedented look into the connections between young stars, H ii regions, and cold molecular gas in these nearby star-forming galaxies. Accurate distances are needed to transform measured observables into physical parameters (e.g. brightness to luminosity, angular to physical sizes of molecular clouds, star clusters and associations). PHANGS-HST has obtained parallel ACS imaging of the galaxy haloes in the F606W and F814W bands. Where possible, we use these parallel fields to derive tip of the red giant branch (TRGB) distances to these galaxies. In this paper, we present TRGB distances for 10 PHANGS galaxies from similar to 4 to similar to 15 Mpc, based on the first year of PHANGS-HST observations. Four of these represent the first published TRGB distance measurements (IC 5332, NGC 2835, NGC 4298, and NGC 4321), and seven of which are the best available distances to these targets. We also provide a compilation of distances for the 118 galaxies in the full PHANGS sample, which have been adopted for the first PHANGS-ALMA public data release.

The processes of star formation and feedback, regulating the cycle of matter between gas and stars on the scales of giant molecular clouds (GMCs; similar to 100 pc), play a major role in governing galaxy evolution. Measuring the time-scales of GMC evolution is important to identify and characterize the specific physical mechanisms that drive this transition. By applying a robust statistical method to high-resolution CO and narrow-band H alpha imaging from the PHANGS survey, we systematically measure the evolutionary timeline from molecular clouds to exposed young stellar regions on GMC scales, across the discs of an unprecedented sample of 54 star-forming main-sequence galaxies (excluding their unresolved centres). We find that clouds live for about 1-3 GMC turbulence crossing times (5-30 Myr) and are efficiently dispersed by stellar feedback within 1-5 Myr once the star-forming region becomes partially exposed, resulting in integrated star formation efficiencies of 1-8 per cent. These ranges reflect physical galaxy-to-galaxy variation. In order to evaluate whether galactic environment influences GMC evolution, we correlate our measurements with average properties of the GMCs and their local galactic environment. We find several strong correlations that can be physically understood, revealing a quantitative link between galactic-scale environmental properties and the small-scale GMC evolution. Notably, the measured CO-visible cloud lifetimes become shorter with decreasing galaxy mass, mostly due to the increasing presence of CO-dark molecular gas in such environment. Our results represent a first step towards a comprehensive picture of cloud assembly and dispersal, which requires further extension and refinement with tracers of the atomic gas, dust, and deeply embedded stars.

We developed control software for an enclosure system of the SDSS-V Local Volume Mapper (LVM) which provides a contiguous 2,500 deg2 integral-field survey. The LVM enclosure, located at the Las Campanas Observatory in Chile, is a building that hosts the LVM instruments (LVM-I). The enclosure system consists of four main systems: 1) a roll-off dome, 2) building lights, 3) a Heating, Ventilation, and Air Conditioning (HVAC) system, and 4) a safety system. Two Programmable Logic Controllers (PLCs) as middleware software directly operate complex mechanisms of the dome and the HVAC via the Modbus protocol. The LVMECP is implemented by Python 3.9 following the SDSS software framework which adopted a protocol, called CLU, with message passing based on the RabbitMQ and Advanced Message Queuing Protocol (AMQP). Also, we applied asynchronous programming to our system to process multiple requests simultaneously. The Dome PLC system remotely sends commands for the operation of a roll-off dome and enclosure lights. The HVAC PLC system keeps track of changing environmental values of the HVAC system in real-time. This software provides observers with remote access by high-level commands.