By Iván Lacerna, MAS and AIUC´s postdoctoral researcher

Sloan Digital Sky Survey (SDSS) is an international project that has made the most detailed 3D maps of our Universe, with multi-color images of a third of the sky, and obtained spectra for more than 3 millions of astronomical objects in the last 20 years. The data have been used in different disciplines of astronomy, including asteroids and small bodies of the Solar System,  structure and stellar population of the Milky Way, properties of galaxies, transient objects such as supernovae and in topics related with dark matter and dark energy of the Universe. SDSS is one of the most successful surveys in the history of astronomy considering the huge number of scientific publications using SDSS data, along with the number of citations of these papers.

SDSS was born with the ambitious aim of observing the sky with multiple scans using a dedicated 2.5 meter telescope located at Apache Point Observatory in New Mexico (USA).

SDSS operations started in 1998 and have had four stages. Several programs or surveys with different instruments have existed at each stage. We present in this work (….154…28B) an overview of the fourth stage of the project (SDSS IV), in which the Chilean astronomical community is involved directly for the first time.

SDSS IV started in 2014 and it is expected to finish in 2020. For first time one of the SDSS programs, the Apache Point Observatory Galactic Evolution Experiment 2 (APOGEE-2), is using a second telescope in the southern hemisphere, the 2.5 meters du Pont telescope at Las Campanas Observatory in Chile. 

APOGEE-2 has two spectrographs, which are instruments that decompose the light in different wavelengths (a spectrum). The rainbow, as example, is the spectrum of the visible light of the Sun. Spectra are useful to identify the chemical composition of astronomical objects and to estimate their distances. There is an spectrograph installed in each hemisphere to obtain spectra for thousands of stars of the Milky Way to understand the formation history of our galaxy. A twin instrument was built and later installed in Chile in the beginning of this year since from this side of the planet it is possible to see the central regions of our galaxy, which are not accessible from the northern hemisphere. Furthermore, we can also observe the Magellanic Clouds, two satellite galaxies of the Milky Way, only from the southern hemisphere. Observing stars from the closest galaxies to the Milky Way may provide valuable information on stellar formation in general and the evolution of our galaxy. 

These tasks are challenging, though. For example, most of the stars and dust are confined in the central part of the Milky Way, which is called bulge. Therefore, studying the inner structure is difficult due to both the crowding of stars and the obscuration by dust when observing in the optical range. For this reason, APOGEE-2 spectrographs use the infrared, which is less affected by dust. In other words, we can see through the dust in the infrared wavelength range. In addition, an upgrade of the du Pont telescope has been made in the last two years due to the technical requirements of the new instrument. APOGEE-2 observations from Chile started in April 2017 and are expected to finish by 2020.

From its very start many years ago, SDSS has been made up by different institutions around the world. Since APOGEE-2 operations are performed also in Chile, in this fourth stage the Chilean astronomical community is part of SDSS IV through the Chilean Participation Group. Several researches and students from different Chilean institutions work within SDSS IV, both as observers and in the operation of the spectrograh, and also in research using data from different programs of the project, such as APOGEE-2 with spectra of around 400 thousands stars, MaNGA with the bigger survey of integral field spectroscopy for galaxies, and eBOSS for cosmological measurements of dark matter and dark energy.

One of the main characteristics of SDSS is that the data are made public yearly. The most recent is Data Release 14 ( The data obtained from Chile is expected to be made public in 2019.


“First light” of APOGEE-2 operated from Chile. It was observed the 30 Doradus region in the Large Magellanic Cloud. The white circle shows the region where nearly 300 stars were observed simultaneously (shown with colored circles). Spectra from four stars are shown as examples.


The twin spectrographs of APOGEE-2 are located in each hemisphere. The instrument in the north is installed at the Sloan telescope in New Mexico, United States. In the south, the instrument is at the du Pont telescope of Las Campanas Observatory, Chile. It is possible to view the whole Milky Way with observations from both hemispheres.