The SDSS Legacy Spectra Survey provides 1D optical spectra of stars, quasars, and galaxies used to map the local and high-redshift Universe.  This article provides descriptions, tutorials, and reference material that will help users of SDSS Legacy data to identify and retrieve relevant products for scientific analysis, using the MAST Portal or one of the Application Programming Interfaces.

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Legacy Spectra Overview

Survey Summary

The SDSS Legacy Spectra Survey is an optical-wavelength spectroscopic survey of millions of stars,  galaxies, and quasars. This includes data from the original SDSS Legacy Spectra survey which targeted galaxies and quasars to map the large-scale structure of the Universe, the SDSS Supernova Survey observing time series of supernovae through repeat observations, and the Sloan Extension for Galactic Understanding and Exploration (SEGUE) which targeted stars in the Milky Way with a goal of mapping the out the galactic disk in abundance space. Together, the SDSS Legacy Spectra and SEGUE extension collected data between the years of 2000-2009 as part of the SDSS-I, -II, and -III projects. All observations have been reduced and processed with either the Galaxy Parameter Pipeline to derive classifications and emission line measurements for galaxies or the SEGUE Stellar Parameter Pipeline to derive stellar parameters and chemical abundances.  Figure 1 shows a graphical overview of the SEGUE survey, demonstrating how stars are used to map the Milky Way.

An optical image of the Milky Way, with the Sun's location labeled. Two inset plots show metallicity as a function of distance from the Galactic Center. Close to the midplane, metallicity declines from the inner disk to the outer disk, and the Sun lies somewhat above the trend-line. Far from the midplane, metallicity is lower, and is about constant or slightly increasing from the inner disk to the outer disk.

Figure 1 - Milky Way science using SEGUE: the radial metallicity gradient of the Galactic disk (Cheng et al. 2012)

Telescopes & Instrumentation

SDSS Legacy Spectra data were obtained at the Apache Point Observatory in New Mexico using the SDSS-2.5m telescope (Gunn et al. 2006), using the original SDSS spectrograph, which was eventually rebuilt into the BOSS spectrograph.  The SDSS spectrograph is a 640 fiber-fed spectrograph with wavelength coverage between 380 nm to 920 nm and resolution (λ/δλ) of roughly 2000.   SDSS observed using plug plates, aluminum plates with holes drilled corresponding to individual astronomical objects or random blank areas of sky.  A plug plate covers a 3-degree diameter circle on the sky, with each fiber having a 2-arcsec diameter.  Each of the 640 fibers were plugged into individual holes for each plate for an observation, allowing for 640 spectra to be taken simultaneously. 


Data Products and Access

The SDSS Legacy Spectra data products are available for download through several MAST interfaces, including the MAST Portal and programmatically in Python through astroquery.mast. For information on how to access and download Legacy Spectra data through MAST,  please refer to the Data Access page. 

MAST has archived the high-level "science ready" data products from the SDSS Legacy Spectra and SEGUE surveys. The Legacy Spectra Data Products page contains details on which files are available at MAST, which include the summary catalog and the 1D extracted spectra from the SDSS Data Reduction Pipeline.

Caveats

  • the original SDSS "spec-lite" files, e.g. spec-3295-54924-0002.fits  have been renamed at MAST to spec-lite-3295-54924-0002.fits  to differentiate them from the "full" spec files.  
  • The data hosted at MAST are copies of the SDSS DR17 release, which includes all data from previous data releases.  Many data, such as from the BOSS survey, have been fully reprocessed in newer releases. However, users should be aware that the Legacy Spectra have not been reprocessed since their original release, which was typically DR7 (in October 2008) or DR8 (in January 2011).
  • One PLATE-MJD combination, 2840-54441, does not have an spZall Catalog. This file does not exist due to a processing error from the pipeline (refer to details on the SDSS Caveats page for more information). To retrieve the best fit results from this PLATE-MJD,  the information can be read from the specObj catalog, or by downloading the spZBest file from directly from the the SDSS Science Archive Server for this plate: spZbest-2840-54441.fits. The spZBest files are not archived at MAST but contain the best-fit outputs from the 1D spectroscopic pipeline.

For Further Reading...

Citations and Acknowledgements

Refer to the SDSS Surveys page for instructions on how to cite this document and acknowledge the use of data obtained from MAST in publications.

For other relevant SDSS Legacy Spectra citations, please refer to the relevant section of the SDSS Technical Papers list. These include: