This article describes the SDSS Legacy Spectra and SEGUE data products hosted at MAST.

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Summary

The primary data products from the SDSS Legacy Spectra survey and SEGUE extensions are optical-wavelength (380- 920 nm) spectra with a resolution of (λ/δλ)~2000 obtained using the SDSS-I/-II Spectrograph on the SDSS-2.5m telescope (Gunn et al. 2006) at Apache Point Observatory. SDSS observed using plug plates, aluminum plates with holes drilled corresponding to individual astronomical objects or random blank areas of sky.  Each of the SDSS Spectra plug plates covers a 3 degree diameter circle on the sky, and contained 640 individual fibers resulting in up to 640 spectra per plate. Targets for the SDSS Legacy Survey include stars, galaxies, quasi-stellar objects, and blank sky. 

An example SDSS Legacy spectrum is shown in Figure 1, corresponding to the data in the "spec-3295-54924-0002.fits" and "spec-lite-3295-54924-0002.fits" spectrum files. For information on how to access and download SDSS Legacy Spectra data through MAST,  please refer to the Data Access page. 

An example spectrum of a star from the SEGUE survey. The image shows a black-and-white plot showing wavelength (on the x-axis) and flux (on the y-axis). The location of several absorption lines labeled on the spectrum.

Figure 1 - Example SDSS Spectra preview file, "spec-image-3295-54924-0002.png".

Data Products at MAST

The SDSS Legacy Spectra data products available at MAST are listed below. All SDSS Legacy Spectra observations in MAST follow the naming scheme:

sdss_spectro_[PLATEID]-[MJD]-[FIBER4]

where all observation names start with the literal string "sdss_spectro_" and the other parameters are defined as:

  •  [PLATEID]  is the name of the plate used for that observation. This is a zero-padded four-digit number between "0266" and "3480".
  •  [MJD] is the integer Modified Julian Date of the observation.
  •  [FIBER4] is the zero-padded four-digit number of the fiber used for that spectrum (a four digit number between "0001" and "0640").

An example SDSS spectrum observation name in MAST would be, "sdss_spectro_3295-54924-0002". Observation names are all lowercase. Each observation represents one  target, and contains both spectrum products and the preview image files. More documentation on for all SDSS data products can be found on the SDSS webpages describing the Optical Spectra survey and Getting Started with the SDSS spectra.

Full Spectrum Files

The "full" spectrum files contain the combined spectrum, associated metadata, and the individual exposures.

Lite Spectrum Files

The "lite" spectrum files contain the combined spectrum and associated metadata but not the individual exposures.

Note: The original SDSS names for the  "spec-lite" files did not include the "-lite" extension, e.g. spec-3295-54924-0167.fits . These products have been renamed at MAST to spec-lite-3295-54924-0167.fits  to differentiate them from the full spectrum files. All other SDSS data products preserve their original filenames at MAST.

Preview Images

  • spec-image-{PLATEID}-{MJD}-{FIBER4}.png
  • spec-image-{PLATEID}-{MJD}-{FIBER4}.thumb.png
  • SDSS Data Model

The preview files and png images showing a preview of each spectrum. An example preview file is shown in Figure 1 above: the file is named "spec-image-3295-54924-0002.png" and it corresponds to the spectrum file "spec-3295-54924-0002.fits" and "spec-lite-3295-54924-0002.fits". 


Catalog Files

The SDSS Legacy Spectra survey provides several different catalog products containing various observation information and analysis properties for the survey targets. Refer to the Data Access at MAST page for more information on how to search and download SDSS spectra data at MAST.

The catalog file types hosted at MAST are described below:

 specObj

The "specObj" catalog provides collated summary tables on the observations and data processing from the SDSS Legacy Spectra survey, including targeting information, spectroscopic classifications, and redshifts for every observation. This catalog contains all observations from SDSS Legacy Spectra, SEGUE, and also the BOSS and eBOSS surveys. To select just the Legacy Spectra and SEGUE data, users can filter the table using the "SURVEY" column, or filter out all BOSS/eBOSS observations by selecting rows where the RUN2d column is not equal to "v5_13_2".  

spPlate 

There is one "spPlate" catalog for each plate-MJD, containing the combined spectra and targeting information for all observations (620 fibers) on a single plate.

spZall 

The "spZall" catalogs, for each plate-MJD, containing the first, second, third, etc. best fit spectral redshift and classification measurements. The best fit is included in the spAll file. It is recommended to look at the second-best fit here if best fit looks bad or has bad chi-squared.

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.

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