Site Map

Spectroscopic Data Processing

Introduction and Resources

The Early Data Release (EDR) paper is the fundamental resource for understanding the processing and data products from the SDSS, describing the pipelines and contents of generated data products. Successive data release papers: DR1, DR2, DR3, DR4, DR5, DR6 and DR7, describe changes between data releases. The technical summary provides more general information on the survey. The SDSS Project Book, written for a funding proposal early in the survey, is the most exhaustive description of the survey but was last updated in 1997. The SDSS publications web page (UPDATE LINK?) includes a list of additional SDSS technical papers.

The algorithms page includes links to pages describing algorithms used by the data reduction pipelines, and the SAS datamodel has a table of the most commonly useful files in the SAS.

The remainder of this page starts with a brief overview of spectroscopic data processing, followed by sections that describe the steps in data processing in detail. In addition to the more detailed description, each detail section privedes references to papers that give additional details, and a table of the files associated with that step of the pipeline that can be found in the SAS. These tables include links to descriptions of the formats of those files and templates that can be used to generate SAS URLs for those files. The templates are in "C printf" format, and can be used in C, bash, Python, and many other languages to automatically generate URLs.

Most of the catalog data (but not the images themselves) have been loaded into the Catalog Archive Server (CAS) database. Users are often better off obtaining SDSS data through a carefully constructed CAS query than they are downloading the data files from the SAS. Simple queries can be used to select just the objects and parameters of interest, while more complex queries can be used to do complex calculations on many objects, thereby avoiding the need to download the data on them at all.



Spectroscopic Observing

Plate Plugging (plug)

When the observatory is ready to observe a plate, the observatory staff plugs optical fibers into the holes drilled into the plates, and maps which fiber correponds to which hole (and therefor which object) by shining light through each fiber. This data is incorporated into one of the HDUs of the spPlate file described below.

Data Collection

Observers mount cartridges containing the drilled, plugged plates on the telescope, and collected exposures on each plate until it reached a threshold estimated signal to noise and at least three exposures had been collected.

SAS files generated in spectroscopic data collection
File Type in/out Description URL format format parameters
sdReport out records exposures collected on a night Not public
sdR out raw spectroscopic data frames common/sdss-spectro/raw/%d/ mjd, CCD (r or b), camera (1 or 2), exposure id

Spectroscopic Data Reduction

Two-dimensional Pipeline (spectro2d, also called idlspec2d)

References: 2002AJ....123..485S, section 4.10.1

The spectro2d pipeline reads science and calibration exposures from the spectrographs, reduces and calibrates the science exposures, extracts the one dimensional spectra from the two dimensional exposures, stacks multiple exposures into combined spectra, and produces corresponding masks and noise estimates.

SAS files used or generated by the spectro2d pipeline
File Type in/out Description URL format format parameters
spPlan2d in the spectro2d processing plan common/sdss-spectro/redux/%d/%04d/spPlan2d-%04d-%d.par rerun, plate, plate, mjd
spPlancomb in the processing plan for combining spectra common/sdss-spectro/redux/%d/%04d/spPlancomd-%04d-%d.par rerun, plate, plate, mjd
plPlugMapM in records which fiber corresponds to which hole in a plate (and therefore objects, and what coordinates on the sky) Not public
sdReport in records exposures collected on a night Not public
sdR out raw spectroscopic data frames common/sdss-spectro/raw/%d/ mjd, CCD (r or b), camera (1 or 2), exposure id
spCFrame out calibrated spectra for a single CCD and exposure common/sdss-spectro/redux/%d/%04d/spCFrame-%c%d-%08d.par rerun, plate, CCD (r or b), camera, exposure id
spPlate out the 640 combined flux- and wavelength-calibrated spectra over all exposures (potentially spanning multiple nights) for a given mapped plate common/sdss-spectro/redux/%d/%04d/spPlate-%04d-%d.par rerun, plate, plate, mjd

One-dimensional Pipeline

The idlspec2d product incorporates the Princeton-1D pipeline (sometimes called specBS) which produces the following files:

SAS files generated by the Princeton 1D pipeline
File Type in/out Description URL format format parameters
spZline out emission line fits common/sdss-spectro/redux/%d/%04d/spZline-%04d-%d.fits rerun, plate, plate, mjd
spZall out all spectroscopic classifications and redshifts common/sdss-spectro/redux/%d/%04d/spZall-%04d-%d.fits rerun, plate, plate, mjd
spZbest out spectroscopic classifications and redshifts common/sdss-spectro/redux/%d/%04d/spZbest-%04d-%d.fits rerun, plate, plate, mjd

Stellar Parameters Pipeline (sspp)

References: 2008AJ....136.2022L, 2008AJ....136.2050L, 2008AJ....136.2070A

The SEGUE stellar parameters pipeline produces a number of files, stored together:

SAS files used or generated by the SSPP pipeline
File Type in/out Description URL format format parameters
ssppOut out SSPP stellar parameters ([Fe/H], log g, etc.) groups/segue2/sspp/%d/%04d/output/param/ rerun, plate, plate, mjd
ssppOut_lineindex out SSPP line indices groups/segue2/sspp/%d/%04d/output/param/ rerun, plate, plate, mjd