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.

Overview

Spectroscopic Observing
The spectrographs mounted on the primary 2.5m telescope collected spectra from each plate. There were two spectrographs, each of which collected data from 320 fibers. Each spectrograph had two CCDs, so the instrument produced a total of four images for each exposure.
Spectroscopic Data Reduction
The spectroscopic pipelines extracted one dimensional spectra from the raw exposures produced by the spectrographs, calibrated them in wavelength and flux, measured features in these spectra, measured redshifts from these features, and classified the objects as galaxies, stars, or quasars.

Notes

In the tables below, add 'http://data.sdss3.org/sas/dr8/' to all 'URL format' values to get the full URL.
Data reduction could occur multiple times for both images and spectra. Each time we repeated a data reduction, we labeled the output from that reduction process with a distinct rerun number.
The target selection algorithms page describes how the pipeline performs target selection, including selection of Main Galaxy Sample, Luminous Red Galaxies (LRG), Quasars, Stars, ROSAT All-Sky Survey sources, Serendipity, and SEGUE targets. The target selection pipeline extracts calibrated catalogs of objects and corresponding field by field statistics from the operations database.
The tiling algorithms page describes the process by which the spectroscopic plates are designed and placed relative to each other.

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/sdR-%c%d-%08d.fit.gz	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/sdR-%c%d-%08d.fit.gz	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/ssppOut-%04d-%5d.fit	rerun, plate, plate, mjd
ssppOut_lineindex	out	SSPP line indices	groups/segue2/sspp/%d/%04d/output/param/ssppOut-%04d-%5d.lineindex.fit	rerun, plate, plate, mjd