Spectra

About spectroscopic samples

The spectroscopic survey is predominantly a survey for

• Bright galaxies, the "main" sample, with Petrosian r-band magnitude < 17.77 (see this pie plot showing the redshift distribution of 78275 DR1 galaxies with -15° < DEC < 20°).
• Luminous Red Galaxies (LRGs) with Petrosian r-band magnitude down to 19.5 (see this redshift distribution of DR1 "main" galaxies and LRGs), and
• Quasars, targetting quasar candidates as faint as i < 20.2 (see this DR1 quasar redshift distribution).

There is also a large number of spectra of stars. Brown dwarfs, ROSAT and FIRST counterparts are targetted as well. Refer to the target selection quality or target selection algorithm for details about the spectroscopic survey targets.

Getting and using spectra

The spectra distributed by the SDSS have been sky subtracted, corrected for telluric absorption (which effectively also corrects for galactic extinction; see extinction note below), and spectrophotometrically calibrated by the Spectro2d pipeline. Spectral classification, redshift determination, and emission and absorption line measurements are carried out by the Spectro1d pipeline. The algorithms page contains details about spectroscopic data processing.

The Data Archive Server provides the same spectrum in two files:

• Spectro2d spPlate*.fits: all calibrated spectra from a single plate, without any parameters or continuum fits.
• Spectro1d spSpec*.fits: multiple fits extensions (images and tables) with the calibrated spectrum, a continuum-subtracted spectrum, and all measured parameters (redshift, line fits, derived quantities). See how to read an spSpec file.

The data access page contains various query forms to get spectra by coordinates, or to search for spectra by redshift, object magnitude, color etc., and to retrieve them from the archive. The quickest way to search spectra is the Spectro Query Server form.

Caveat for using SDSS-measured redshifts: Only 1% of the objects have an "unknown" classification. The redshifts of all but a few tenths of a percent of the remainder are believed to be correct. To identify the few objects with unreliable redshifts, be sure to consider the confidence we have assigned to each redshift (z_conf in the spSpec*.fits primary header, and the status of the redshift measurement zStatus and zWarnin, which may have failed.

Imaging information associated with each spectrum

Very often one wishes to obtain all the SDSS photometric (imaging camera) data associated with each spectrum. One can obtain this information most easily for nearly all plates through the SQS interface by selecting imaging parameters to return with your selected spectra. There are also available, for those who prefer working with the full fits files of spectra and imaging catalogs, a special set of files (spObj-*.fit) containing a bundled version of the spectroscopic data and all the corresponding imaging data. These files are available via the DAS by requesting the data product tsObjFromMap (or spObj) for the plates you wish. To get these files in bulk for the entire survey, see getting SDSS spectra and corresponding imaging data.

One caveat is (see below) is that a few plates in DR1 target objects beyond the survey limits for a particular survey stripe. Their corresponding imaging data is not available via a SQS query and thus is most easily available through the spObj-*.fit files, which are available for all plates.

Quality of spectra

• spectroscopic signal-to-noise as function of magnitude
• Details about the quality of spectrophotometry

About the spectra

For details about the spectrographs, see the spectrograph page.

Further details

• Error and mask arrays are available (see the data model.) The sky spectrum subtracted from each object spectrum is not currently available.
  
• Spectroscopic observations are undertaken in non-photometric conditions when the imaging camera is not in use. At least three fifteen-minute exposures are taken until the cumulative mean S/N per pixel exceeds 4 for a fiducial fiber magnitude of g = 20.2 and i = 19.9.
• We provide the cross-correlation templates used to obtain cross-correlation redshifts.

Caveats

Redshift status

Only 1% of the objects have an "unknown" classification. The redshifts of all but a few tenths of a percent of the remainder are believed to be correct. To identify the few objects with unreliable redshifts, be sure to consider the confidence we have assigned to each redshift (z_conf in the spSpec*.fits primary header, and the status of the redshift measurement zStatus and zWarnin, which may have failed.

Extinction correction

Spectra are reddening-corrected and flux-calibrated using reddening and spectrophotometric standards. This is done under the assumption that the reddening to the spectrophotometric and reddening stars is appropriate for every object in the plate (i.e., that the reddening is uniform on the scale of the plate). The standard stars are behind most of the Galactic dust causing the Galactic extinction. Therefore, independently of the precision of our spectrophotometry, this procedure effectively removes not only telluric, but also the plate-averaged Galactic extinction. However, although the Galactic dust is known to vary across a spectroscopic plate (3 degree field-of-view), no attempt is made to correct for this.

Thus, although there is not much Galactic extinction in the survey area, applying a further Galactic reddening correction to extragalactic spectra based on extinction maps will artificially both increase the flux and make the spectrum bluer, introducing systematic errors. Users should not apply any reddening correction to extragalactic spectra. Conversely, nearby stars have incorrectly been dereddened by this procedure.

Night sky emission lines

Beware of night sky emission lines in spectra at 5577Å, 6300Å, and 6363Å which may occasionally be very strong.

Unmapped fibers

In a few cases, the fiber mapping failed which identifies which fiber has been plugged into which hole. When this happens for two or more objects on the sample plate, there is the possibility of wrong matches between spectra and photometric objects. There should be no more than a handful of these cases; we are working on the compilation of a list.

Velocity dispersion measurements

The velocity dispersion measurements distributed with SDSS spectra use template spectra convolved to a maximum sigma of 420 km/s. Therefore, velocity dispersion sigma > 420 km/s are not reliable and must not be used. There is a postscript file showing the quality of velocity dispersion error estimates.

We recommend the user to not use SDSS velocity dispersion measurements for:

• spectra with S/N < 10
• velocity dispersion estimates smaller than about 70 km s^-1 given the typical S/N and the instrumental resolution of the SDSS spectra

Also note that the velocity dispersion measurements are not corrected to a standard relative circular aperture.

See the velocity dispersion algorithm for details.

"Bonus" plates beyond the survey limits

A few plates in DR1 target objects beyond the survey limits for a particular survey stripe, stripe 10 (see survey coverage page). These plate/MJD combinations are:

Their corresponding imaging data is not available via a regular SQS query (which only contains imaging information on PRIMARY objects within the survey limits) and thus is most easily available through the spObj-*.fit files, which are available for all plates. These files are available via the DAS by requesting the data product tsObjFromMap (or spObj) for the plates you wish. They are also available for bulk rsync or wget download via DAS through http (in spectro/ss_SPRERUN/PLATE/spObj-*).

Radial velocity accuracy of stellar redshifts in DR1

Spectra for approximately 17,600 Galactic stars of all common spectral types are available with DR1. Radial velocities (RVs) are stored as redshifts (multiply by the speed of light to get the RV in km/s) and were measured by cross-correlation to a set of stellar templates. Also tablulated are measures of individual common spectral absorption and emission lines, such as Ca II K (3933), the Balmer series of Hydrogen, and Na (5896), and the offsets of these individual lines from their vacuum rest wavelengths may be also used to may be used to estimate a radial velocity (lines are tabulated with a significance indicator, nSigma, and generally speaking, only lines above a threshold, such as nSigma > 7, should be considered reliable). The resolution of the spectrographs (1800 < R < 2250) suggests that radial velocities should be measurable with one errors of about 8 km/s for objects with reasonable S/N in the SDSS. The current DR1 dispersion solutions for individual objects have been demonstrated to be good to better than 5 km/s, including correction for such effects as flexure of the telescope. This dispersion error has been determined by examining multiple observations of the same astrophysical object on a variety of plates under a variety of observing conditions and showing that RVs were reproducible to this accuracy.

The DR1 templates used for stellar RVs have shifts due to zeropointing and (low) signal-to-noise problems with the templates themselves. This introduces systematic errors in addition to possible dispersion errors, depending on spectral type. The templates used for DR2 and beyond will be corrected for these zeropoint errors. The error as a function of template are given below, along with the identity of the actual template star (which may be examined). The 'Template Number' for any spectrum may be found under the keyword 'BESTTEMP' in the PDU header of a SDSS spSpec-$mjd-$plate-$fiberid.fit file. A better estimate of the true RV of a particular object may be found by RV(true) = cz(tabulated)+ RVshift(below). Estimates for White Dwarf and M,L,T star Radial velocities are not accurately quoted for DR1. Interested scientists are encouraged to compute their own RVs off of the extracted spectra, keeping in mind that the wavelengths measured of individual lines measured are on a vacuum system.

Template number	Sp Type	Plate	MJD	FiberID	RVshift	Note
1	sdO	593	52026	265	+21
2	sdB	301	51942	431	+13
3	sdOB	282	51658	110	-59
4	BHB	290	51941	301	-170	Bi-modal RVshift
5	A	300	51666	128	-22
6	early F	289	51990	5	-23	color dependent RVshift
7	early G	306	51637	295	-13
8	late F	273	51957	304	+7
9	G	310	51990	356	+1.5
10	G/K	396	51816	605	-5	color dependent
11	early M	402	51793	204	+28
12	M	367	51997	593	-5