Spectra
Up: Data Products Sections: Images - Object lists - Spectra - Tiling
About spectroscopic samples
The spectroscopic survey is predominantly a survey for
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 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. This file also contains
the resolution at each pixel as measured from arc spectra (see
Data model for spPlate*.fits
resolution HDU #4).
- 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. In particular, the Catalog Archive Server provides
a fast search capability for object lists and spectroscopic parameters
as well as pointers to the files in the Data Archive Server. The Spectro
Query Server query form is dedicated to the search of the
spectroscopic database.
Please note the caveats below, which are
essential to obtain meaningful scientific results from analysing SDSS
spectra, in particular the redshift status caveat.
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.
For those who prefer working with the full fits files of spectra
and imaging catalogs, we will make available a special set of files
(spObj-*.fit ) containing a bundled version of the
spectroscopic data and all the corresponding imaging data (as for DR1)
in the near future
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.
Quality of spectra
About the spectra
For details about the spectrographs, see the spectrograph
page.
Plate diameter | 3 degrees |
Fiber diameter | 3 arcsec |
Wavelength coverage | 3800 - 9200 Å |
Wavelength calibration | better than 5 km/s |
Wavelength reference | heliocentric vacuum wavelengths |
Binning | log-lambda, 69 km/s per pixel |
Resolution | 1850 to 2200; value for each spectrum in spPlate*.fits |
Flux Units | 10-17 erg/s/cm2/Å |
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, usually
because of low signal-to-noise ratio or completely featureless
spectra. 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 zStatus
and zWarnin
of the redshift measurement, which may have failed.). A
useful cut on the redshift confidence is z_conf > 0.35
(or zConf > 0.35 in the SQL database).
Galactic extinction correction
The EDR and DR1 data nominally corrected for galactic extinction.
The spectrophotometry in DR2 is vastly improved compared to DR1, but
the final calibrated DR2 spectra are not corrected for
foreground Galactic reddening (a relatively small effect; the median
E(B-V) over the survey is 0.034). This may be changed in
future data releases. Users of spectra should note, though, that the
fractional improvement in spectrophotometry is much greater than the
extinction correction itself.
Night sky emission lines
The night sky emission lines at 5577Å, (when there is auroral
activity) at 6300Å, 6363Å, and in the OH forest in the red
can be very strong, and leave significant residuals in the spectra
whose amplitude is occasionally underestimated by the noise model. Be
cautious about interpreting the reality of weak features close to
these lines.
Plates with not-quite-perfect spectrophotometry
A small number of plates, given in the list
of not-quite-perfect plates, suffered from a variety of minor
problems. The CCD frames for several plates suffered from a transient
electronic problem in the red camera in Spectrograph 2, causing the
columns of the CCD to be misaligned on readout. This was fixed in
software, and we believe the data to be reliable. Another set of
plates labeled "Spectrograph Collimation Problem" suffered
from having the spectrograph collimator improperly focused. This
problem caused a mismatch between the flatfields and the science
exposure instrumental profile shapes on the CCD in both the spatial
and wavelength directions, causing the optimal extraction process to
reject an excessive number of pixels. This problem was fixed in
software, and comparing overlapping objects from adjacent plates
confirms that the redshifts from these problematic plates are
unbiased. However, the spectra themselves should not be used for
precision work or spectrophotometry. Finally, during the exposure of
one plate, light from an LED somewhere on the telescope found its way
to the spectrographs, resulting in an artificial excess of light
centered roughly at 6500Å; the spectrophotometry of this plate
is quite poor.
Mismatches between spectra and photometric data
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 are 62 objects
for which the mapping between object and spectrum cannot be
established ("unmapped fibers"); their ra/dec is listed as
-9999 in the spectroscopic data set.
Errors in the deblending algorithm in the target reductions
caused spectroscopy to be carried out occasionally on non-existent
objects (e.g., diffraction spikes of bright stars or satellite
trails). Many of these objects no longer exist in the best
imaging reductions with its improvements to the deblender. In other
cases, the photometric pipeline timed out during the best
imaging reductions in fields for which target imaging proceeded
without problem, so that the best photometry is missing for
bona-fide objects. This predominantly happens in fields close to a few
very bright stars. We expect to recover objects from these ``timeout
holes'' in future data releases.
A total of 663 spectroscopic objects therefore do not have a
counterpart in the best catalogs, 0.2% of the total. Of
these, 80 (including the 62 unmapped fibers) can only be retrieved
from the table specObjAll in the database. The remaining
583 objects are contained in the default spectroscopic table
specObj , but will not be found by queries requesting both
photometric and spectroscopic information.
Galaxy 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 median per-pixel 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 DR2 target objects beyond the survey limits for a particular
survey stripe, stripe 10 (see survey coverage page). These
plate/MJD combinations are:
Plate | MJD |
343 | 51692 (straddles stripe limits) |
344 | 51693 |
345 | 51690 |
346 | 51693 |
348 | 51671 |
364 | 52000 |
Their corresponding target imaging data does not
contain any PRIMARY objects. Objects from this region are
therefore not available via a normal search of the target
object lists. To find the target data, select on status &
0x402 (both GOOD and OK_SCANLINE )
instead of selecting on the PRIMARY flag being set in
status . The information is also contained in 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-* ).
This problem does not occur for DR2 best data.
Accuracy of stellar radial velocities
The accuracy of stellar radial
velocities in DR2 is described on a separate page.
Last modified: Mon Apr 19 11:33:28 CDT 2004
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