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Some selected SDSS spectra.
Some selected SDSS spectra.

Here we describe details of the spectra from the SDSS spectrograph and the tools available to use them. See the spectroscopic data access documentation for how to access the data. Before proceeding, make sure you have learned the basics of SDSS spectra.

Spectrum information

As described in the data access description, the spectra themselves are provided in spPlate files in FITS format. The primary HDU of each file contains an image which yields all 640 spectra on each plate, each as a row in the image. These spectra are flux- and wavelength-calibrated. Further HDU contain error masks and other information.

Note that the wavelength grid for each fiber on a given plate is the same; however, the wavelength grid differs from plate to plate. Similarly, the exact spectral coverage varies slightly from plate to plate due to differing observing conditions.

The table below yields some pertinent information regarding these spectra:

Fibers per plate 640
Pixel spacing log-wavelength (10-4 dex)
Units 10-17 erg cm-2 s-1 Å-1
Wavelength calibration < 5 km/s
Wavelength reference heliocentric vacuum wavelengths
Fiber diameter 3 arcsec (180 microns)

See the detailed instrument specifications at the SDSS DR7 site for more information.

In further HDU the spPlate files store the error and mask information. HDU1 stores the "inverse variance" of the uncertainties (one over sigma-squared, that is). This quantity may be used, for example, in model fits to the spectra. It is set to zero for pixels that should be ignored entirely (another way of thinking about it is that they have infinite error). For example, in the spectra shown above the errors are shown as the grey band surrounding the spectrum; for masked pixels the grey band becomes vertical.

The pixel mask information is stored in HDU2 and HDU3. These images yield a bitmask for each pixel, in particular the SPPIXMASK bitmask. Since the final spectrum is a combination of individual exposures, it may be that some bits were flagged in some exposures but not in others. HDU2 is the "and mask", which lists all the bits that were set for that pixel in all exposures. HDU3 is the "or mask", which lists all the bits that were set for that pixel in any one (but not necessarily all) of the exposures. The "and mask" (HDU2) is the mask of greatest use.

Conversion between vacuum and air wavelengths

The SDSS data are stored in vacuum wavelengths. However, most optical astronomers know the wavelengths of transitions as measured at S.T.P., which is how the CRC lists them for any transitions redward of 2000 Angstroms.

The IAU standard for conversion from air to vacuum wavelengths is given in Morton (1991, ApJS, 77, 119). For vacuum wavelengths (VAC) in Angstroms, convert to air wavelength (AIR) via:

AIR = VAC / (1.0 + 2.735182E-4 + 131.4182 / VAC^2 + 2.76249E8 / VAC^4)

These are the air and vacuum wavelength of some common transitions:

[O III]4958.9114960.295
[O III]5006.8435008.239
[N II]6548.056549.86
[N II]6583.456585.27
[S II]6716.446718.29
[S II]6730.826732.68

Note that the wavelengths are also shifted such that measured velocities will be relative to the solar system barycentric at the mid-point of each 15-minute exposure (using TAI-BEG + 0.5 * EXPTIME from the header).