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Some selected SDSS spectra.
Some selected SDSS spectra.

NOTE: this page is out of date! Information from here has been spread out among the Spectroscopic data index page, Understanding SDSS spectroscopic data, and Available spectroscopic data. We're keeping it here just for now to preserve existing links. If you've gotten to this page, fix the link that brought you here!

Here we describe details of the spectra from the SDSS and BOSS spectrographs 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 (SDSS) or 1000 (BOSS) spectra on each plate, each as a row in the image. These spectra are flux- and wavelength-calibrated. Additional HDUs contain the wavelength solution, variances, masks, and other information.

The logarithmic wavelength grid spacing is the same for all plates (log10 λi+1 - log10 λi = 0.0001) but the starting wavelength differs from plate to plate. All fibers on the same plate share exactly the same grid.

The table below yields some pertinent information regarding these spectra:

Fibers per plate SDSS: 640
BOSS: 1000
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 SDSS: 3 arcsec (180 microns)
BOSS: 2 arcsec (120 microns)

For more information, see the detailed descriptions of the BOSS spectrograph and the SDSS-I/-II spectrograph

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).