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Wavelength Calibration

The spectroscopic wavelength calibration is done quite accurately in SDSS, with typical errors of 2 km s-1 or better. As the DR6 paper describes, however, detailed analyses of stellar spectra revealed occasional errors that were substantially larger than this, especially in the blue end of the spectrum. The algorithms for fitting arc and sky lines were made more robust for DR6, and this improved the situation considerably. Two further improvements were implemented for DR7 and DR8:

The stellar spectral template library which gives the best radial velocity estimates is based on the ELODIE library (Prugniel & Soubiran 2001). We have removed one additional ELODIE template that gave velocities with a consistent offset from the rest of the library, as measured using the sample of ~ 5000 stars with duplicate observations on each SEGUE plate pair. In order to provide more complete coverage in effective temperature, surface gravity, and metallicity for hot stars, we generated a grid of synthetic spectra from the models of Castelli & Kurucz (2003) over the same wavelength range and at the same resolving power as the spectra in the ELODIE library. This blue grid spans 6000-9500 K in 500 K increments, -0.5 > [Fe/H] > -2.5 in increments of 0.5 dex, and log g of 2 and 4. We also added a grid of synthetic carbon enhanced spectra (Plez, private communication, using the stellar atmospheric code described by Gustafsson et al. 2008) at values of [Fe/H] between -1 and -4, [C/Fe] between 1 and 4, log g values between 2 and 4, and Teff in the range 4000 K - 6000 K. With these improvements, the radial velocity scatter in repeat observations for objects that match the Carbon star templates is now the same as for the full sample.

The DR6 paper describes a 7 km s-1 systematic error in the radial velocities of stars (in the sense that the pipeline-reported velocities are too small). This is still with us in DR7; a correction is put into the outputs of the SEGUE Stellar Parameter Pipeline (SSPP; Lee et al. 2008) but not elsewhere in the CAS or DAS. Beyond this problem, the plate-to-plate velocities of SEGUE stars have systematic errors of about 2 km s-1 in the mean. The rms velocity error of any given SEGUE star observation is about 5.5 km s-1 at g = 18.5, degrading to 12 km s-1 at g = 19.5.

*Text and figures on this page are taken from the Data Release 7 paper. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it.