The spectroscopic wavelength calibration is done quite accurately in SDSS and BOSS, with typical errors of 2 km s-1 or better. The wavelength calibration is established using arc calibration frames taken at altitude immediately before or after each contiguous sequence of science observations. Low-order adjustments to correct for flexure over the course of the observation sequence are made based on the positions of night-sky emission lines in each individual exposure.
Additional DR6 and DR7 (SEGUE-specific) algorithms
The wavelength calibration codes for DR6 and DR7 incorporate additional strategies that are not used either in earlier data releases or for BOSS spectrograph data (DR9 through DR12). As the DR6 paper describes, 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:
- SEGUE spectroscopy was often done on nights with a moderate amount of moon. The bluest sky line used for wavelength calibration is a Hg line at 4046Å, which is very close to a strong Fe I absorption line in the solar spectrum. Thus when there is substantial moonlight in the sky spectrum, a fit to what is assumed to be an isolated emission line can be significantly biased, systematically skewing the wavelength solution at the blue end by as much as 20 km s-1. In DR7, we now fit this line to a linear combination of a Gaussian plus a stellar template including the absorption line, giving an unbiased estimate of the wavelength of the line. In practice, bright moon affected 10 plates (listed in Yanny et al. 2009) out of a total of 410 SEGUE plates.
- The sky and arc lines for each fiber are fit to a wavelength solution; this is done independently for each fiber. This works well for the vast majority of plates. However, for a small fraction of plates, the arcs are weak (perhaps because the arc lamps themselves were faulty at that time, or because the telescope top petals off of which the arc lamp light reflects were not properly deployed), and the wavelength solution is poorly constrained. We therefore required that second- and higher-order terms in the wavelength solution be continuous functions of the fiber number, to constrain the solution. We found that this produces much more robust wavelength solutions for those plates with weak arc observations, and has no substantial effect on the remaining plates.
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.