Stars Across the SDSS


Photo of Connie Rockosi
Connie Rockosi
UC Santa Cruz / Lick Observatory


A spectroscopic survey designed to derive stellar parameters for stars seen in multiple SDSS surveys using consistent methods

Finding Targets

An object whose ANCILLARY_TARGET2 value includes one or more of the bitmasks in the following table was targeted for spectroscopy as part of this ancillary target program. See SDSS bitmasks to learn how to use these values to identify objects in this ancillary target program.

Program (bit name) Bit number Target Description Spectra Primary Objects
GES 35 Star observed by the Gaia/ESO Survey (GES) 413 410
SEGUE1 36 Star observed by the prior SDSS spectroscopic SEGUE-1 survey 5,289 5,195
SEGUE2 37 Star observed by SEGUE-2 2,144 1,998
SDSSFILLER 38 Star in the GES survey area, targeted from prior SDSS photometry 4,724 4,694
COROTGESAPOG 48 Star observed by both the CoRoT survey and by APOGEE 2 2
COROTGES 49 Star observed by both CoRoT and GES 47 47
APOGEE 50 Star in the CoRoT survey area, not observed by CoRoT but with an APOGEE spectrum 145 145
2MASSFILL 51 Star in the CoRoT survey area targeted from 2MASS photometry 324 324


This project provides a cross-calibration of several large spectral surveys, which together are giving us a detailed map of the different stellar populations in the Milky Way. Dedicated stellar spectroscopic surveys such as SEGUE (Yanny et al. 2009), the RAdial Velocity Experiment (RAVE; Steinmetz et al. 2006), APOGEE, the Gaia/European Southern Observatory Survey (GES; Gilmore et al. 2012), and the massive Gaia survey itself (de Bruijne 2012) provide kinematic information and chemical diagnostics for large samples of stars. In addition, there are over 250,000 BOSS spectra of stars, mostly targeted as quasar candidates.

Derived stellar parameters, such as effective temperatures, surface gravities, and metallicities must be robust and consistent between surveys to use them jointly to build a coherent picture of our Galaxy. Because each survey targets a particular magnitude range, one must be careful that stars at different distance ranges will not each have their own systematic errors in stellar parameters.

This program obtained spectra in eight SEGUE-1 and/or SEGUE-2 fields, eight GAIA/ESO survey fields, and one COROT field. Using the BOSS spectrometer to obtain spectra in all these fields allows all these surveys to be cross-calibrated, resulting in more consistent measurements of stellar parameters.

These better stellar parameters will in turn lead to better measurements of important Milky Way disk and halo parameters. For example, the combined samples will allow analysis of gradients in [Fe/H] and [alpha/Fe] to constrain models of timing and rate of star formation in the Milky Way.

Target Selection

This program obtained BOSS spectra of stars observed by the SEGUE-1 and SEGUE-2 surveys on eight plates (target classes SEGUE1 and SEGUE2, plates 7253-7256, 7454-7457), GES targets in eight plates (target class GES, plates 7330-7333, 7450-7453), and one plate (7258) dedicated to stars from the COnvection, ROtation, and planetary Transit mission (CoRoT; Baglin et al. 2002) that had also been observed by GES and APOGEE (target classes COROTGES, COROTGETAPOG). As many CoRoT and GES stars were given fibers as possible, restricted only with the bright magnitude limit of i > 14 to avoid saturation in the spectrographs.

There were sometimes not enough targets to fill all the fibers on the BOSS plates, particularly in areas where the GES fields did not overlap with SEGUE-1 or SEGUE-2 plates; therefore, the eight GES plates also targeted stars selected from the SDSS photometry (target class SDSSFILLER) with the following selection cuts. These cuts were chosen to ensure good SNR and to avoid very cool stars, for which it is more difficult to obtain accurate stellar parameter measurements with the SSPP:

  • 0 < g – r < 1.25
  • g < 19
  • i > 15
  • r > 15

The CoRoT plate had targets chosen from APOGEE (target class APOGEE) and 2MASS (target class 2MASSFILL) as well. As much as possible, stars were targeted to sample the full parameter space of effective temperature, metallicity, and log(g).

The GES project (Milky Way survey) targeted stars with 0 < J – K < 0.7 and 12.5 < J < 17.5, with near-infrared photometry from the Visible and Infrared Survey Telescope for Astronomy (VISTA; Emerson et al. 2004). In total, the eight GES plates gave spectra with SNR high enough to provide acceptable SSPP parameters for 296 stars with -0.25 < g – r < 1.5 and 14 < g < 19.


Baglin, A., et al. 2002, ESASP, 485, 17

de Bruijne, J. H. J. 2012, Ap&SS, 341, 31

Gilmore, G., et al. 2012, Msngr, 147, 25

Steinmetz, M., et al. 2006, AJ, 132, 1645

Yanny, B., et al. 2009, AJ, 137, 4377