Extra-galactic Observing Programs

SDSS has conducted a number of extragalactic single-fiber spectroscopic programs (or surveys) over the years, from SDSS-I/II through to SDSS-IV. This page summarises the main features and distinctions between these programs, namely the Legacy Survey, BOSS, SEQUELS and eBOSS (including TDSS and SPIDERS).

For a similar list of single-fiber spectroscopic programs/surveys within the Milky Way, see SDSS optical Milky Way observing programs.

Legacy Survey

What is now known as the Legacy Survey refers to the observing programs carried out during SDSS-I/II (2000-2008). The spectroscopic part of the legacy survey targeted galaxies and quasars, over a photometric footprint spanning in excess of 7500 square degrees. The complete spectroscopic samples of the Legacy Survey were released in DR7.

Legacy Galaxies

The galaxies in the legacy survey were targeted via two main selections. The Main Galaxy Sample (MGS) (Strauss et al. 2001) is a highly complete spectroscopic sample, essentially magnitude limited by $r_p < 17.77$. The Luminous Red Galaxies (LRG) sample (Eisenstein et al. 2001) is a heavily colour and magnitude selected spectroscopic sample that targets the brightest and reddest galaxies out to a redshift of 0.5 (the LRG sample itself uses two separate cuts to target galaxies at low and high redshift).

Legacy Quasars

QSOs targeted during the legacy survey were primarily identified as outliers from the stellar locus. The selection is documented in Richards et al. 2002, and a final quasar catalogue was released in Schneider et al. (2010).


BOSS was the only extragalactic survey in SDSS-III (2008-2014), and it targeted both galaxies and quasars. BOSS benefited from a new spectrograph and new optical fibres, featuring a smaller aperture (2 arc-seconds instead of 3 arc-seconds). BOSS data was first released in DR9 and the complete BOSS dataset was released in DR12.

BOSS Galaxies

BOSS galaxy target selection was designed around its main science requirements - a measurement of the cosmic distance scale using baryon acoustic oscillations (BAO) with a precision of 1% at redshifts z = 0.3 and z = 0.55. This required two galaxy target algorithms: LOWZ (0.15 < z 0.45) and CMASS (0.4 < z < 0.7). The LOWZ selection is a simple extension of the Legacy Survey LRG Cut-I selection, extended primarily towards fainter targets in order to increase the spatial number density by approximately a factor of three. The CMASS selection is an extension of the Legacy Survey Cut-II selection, extended both by including fainter and bluer targets in order to increase the spatial number density.

BOSS Quasars

BOSS quasar target selection (Ross et al. 2012) was designed to select high-redshift (z > 2.1) objects as tracers of the large-scale structure via the Ly$\alpha$ forest. As such, it differed substantially from the target selection used in the Legacy Survey.


SEQUELS is a stand-alone ancillary observing program that spans SDSS-III and SDSS-IV. It was designed as a pilot program for eBOSS, targeting higher-redshift LRG, emission line galaxies (ELGs) and quasars. In total, 117 LRG and quasar plates were observed (66 during SDSS-III, released in DR12 and 51 during SDSS-IV, released in DR13).


Pushing the targeting of LRGs beyond the redshifts probed by the Legacy LRG and BOSS samples requires the use of photometry redward of the SDSS z-band. SEQUELS tested two different target algorithms using data from the Wide-field Infrared Survey Explorer (WISE) with the goal of extending the redshift peak of the LRG sample to z~0.7. See Prakash et al (2016) for details on targeting.


In addition to Ly-$\alpha$ quasars at z > 2.1, SEQUELS for the first-time targets QSO to be used as direct probes of the large-scale structure at 0.8 < z < 2.2. This requires a number density of roughly 70 per square degree. Quasar targeting for SEQUELS and eBOSS is described in Myers et al. (2015).


SEQUELS tested new selection algorithms to identify ELGs using SCUSS u-band and SDSS imaging, as detailed in Section 2.5.2 of Comparat et al. (2015). These fields were observed separately from the main LRG/quasar SEQUELS fields.


eBOSS (2014-2019) is an SDSS-IV program, targeting LRGs, ELGs and QSOs (Dawson et al. 2016). eBOSS focuses on measuring the BAO acoustic scale at redshifts left unexplored by BOSS, namely the gap between CMASS and the Ly$\alpha$ quasars (0.6here and in the accompanying links. Although the targeting for all the main classes was slightly modified from SEQUELS to eBOSS, the main difference is in the final eBOSS ELG targeting that is based on DECaLS imaging. See Raichoor et al (2017) and Comparat et al. (2016).


The Time-Domain Spectroscopic Survey (TDSS) is a sub-program of eBOSS, selecting time-variable targets for spectroscopic follow-up - see the following references:


SPIDERS (SPectroscopic IDentifcation of ERosita Sources) is a sub-program of eBOSS, targeting X-ray selected clusters, see Clerc et al. (2016) and AGN Dwelly et al. (2017).


Eisenstein et al. 2001
Clerc et al. 2016
Comparat et al. 2015
Comparat et al. 2016
Dwelly et al. 2017
Dawson et al. 2016
Green et al. 2015
Morganson et al. 2015
Myers et al. 2015
Prakash et al 2016
Raichoor et al 2017
Ross et al. 2012
Schneider et al. 2010
Strauss et al. 2002