Sloan Digital Sky Survey spectrograph

Sloan Digital Sky Survey spectrograph
Name	Sloan Digital Sky Survey spectrograph
Manufacturer	Apache Point Observatory / University of Chicago / Johns Hopkins University
Introduced	2000
Wavelength range	380–920 nm (original)
Resolution	R ≈ 1800–2200
Telescope	Apache Point Observatory Hobby-Eberly Telescope?
Survey	Sloan Digital Sky Survey

Sloan Digital Sky Survey spectrograph The Sloan Digital Sky Survey spectrograph is the multi-object optical spectrograph developed for the Sloan Digital Sky Survey to obtain millions of spectra for the Large Scale Structure of the Universe, Galactic archaeology, and extragalactic astronomy programs. Designed and built through collaborations among institutions including Fermilab, University of Chicago, Johns Hopkins University, and Apache Point Observatory, the instrument enabled systematic spectroscopic follow-up of targets selected from the Sloan Digital Sky Survey imaging database. Its deployment transformed observational campaigns such as SDSS-I, SDSS-II, and later phases by pairing wide-field imaging from the 2.5-meter telescope with efficient fiber-fed spectroscopy.

Overview and Purpose

The spectrograph was conceived to provide moderate-resolution spectra across the optical band for large numbers of objects identified by the Sloan Digital Sky Survey imaging pipeline, supporting science goals tied to the Lambda-CDM cosmological model, measurements of the baryon acoustic oscillation scale, and redshift surveys similar in ambition to projects like 2dF Galaxy Redshift Survey and DEEP2 Galaxy Redshift Survey. It served programs including the Baryon Oscillation Spectroscopic Survey and SEGUE for mapping the Milky Way stellar populations, enabling redshift catalogs used by teams associated with Princeton University, Yale University, and University of Washington.

Design and Optical Components

The instrument employs a dual-beam design with dichroic splitting and separate red and blue channels, using components developed in collaboration with engineering groups at Lawrence Berkeley National Laboratory and optical vendors familiar with projects like Hubble Space Telescope instruments. Key optical elements include a focal-plane assembly at the 2.5-meter telescope prime focus, a dichroic mirror to separate bands, volume-phase holographic gratings reminiscent of those used on the Subaru Telescope spectrographs, and lenses optimized for the targeted wavelength range. The design balances throughput and spectral resolution to meet requirements driven by surveys led by teams at University of Chicago and Johns Hopkins University.

Fiber-Optic System and Plugplates

An essential feature is the fiber-fed system that routes light from the telescope focal plane to the spectrographs, using steel plugplates drilled for targets selected by imaging algorithms developed at Princeton University and Apache Point Observatory. Each plate accommodates 640 fibers (in early configurations) plugged by staff and students from institutions such as University of Arizona and Carnegie Mellon University, a workflow influenced by fiber systems used on instruments at Cerro Tololo Inter-American Observatory and Anglo-Australian Observatory. The plugplate system enabled rapid field changes and high multiplexing, facilitating surveys administered by consortia including SDSS Consortium partners.

Detectors and Electronics

The detectors consist of charge-coupled devices optimized for the blue and red channels, with electronics for readout and control developed alongside groups at Fermilab and Lawrence Livermore National Laboratory. CCD packages share heritage with devices used on projects like the Hubble Space Telescope Advanced Camera for Surveys and ground-based imagers at Kitt Peak National Observatory. Instrument control software interfaces with data acquisition systems influenced by architectures from National Optical Astronomy Observatory projects and allowed coordination with observatory operations teams from Apache Point Observatory.

Calibration and Data Reduction

Calibration strategies combine arc-lamp exposures, flat fields, and sky fibers to correct throughput and wavelength solutions, methods paralleling best practices from the European Southern Observatory community and the Keck Observatory instrument teams. The data reduction pipeline, developed by software groups at Princeton University and Johns Hopkins University, produced calibrated one-dimensional spectra and redshift estimates using algorithms comparable to those implemented in the Spec2d and Spec1d packages, feeding catalogs curated by collaborations with Yale University and University of Washington.

Performance and Survey Contributions

Operating since 2000, the spectrograph achieved the throughput, resolution, and stability needed to deliver millions of galaxy, quasar, and stellar spectra, underpinning high-impact results such as precise measurements of the baryon acoustic oscillation scale, the Sloan Digital Sky Survey Quasar Catalog, and stellar population maps used in studies of the Galactic halo and thin disk. Data products fed follow-up science at institutions including Princeton University, Harvard University, Massachusetts Institute of Technology, and University of California, Berkeley, influencing cosmology results later compared with probes like Planck (spacecraft) and missions such as Gaia.

Upgrades and Successor Instruments

The original spectrograph underwent upgrades to increase fiber count and improve detectors for later SDSS phases, paving the way for successor spectrographs deployed in SDSS-III and SDSS-IV projects like the Baryon Oscillation Spectroscopic Survey and eBOSS. Lessons from its fiber and optical design informed new instruments at facilities including the Dark Energy Spectroscopic Instrument and spectrographs installed at observatories such as Kitt Peak National Observatory and Cerro Tololo Inter-American Observatory, with ongoing collaborations among institutions like Lawrence Berkeley National Laboratory, University of Chicago, and Fermilab.

Category:Spectrographs