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The Multi-Object, Fiber-Fed Spectrographs for SDSS and the Baryon Oscillation Spectroscopic Survey

Stephen Smee, James E. Gunn, Alan Uomoto, Natalie Roe, David Schlegel, Constance M. Rockosi, Michael A. Carr, French Leger, Kyle S. Dawson, Matthew D. Olmstead, Jon Brinkmann, Russell Owen, Robert H. Barkhouser, Klaus Honscheid, Paul Harding, Dan Long, Robert H. Lupton, Craig Loomis, Lauren Anderson, James Annis, Mariangela Bernardi, Vaishali Bhardwaj, Dmitry Bizyaev, Adam S. Bolton, Howard Brewington, John W. Briggs, Scott Burles, James G. Burns, Francisco Castander, Andrew Connolly, James R. Davenport, Garrett Ebelke, Harland Epps, Paul D. Feldman, Scott Friedman, Joshua Frieman, Timothy Heckman, Charles L. Hull, Gillian R. Knapp, David M. Lawrence, Jon Loveday, Edward J. Mannery, Elena Malanushenko, Viktor Malanushenko, Aronne Merrelli, Demitri Muna, Peter Newman, Robert C. Nichol, Daniel Oravetz, Kaike Pan, Adrian C. Pope, Paul G. Ricketts, Alaina Shelden, Dale Sandford, Walter Siegmund, Audrey Simmons, D. Smith, Stephanie Snedden, Donald P. Schneider, Michael Strauss, Mark SubbaRao, Christy Tremonti, Patrick Waddell, Donald G. York

TL;DR

The paper documents the SDSS twin fiber-fed spectrographs, detailing their original design, anticipated performance, and the substantial 2009 BOSS upgrades that expanded fiber multiplexing to 1000 per plate, extended the wavelength range to 3560–10,400 Å, and boosted peak throughput via VPH gratings and modern CCDs. It presents a comprehensive account of the optical, mechanical, and detector architectures, calibration, flexure control, and data acquisition, along with measured performance from pipeline reductions and survey data. Key outcomes include successful mapping of large-scale structure, the first BAO detections, and a robust, modular instrument design that has enabled high-quality redshift measurements for hundreds of thousands of galaxies and quasars. The work demonstrates how thoughtful integration of fibers, VPH gratings, and advanced detectors can deliver high-throughput, wide-field spectroscopy suitable for cosmology and extragalactic science, with lasting impact on surveys like SDSS-III/BOSS and beyond.

Abstract

We present the design and performance of the multi-object fiber spectrographs for the Sloan Digital Sky Survey (SDSS) and their upgrade for the Baryon Oscillation Spectroscopic Survey (BOSS). Originally commissioned in Fall 1999 on the 2.5-m aperture Sloan Telescope at Apache Point Observatory, the spectrographs produced more than 1.5 million spectra for the SDSS and SDSS-II surveys, enabling a wide variety of Galactic and extra-galactic science including the first observation of baryon acoustic oscillations in 2005. The spectrographs were upgraded in 2009 and are currently in use for BOSS, the flagship survey of the third-generation SDSS-III project. BOSS will measure redshifts of 1.35 million massive galaxies to redshift 0.7 and Lyman-alpha absorption of 160,000 high redshift quasars over 10,000 square degrees of sky, making percent level measurements of the absolute cosmic distance scale of the Universe and placing tight constraints on the equation of state of dark energy. The twin multi-object fiber spectrographs utilize a simple optical layout with reflective collimators, gratings, all-refractive cameras, and state-of-the-art CCD detectors to produce hundreds of spectra simultaneously in two channels over a bandpass covering the near ultraviolet to the near infrared, with a resolving power R = λ/FWHM ~ 2000. Building on proven heritage, the spectrographs were upgraded for BOSS with volume-phase holographic gratings and modern CCD detectors, improving the peak throughput by nearly a factor of two, extending the bandpass to cover 360 < λ< 1000 nm, and increasing the number of fibers from 640 to 1000 per exposure. In this paper we describe the original SDSS spectrograph design and the upgrades implemented for BOSS, and document the predicted and measured performances.

The Multi-Object, Fiber-Fed Spectrographs for SDSS and the Baryon Oscillation Spectroscopic Survey

TL;DR

The paper documents the SDSS twin fiber-fed spectrographs, detailing their original design, anticipated performance, and the substantial 2009 BOSS upgrades that expanded fiber multiplexing to 1000 per plate, extended the wavelength range to 3560–10,400 Å, and boosted peak throughput via VPH gratings and modern CCDs. It presents a comprehensive account of the optical, mechanical, and detector architectures, calibration, flexure control, and data acquisition, along with measured performance from pipeline reductions and survey data. Key outcomes include successful mapping of large-scale structure, the first BAO detections, and a robust, modular instrument design that has enabled high-quality redshift measurements for hundreds of thousands of galaxies and quasars. The work demonstrates how thoughtful integration of fibers, VPH gratings, and advanced detectors can deliver high-throughput, wide-field spectroscopy suitable for cosmology and extragalactic science, with lasting impact on surveys like SDSS-III/BOSS and beyond.

Abstract

We present the design and performance of the multi-object fiber spectrographs for the Sloan Digital Sky Survey (SDSS) and their upgrade for the Baryon Oscillation Spectroscopic Survey (BOSS). Originally commissioned in Fall 1999 on the 2.5-m aperture Sloan Telescope at Apache Point Observatory, the spectrographs produced more than 1.5 million spectra for the SDSS and SDSS-II surveys, enabling a wide variety of Galactic and extra-galactic science including the first observation of baryon acoustic oscillations in 2005. The spectrographs were upgraded in 2009 and are currently in use for BOSS, the flagship survey of the third-generation SDSS-III project. BOSS will measure redshifts of 1.35 million massive galaxies to redshift 0.7 and Lyman-alpha absorption of 160,000 high redshift quasars over 10,000 square degrees of sky, making percent level measurements of the absolute cosmic distance scale of the Universe and placing tight constraints on the equation of state of dark energy. The twin multi-object fiber spectrographs utilize a simple optical layout with reflective collimators, gratings, all-refractive cameras, and state-of-the-art CCD detectors to produce hundreds of spectra simultaneously in two channels over a bandpass covering the near ultraviolet to the near infrared, with a resolving power R = λ/FWHM ~ 2000. Building on proven heritage, the spectrographs were upgraded for BOSS with volume-phase holographic gratings and modern CCD detectors, improving the peak throughput by nearly a factor of two, extending the bandpass to cover 360 < λ< 1000 nm, and increasing the number of fibers from 640 to 1000 per exposure. In this paper we describe the original SDSS spectrograph design and the upgrades implemented for BOSS, and document the predicted and measured performances.

Paper Structure

This paper contains 101 sections, 42 figures, 4 tables.

Table of Contents

  1. Introduction
  2. SDSS Spectrograph Design
  3. Design Requirements
  4. Telescope Design
  5. Number of Fibers
  6. Fiber Diameter
  7. Wavelength Range
  8. Resolving Power
  9. Throughput and Signal to Noise Ratio
  10. Fiber System Design
  11. Overview of Fiber System
  12. Cartridges
  13. Optical Fiber
  14. Fiber Harnesses
  15. Slitplates
  16. ...and 86 more sections

Figures (42)

  • Figure 1: Rendering of a fiber cartridge. The fiber cartridge consists of a cast aluminum body that supports the fiber harness, the two slitheads, and the plug-plate, which has a diameter of 800 mm. The slitheads are attached to the cartridge body with a spring-loaded seating system that provides alignment for insertion into the spectrograph bodies, but then allows the slithead to float free from the cartridge body and engage the slithead-to-spectrograph kinematic mounting system. Kinematic mounts around the periphery of the cartridge casting ensure accurate and repeatable placement of the cartridge with respect to the telescope.
  • Figure 2: Photograph of a BOSS fiber cartridge. Fibers plugged into the back of the plug-plate are routed in bundles to the slitheads (the two boxes standing upright at the left and right side of the cartridge). The design shown is identical to that used for SDSS except for the number and size of the fibers. For SDSS, 320 fibers are routed to each slithead, while for BOSS each slithead carries 500 fibers.
  • Figure 3: Photograph showing a fiber cartridge being installed on the telescope. The twin spectrographs are the green instruments on either side of the focal plane. The cartridge is raised by a hydraulic lift in the floor below the primary cell. When raised, the cartridge engages kinematic mounts for precise location. At the same time, the two slitheads engage the spectrographs, each of which is located by its own kinematic mounting features integral to the slithead and spectrograph optical bench. Installation takes approximately three to five minutes.
  • Figure 4: Two schematic views of the cartridge mounted on the telescope. Top: a cutaway side view showing the slitheads inserted into the spectrographs (only 9 fiber harnesses are shown). Bottom: top view showing the cartridge located between the two spectrographs which are mounted to the instrument rotator (depicted as the large outer circle).
  • Figure 5: Fiber harness for the SDSS spectrographs. The ends, which are plugged and unplugged during operations, are protected by tough nylon tubing, which terminates in the anchor block. The lens enlarges the view of the v-groove block termination at the slit end.
  • ...and 37 more figures