ASTR 511 (O'Connell) Lecture Notes


2dF on AAT

Two Degree Field Fiber Spectrograph on AAT (J. Pogson)

Multi-beam spectrographs are among the most sophisticated and expensive instrumentation used by astronomers today, costing upwards of $5M on a large telescope. Here are some state-of-the-art examples. The primary application of these systems has been to large samples of faint galaxies.


Fiber-optic transfer devices typically offer small (2-4 arcsec) entrance apertures for each target. Fibers must be repositioned with high precision for each new field. This is usually done by mechanical robots. In most designs, individual fiber apertures are clamped magnetically on a flat focal-plane plate. Output of the fiber unit is usually a linear (slit-like) array at the spectrograph input, yielding a fixed position on the focal plane for each spectrum.

  • SDSS-I Spectrograph (2.5-m)

      Cassegrain mounting

      Two dual spectrographs (R ~ 2000). Red/blue light split in each by diagonal dichroic mirror [red light transmitted, blue reflected]

      640 fibers (320 each spectrograph) manually plugged into precision drilled aperture plate covering 3 degree FOV

SDSS Fibers


Aperture-plate spectrographs use small apertures in a focal-plane mask at the spectrograph entrance to transmit light from selected targets. Usually, computer-controlled devices (mechanical cutters, lasers) are used to cut slits in a thin, shaped, metallic mask. Early designs used photographic masks. The slits can be of arbitrary shape and length within overall constraints set by the spectrograph focal plane. The distribution of spectra in the focal plane depends on the distribution of targets in the field.

Main operational problem is to avoid overlap of spectra and to maximize use of the detector area in a given field; this requires special optimizing software. In principle, aperture plate designs should have better throughput, better sky background subtraction, and better flux calibration than fiber designs. Fiber designs can accommodate more targets, however (because the output format on the detector is fixed and optimally packed).


An integral field unit (IFU) produces distinct spectra for many contiguous elements in a given compact field. Powerful for the study of extended objects like globular clusters or nearby galaxies. Relative aperture positions and sizes are fixed and generally cover a square/retangular area. IFU's have been designed using fiber bundles, lenslet arrays, and configurable microaperture or micromirror arrays.


The most conspicuous use of high precision spectroscopy has been in the detection of extra-solar planets through stellar reflex Doppler shifts, where velocity differences of order 5 m/s must be measured. Requires both high spectral resolution and great mechanical/optical stability in spectrograph. Suitable designs employing digital detectors have been around since the late 1970's but were not energetically exploited until the surprising detection of a Jupiter mass planet in a sub-AU orbit made in 1995 by Mayor & Queloz at Haute-Provence Observatory.


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Last modified August 2010 by rwo

Text copyright © 2001-2010 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 511 at the University of Virginia.