Multi-Object Spectroscopy

Multi-object spectroscopy is a technique for obtaining detailed spectra for multiple targets simultaneously. The two favoured approaches for obtaining spectroscopy of faint targets, where the removal of the contaminating sky background is critical, are (i) multi-slit spectroscopy and (ii) multi-fibre spectroscopy. CfAI has been involved in pioneering instruments using both of these techniques, with recent examples achieving multiplexes of several thousand, thus massively increasing the speed at which statistical surveys of large samples of stars or galaxies can be completed.

Various stages of the production of one (out of 10) 500-fibre cables for the Dark Energy Spectroscopic Instrument developed at CfAI for the Lawrence Berkeley National Laboratory in the US.

Multi-slit spectroscopy uses a focal plane mask containing multiple short slitlets to isolate the object spectrum and local sky background for each target. The associated spectrographs are often of transmissive design (using grisms rather than gratings as dispersing elements) to allow a compact instrument which can be mounted on the telescope. An early example of this type of instrument built at CfAI was the Low Dispersion Survey Spectrograph [1] for the 3.9-m Anglo-Australian Telescope (AAT), which was later duplicated for use on the 4.2m William Herschel Telescope (WHT) and subsequently the 6.5m Magellan Telescope [2].

Multi-fibre spectroscopy uses an array of individual optical fibres to relay light from each target in the focal plane of the telescope to the input slit of a conventional spectrograph. The excellent transmission properties of modern optical fibres allows the use of long fibre runs and the spectrograph can then be mounted in a thermal and gravity stable position off the telescope. The original multi-fibre systems used manual plug-plates to locate the fibres in the focal plane, but these have rapidly been replaced by robotic fibre positioning systems. Durham has been involved in the design and build of a number of these multi-fibre spectrograph systems including Autofib-1 for the AAT [3], Autofib-2 for the WHT [4], FMOS on Subaru [5], and most recently the fibre cable system for the 5000-fibre DESI instrument on the 4.0m Mayall Telescope at Kitt Peak [6]. Some images of the cable-winding procedure for DESI are shown above. Developing new ideas for cooled cryogenic optical fibre cables for infrared wavelengths is an active area of current CfAI research.

[1] A Low Dispersion Survey Spectrograph
[2] A Low Dispersion Survey Spectrograph (LDSS-2) for the William Herschel Telescope
[3] An automated multiobject fibre optic coupler for the Anglo-AustralianTelescope
[4] Autofib–2: an automated fiber positioner for the prime focus of the William Herschel Telescope
[5] Design and construction of the fibre system for FMOS
[6] Fiber system design for the Dark Energy Spectroscopic Instrument (DESI)