Fibre Optics Multi-Object Spectrograph

The Fibre Optics Multi-Object Spectrograph (OPTOPUS) enabled multiple object spectroscopy (MOS) to be carried out at the Cassegrain focus of the ESO 3.6-metre telescope at La Silla Observatory in Chile. The instrument was named after it’s optical fibres which had an octopus-like appearance. OPTOPUS was the first fibre multiple object spectrograph at ESO.

A prototype of OPTOPUS was tested at the ESO 3.6-metre telescope in November 1982. After several tests and improvements, OPTOPUS was made available to ESO astronomers in March 1985. In 1988, OPTOPUS was upgraded to improve the overall efficiency and number of operation modes, becoming known as OPTOPUS2.

The fibre-fed design of OPTOPUS allowed the light from up to 54 sky targets within 25 arc-minutes of the Cassegrain field of view — almost half a degree, to be simultaneously launched to the entrance slit of the Boller and Chivens Spectrograph (B&C). In other words, it enabled the conventional use of the B&C to be extended to multi-object spectroscopy.

To take observations, OPTOPUS used circular aluminium disks with accurately drilled holes called starplates, engineered so that connected fibres collected light from one object at a time. One plate was used for a given field in the sky. Before observing with OPTOPUS, astronomers had to provide the positions of interest for the starplates. For that the user had to identify and measure the positions of the targets on Schmidt plates. A huge X–Y densitometer on a granite table was used to accurately measure the positions of the targets on the sky. The data was reformatted and entered in the new ESO workshop Computer Numerical Control (CNC) milling machine for the drilling of the starplates. Once drilled, each hole was properly labelled for target identification. Two additional holes were made to point to bright stars for alignment and guiding of the starplates on the focal plane of the telescope.

The milling process was expensive and extremely time-consuming. A single observation could require machining 1500 precision-reamed holes. Later, software updates to the machining equipment lowered the preparation time a little. The plates were packed and sent to La Silla well in advance to arrive in time for the observations. It was a very long and careful preparation process at the time, with limited facilities.

In December 1985, OPTOPUS2 was used for the first time by regular visitors at La Silla Observatory. It yielded measurements of roughly 1000 spectra over 9 nights, including quasar candidates, carbon stars in Fornax, planetary nebulae, galaxy clusters and selected regions of the coma of Halley’s Comet.

OPTOPUS2 was decommissioned from the ESO 3.6-metre telescope in 1992 to be replaced by MEFOS, the MOS instrument with deployable arms instead of fixed holes to position the fibres in the field of view of the ESO 3.6-metre telescope prime focus.

OPTOPUS at the ESO 3.6-metre telescope

This table lists the global capabilities of the instrument.

Location: Decommissioned
Telescope: ESO 3.6-m telescope
Focus: Cassegrain
Type: Multi object spectrograph
Wavelength range: 30–850 nm
Fibre aperture on sky: 2.3 arc-seconds
Spectral resolution: < 1000
First light: March 1985
Science goals:
  • Spectroscopy
  • Radial velocity measurement
  • Redshift determination
  • Galaxy surveys
Images taken with the instrument: N/A
Images of the instrument: N/A
Press Releases with the instrument: Link
  • ESO