MEFOS

The Meudon-ESO Fiber Optic Spectrograph (MEFOS) was mounted at the prime focus of the ESO 3.6-metre telescope at La Silla Observatory, in 1991. It was built by the Department of Extragalactic Astrophysics and Cosmology of the Paris-Meudon Observatory in collaboration with the National Center for Scientific Research (CNRS) under an ESO contract.

MEFOS was a fibre positioner with 30 arms (29 dedicated to astronomical object observation and 1 used for guiding), arranged in a circle and that could cover a 1 degree field of view in diameter. This was considered a large field of view compared to OPTOPUS, its predecessor, which covered 0.5 degrees. The main purpose of MEFOS was multi-object spectroscopy of faint objects. Each arm was equipped with an individual viewing system and two spectroscopic fibres: one fibre observed the astronomical object and the other observed the sky and was used for sky subtraction.

MEFOS was built in a way that allowed real time adjustment to compensate for atmospheric refraction. It also had a fast exchange between the fibres, used to record the object and the surrounding sky which led to a better sky subtraction. The whole instrument was controlled by a computer that read the object names and coordinates supplied on a floppy disc by the users. Software assigned an object to each arm using the Hungarian algorithm, in order to find the best match and avoid collisions between the arms.

The first observations with MEFOS were of a field of galaxies between magnitudes 17 and 19.9. Over time, MEFOS was used in many more observation campaigns, from redshift measurements to mapping of merging clusters.

MEFOS was an improved version of OPTOPUS with an automatic starplate system and the arm approach instead of a manual one. MEFOS was connected with the Boller and Chivens Spectrograph (B&C) that was set at the Cassegrain focus through a 21-metre optical fibre.

MEFOS was decommissioned from the ESO 3.6-metre telescope around 1997. It was re-commissioned by LESIA of the Paris Observatory and used in a new instrument called the Multi-object Instrument for Occultations in the SOlar system and TransitorY Systems (MIOSOTYS). MIOSOTYS has since been mounted on the 193-cm telescope at the Haute-Provence Observatory in France, and on the 123-cm telescope at the Calar Alto Observatory in Spain. The goal of the new survey was to characterise the spatial distribution and extension of the Kuiper Belt as well as the physical size distribution of trans-Neptunian objects (TNOs).