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Observatorio del Roque de los Muchachos


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Image of the MERCATOR
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The Mercator Telescope is a 1.2 m quasi-robotic telescope which scientific niche is focussed on monitoring variable celestial phenomena with a large range in typical time-scales (pulsating stars, gravitational lenses, Gamma Ray Bursts, active Galactic Nuclei). Currently, it is equipped with 2 permanently installed instruments, both operating in the visible part of the electromagnetic spectrum. The telescope is located at 2300 m above sea level at the Roque de los Muchachos Observatory on La Palma (Canary Islands, Spain).

An audiovisual about the telescope is available in several languages:

 English, Spanish, Dutch


The Mercator Telescope is a product of the longstanding collaboration between the Institute of Astronomy (University of Leuven, Belgium) and the Observatory of Geneva (Switzerland). Its design dates from the beginning of the 1990's and it was first assembled in Geneva during the years 1998-1999.
The Mercator Telescope was reassembled in the year 2000 on La Palma.

It became operational in May 2001 and is supported by staff of the Institute of Astronomy, Katholieke Universiteit Leuven in collaboration with the Observatory of Geneva.

The telescope was named after the famous Flemish cartographer Gerardus Mercator (1512-1594), who studied and taught at the University of Leuven before moving to Duisburg (Germany).

Mercator is famous for his scheme of projecting a globe on a plane while keeping the angles correct.


The Nasmyth focus of the MERCATOR telescope is equipped with a two-channel photometer called P7-2000. The photometer allows for quasi-simultaneous measurements in the 7 colors of the Geneva photometric system (340nm-590nm) of stars till magnitude 13 with a precision between 0.1% and 1%. Alternatively the instrument can be blocked on a specific filter and fast photometry of relatively bright stars becomes possible.

In the summer of 2004, a CCD camera called MEROPE, has been installed at the Cassegrain focal plane. This camera has a filterwheel with 16 positions and can be used for 2D-photometry and imaging of fainter objects in a broader part of the electromagnetic spectrum (330nm-760nm). The field of view of this instrument is 6.5 arcmin squared with a spatial resolution of 0.2 arcseconds per CCD pixel.

In January 2005 the project to build a high efficiency, high-resolution spectrograph HERMES was approved. The spectrograph will be fibre fed with a wavelength coverage from 380nm-875nm in one shot and a resolution in between 50000 and 90000. The instrument will be installed at the end of 2007 on the telescope.


The primary mirror's diameter measures 1.2 m and in combination with the secondary mirror of 0.3 m, this semi-automatic telescope has a focal length of 14,4 m (plate scale of 14.324 arcsec/mm). There is also a third flat mirror which can occupy 2 positions, one at 45 degrees of the telescopes optical axis to deviate the light to the Nasmyth focus and a second position outside the optical beam to let the light pass to the Cassegrain focus. The optical combination is of a Ritchey-Chretien type with two main hyperbolic mirrors (one concave and one convex).

The unvignetted field of view of the telescope is 20 arcmin or 2/3 of the angular diameter of the moon. Its altazimuth mounting is computer controlled and allows very precise tracking. The motion around the vertical axis occurs on an oil pad of only 25 micrometers thickness. The high quality of the mirrors ensures that the sharpness of the images is only set by atmospheric turbulence (which is usually very low on La Palma). The primary mirror, with a mass of 385 kg, is supported by 15 dorsal and 4 radial pressurized air pads plus 6 fixed points.


The Mercator Telescope has a flexible operational scheme, which allows to optimize detailed monitoring campaigns on very different timescales.

The main science drivers of the telescope and its instruments are therefore related to a wide range of variable phenomena observed in stars and galaxies.

The main astrophysical areas covered by the different programs are the variability studies of stars on or near the main sequence to probe their internal structure parameters (asteroseismology); variability studies of evolved stars to gain insight in their fast evolutionary phase; detailed measurements of gravitational lens time delays of quasars to be used as cosmological probes; detection and follow-up of the optical counterparts of Gamma Ray Bursts for which a rapid response mode was implemented.

- Asteroseismology: By studying the intricate variability pattern some stars exhibit one can derive their internal structure in much the same way geophysicists unravel the internal structure of the earth from earthquake observations. Such studies for stars require many observations that are suitably spred in time. The results have profound implications on our understanding of stellar structure and evolution, and in particular on determining the distance scale and the age of the universe.

- Gravitational lenses of quasars: when a galaxy lies near the line of sight of a quasar, its gravitational field deflects the light to form several images of the same quasar. The study of this phenomenon holds important clues on the quasar as well as the lensing galaxy. Monitoring of gravitational lenses provides an independent way to estimate the distance scale of the universe and so the Hubble constant. This project uses a new image deconvolution technique developed in Liege and exploits the superb seeing qualities of the La Palma site.

- Gamma Ray Bursts: The high energies of the electromagnetic spectrum can best be probed from space. Several X-ray and gamma-ray satellites are now operational, and several more will be launched in the near future. The satellites often detect peculiar sources involving compact objects (neutron stars, black holes) that require rapid follow-up observations from the earth. For such observations a telescope which is available in a flexible way and the scheduling of which enables continuous follow-up, offers important advantages. MERCATOR will be available for the monitoring of gamma-ray bursters, X-ray transients, supernovae..., and thereby usefully complement space projects. Quite often space projects detect the interesting sources, but they are realy understood only after ground-based follow-up.

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