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CERGA Observatory

Gallery of 8 photos for CERGA Observatory

The Observatoire du CERGA is located on the southern edge of the Plaine de Calern. CERGA (Centre d'Etudes et de Recherches Géodynamiques et Astronomiques) does specialized research in measuring movements of the Earth and development of astronomical instruments.

The CERGA site can be visited at any time, to wander amidst the white domes and enjoy the great views and the nature. Information panels posted around the site have pictures and explanations of the local fauna and flora. There are also very interesting and very informative guided tours of the CERGA installations, during July and August, given by the scientists who work there. (The official CERGA web is at "".)

If you visit the site any night that isn't overcast (most of the time), you'll see the erie sight of laser pulses streaking up through the sky towards invisible satellites as they pass over. We experienced this during a few nights during the Spring of 1997 when we (and several other people) were taking pictures of the Haly-Bopp comet.

Schmidt Telescope

site photo The Schmidt-telescope building is located at the edge of the plain, to the left of the road as you drive up. Its white dome is visible from the valley below. The telescope has a 152-cm mirror and a focal distance of 3161 mm. One of three large Schmidt telescopes in the northern hemisphere, this is used to detect normal astronomical objects, catalogue objects inside and outside our "Milkyway" galaxy, precision measurement of weak objects and study the galactic structure. Measurements are made using photographic images (from ultraviolet to near infrared) and CCD mosaics.

Optical Interferometer

An optical interferometer uses a pair of telescopes to measure the diameter of stars.


Laser and IR photo The Soirdete interferometer observes cool stars at infra-red wavelengths, and has a fixed east-west baseline, giving it a changing sky-plane projection due to the earth's rotation. In this photo, the Soirdete is in the foreground, with the moon-laser and satellite-laser behind to the left.

I2T - Interféromètre à 2 Télescopes

The "petit" I2T is used to determine the apparent dimension and structure of stars, and ultimately to photograph them. The light from two telescopes are converged with optical recombination, and the "interference zones" (I need some help here) are analyzed.

Cerga's I2T interferometer uses two 26-cm telescopes separated by 140 m on a north-south baseline. During the observation, real-time corrections for atmospheric turbulence are made in thousands of a second. The "petit" telescopes are housed in a plain little building perched in the center of the long north-south rails.

GI2T - Grande Interféromètre à 2 Télescopes

site photo The GI2T (the two "jars" behind the working quarters in our photo) has a pair of 1.52-m telescopes mounted on rails oriented north-south to give them a separation of 13 to 65 metres. The GI2T observes at visible wavelengths, and studies the gaseous envelopes around hot stars and very-close-together double stars. Other active studies include the variation of the angular diameter of pulsating stars (cepheid type stars), the study of the gas kinematics around hot stars, and the measurement of limb-darkening of stellar photospheres.

site photo Results of its studies are being used in two ambitious projects:
• The OVLA (Optical Very Large Array) project (of A. Labeyrie) intends to distinguish the details of the surface of stars early in the next century.
• VLT, the European Very Large Telescope project.
Our second photo here is a front view of the GI2T with the Schmidt in the background.

Laser Telemetry

In laser telemetry, a stream of laser pulses from Earth are bounced off reflectors on artificial satellites or the moon, and back to the Earth. The precise distance between Earth and satellite is calculated by measuring the round-trip time of each pulse. CERGA has two laser units mounted at opposite ends of the same building .

Satellite Laser

site photo The "satellite" laser in the rectangular housing has a 1-m mirror. Measuring the precise position of a satellite, who's position is already known, gives a precise measurement of a position on the earth. From that, information such as the movement of tectonic plates can be determined.

We also have a photo of the actual laser unit (31 k) inside.

Moon Laser

site photo The "moon" laser (in this photo) has a 1.5-m mirror, and bounces laser pulses off reflectors placed on the moon by American and Russian space missions. Observations taken together over the past twenty years by the CERGA observatory and the observatory at McDonald, Texas have been used to test certain theories, such as Einstein's theory of relativity.

Solar Astrolabes

A solar astrolabe can be used to measure the position of stars, determine the precise orbit of the earth around the sun, or measure the "diameter" of the sun. The planispheric astrolabe was used by the Greeks in the 1st century BC, and was the most important instrument used by astronomers and navigators from medieval times through the 16th century.

The astrolabes at Calern have measured the position of over 120 000 stars, with a precision of 1/100-second of a degree, since 1989. The astrolabe uses double reverse reflections, with a mercury bath as the bottom mirror. When the two images of the celestial object (such as a star) cross in the center of the objective, the object is at the precise angle of the reflector prism. Since 1987, the main Calern astrolabe as 11 prisms to measure angles from 30° to 70°. The astrolabs are house in unassuming little buildings with metal sides.

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