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LUCAS experiment: Observations of Earthshine in Antarctica

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    1. LUCAS experiment: Observations of Earthshine in Antarctica Danielle BRIOT (Paris-Meudon Observatory)

    3. Restrictions of Earthshine observations As it is well known, at mean or low latitudes, Earthshine observations are are observations of twilight : just after the sunset or just before the sunrise. So observations can only be made during a short time. And roughly speaking, for one telescope, only two enlighted parts of Earth can be facing the Moon : either the part located at the West of the observing telescope for evening observations (begginning of the lunar cycle), or the part of Earth located at the East of the observing telescope for morning observations (last days of the lunar cycle).

    4. New perspectives However, there other possibilities. From an idea of Jean Schneider (2002), if observations are made from a site located at a high latitude, conditions of Earthshine observations are different. From six to eight times in a year, around equinoxes, Earthshine can be observed during several hours, and even in very high latitude places during a 24 hour duration (total nycthemere). During these long observing times, and due to the terrestrial rotation, several landscapes alternately face the Moon. Actually, Antarctica offers a very good opportunity for this kind of observations.

    5. The Concordia station The Concordia station is a French-Italian base for scientific research, among them astrophysics, located at Dome C in Antarctica. Latitude : -7506 South Longitude : 12323 East Altitude : 3220 m on a plateau Mean temperature : - 54C Lowest temperature : - 81.9C

    6. The Concordia station Dome C is located (3220 m altitude) on sub-horizontal ice ground with no crevasses. There is no local fauna or flora. The distance is around 1100 km from the French base Dumont dUrville (Terre Adlie) and around 1200 km from Terra Nova bay where is established the Italian station Mario Zucchelli, behind the Transantarctic mountain range.

    7. Dome C location

    8. Antarctic Continent

    9. Observational qualities of the Dome C site (1) The Dome C is a very good site for astronomy, maybe the best in the world. Many testing instruments have been operated during the last years, and specially during the last five years. It is an extremely clear site : instrumentation has given lower limits of 74 - 80% to the percentage of clear skies, and recent estimates put this number to a remarquable 96% over a 5 month period.

    10. Observational qualities of the Dome C site (2) As well as having one of the lowest average wind speed ever recorded at an existing or potential observatory, Dome C also has an extremely stable upper atmosphere and a very low inversion layer. The atmosphere is divided in two regions : (i) a 36 m high surface layer responsible for 87% of turbulence and (ii) a very stable free atmosphere above with a median seeing of 0.360.19 arcsec at an elevation of h = 30m.

    11. Preliminary observations at Concordia Checking the feasability of Earthshine observations considering the darkness of the sky was the first point. Is the sky enough dark to allow observations of Earthshine ? The first tests have been planned during the first winterover campaign in 2005, by Karim Agabi. However, bad weather conditions did not allow some conclusive observations. In 2006, photographic tests made by Eric Aristidi showed that Earthshine observations can be possible.

    12. Then, we decide to make observations to observe Vegetation Red Edge and biomarkers during the southern winter of 2008. It was first necessary to collect some funds and then to design and construct a special instrumentation for Earthshine spectroscopic observations in the extreme conditions corresponding to Concordia station. It was a rather tight schedule !

    13. LUCAS technology Special instrumentation for Earthshine observations : The telescope is a Celestron 8 Diametre : 203 mm The spectrograph was designed by Luc Arnold and Pierre Riaud and built at the Haute-Provence Observatory. Grating : 300t/mm CCD : Camera Audine - Detector KAF 402 ME

    14. 3D assembly diagram of LUCAS

    15. LUCAS instrumentation during testing at the Haute-Provence Observatory

    16. LUCAS technology Acquisition and storage of observational data will be provided by a computer located in a igloo at around twenty meters of the telescope. Testings carried out at the Haute-Provence observatory validated the instrumentation. The mean temperature being - 54C and the extreme temperature being - 81.9C, all the instruments (telescope, camera, spectrograph and all acquisition system) have been to be antarctised that is to say specially adapted for very cold conditions. And Luc Arnold has written a very detailed handbook intented for winterover observers.

    17. Position of the slit on the lunar surface : The observing slit is to be positionned first on both the lighted crescent and the sky, and then on both the Earthshine and the sky.

    18. Example of conditions of astronomical observations at the Concordia station

    19. Before their departure, winterover observers made testing of observations at the Haute-Provence observatory. And the instrumentation was carried to the Dome C. Since the Haute-Provence Observatory (South of France) the way is: Plouzan (Institut Paul Emile Victor, in the western part of France), Roissy (Paris airport), Hong-Kong (China), Hobart (Australia), Dumont-dUrville (Antarctica) and finally attained the Concordia station at Dome C.

    20. Current situation The LUCAS instrumentation arrived at Concordia last week, on the 9th of January. And first observations are planned for the beginning of March 2008 (Southern autumn). We have to note that LUCAS takes place among the first purely astrophysical experiments at the Dome C site.

    21. General schedule January 2008 March 2008 Conveying and installation of instruments at Concordia; testing in real conditions. March 2008 September 2008 First campaign of observations of Earthshine; reception of data Preparation of the 2009 observation campaign: buying of a new mounting; conveying and installation at Concordia. October 2008 Dcember 2009 Analysis and interpretation of data; first publications and communications in scientific meetings. Second campaign of Earthshine observations; reception and analysis of data.