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Proceedings Paper

SkyProbeBV: dual-color absolute sky transparency monitor to optimize science operations
Author(s): Jean-Charles Cuillandre; Eugene Magnier; Dan Sabin; Billy Mahoney
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Paper Abstract

Mauna Kea (4200\,m elevation, Hawaii) is known for its pristine seeing conditions, but sky transparency can be an issue for science operations: 25% of the nights are not photometric, a cloud coverage mostly due to high-altitude thin cirrus. The Canada-France-Hawaii Telescope (CFHT) is upgrading its real-time sky transparency monitor in the optical domain (V-band) into a dual-color system by adding a B-band channel and redesigning the entire optical and mechanical assembly. Since 2000, the original single-channel SkyProbe has gathered one exposure every minute during each observing night using a small CCD camera with a very wide field of view (35 sq. deg.) encompassing the region pointed by the telescope for science operations, and exposures long enough (30 seconds) to capture at least 100 stars of Hipparcos' Tychos catalog at high galactic latitudes (and up to 600 stars at low galactic latitudes). A key advantage of SkyProbe over direct thermal infrared imaging detection of clouds, is that it allows an accurate absolute measurement, within 5%, of the true atmospheric absorption by clouds affecting the data being gathered by the telescope's main science instrument. This system has proven crucial for decision making in the CFHT queued service observing (QSO), representing today 95% of the telescope time: science exposures taken in non-photometric conditions are automatically registered for being re-observed later on (at 1/10th of the original exposure time per pointing in the observed filters) to ensure a proper final absolute photometric calibration. If the absorption is too high, exposures can be repeated, or the observing can be done for a lower ranked science program. The new dual color system (simultaneous B & V bands) will allow a better characterization of the sky properties above Mauna Kea and should enable a better detection of the thinner cirrus (absorption down to 0.02 mag., i.e. 2%). SkyProbe is operated within the Elixir pipeline, a collection of tools used for handling the CFHT CCD mosaics (CFH12K and MegaCam), from data pre-processing to astrometric and photometric calibration.

Paper Details

Date Published: 12 July 2008
PDF: 8 pages
Proc. SPIE 7016, Observatory Operations: Strategies, Processes, and Systems II, 70160U (12 July 2008); doi: 10.1117/12.787300
Show Author Affiliations
Jean-Charles Cuillandre, Canada-France-Hawaii Telescope Corp. (United States)
Eugene Magnier, Institute for Astronomy (United States)
Dan Sabin, Canada-France-Hawaii Telescope Corp. (United States)
Billy Mahoney, Canada-France-Hawaii Telescope Corp. (United States)


Published in SPIE Proceedings Vol. 7016:
Observatory Operations: Strategies, Processes, and Systems II
Roger J. Brissenden; David R. Silva, Editor(s)

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