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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.
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