Speaker
Jean-Charles Cuillandre
(Canada-France-Hawaii Telescope)
Description
Mauna Kea is known for its pristine seeing conditions, but sky transparency
can be an issue for science operations since some 25% of the observable
nights are not photometric, mostly due to high-altitude cirrus. Since 2001,
the original single-channel SkyProbe on the Canada-France-Hawaii Telescope
has gathered one V-band 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 (40 seconds) to capture at least 100 stars of
Hipparcos' Tycho 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 its capacity to derive an accurate
absolute measurement, within a few percents, 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 the majority of the
telescope time: science exposures taken in non-photometric conditions are
automatically registered a new observation later on at 1/10th of the original
exposure time per pointing in the observed filters in photometric conditions
to ensure a proper final absolute photometric calibration. Photometric
standards are observed only when conditions are reported stable by SkyProbe.
The new dual color system (simultaneous B&V bands) will allow a better
characterization of the sky properties atop Mauna Kea and will 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.
Primary author
Jean-Charles Cuillandre
(Canada-France-Hawaii Telescope)