The SkyProbe camera provides a measurement of the atmospheric
attentuation at a frequency of roughly one sample per minute. The
measurement is valid for the field at which the CFHT 3.6m telescope is
currently pointing, and is stable to ~0.3%. The sky transparency is
measured by measuring the flux of 100 - 400 stars in the Tycho
photometric catalog and comparing the results to past observations.
Since other systematic terms are calibrated, apparent variations in
the image zero point are due to presence of clouds or haze attenuating
the observed brightness of the stars in the field.
The system employs a small (500 x 700) SBIG camera with a 50 mm lens,
which observes a large field of view (roughly 5 x 7 degrees) to a
depth around 12 magnitudes. The camera is roughly co-aligned with the
telescope, and takes an image every 60 seconds. An analysis is
performed on the images, which includes flat-fielding, photometry,
astrometry, and matching the stars to the Tycho catalog. The
resulting image zero-points are added to a database, and plots
displaying the results are made available in real time on these web
Select the 'tonight' or 'recent' buttons to see a plot of the sky
transparency for the current night or past three hours. These plots
begin at 18:00 HST and stop at 06:00 HST. If the CFHT dome is closed
or the clouds are too thick, the plots will be marked 'no data
available'. These plots will update automatically every minute.