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 pages.

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.