SkyProbe About

The SkyProbe Camera

SkyProbe consists of a short focal-length lens (aoubt 50 mm) and a small SBIG CCD. It images a 5x7 degree patch of the sky roughly centered on the CFHT field of view. The goal of this camera is to measure the transparency of the sky and to provide in real time a quantitative indication of the current attenuation by clouds.

In the current setup, a 35 second exposure is taken approximately every minute. These images are automatically analysed with tools from Elixir: dark and flat correction with Flips, followed by stellar photometry with Sextractor (E. Bertin). The stellar measurements are calibrated astrometrically and photometrically by reference to the Tycho Catalog. The resulting calibrated measurements are incorporated in an Elixir photometry database.

In a typical image, there will be approximately one to two hundred stars matched with the Tycho Catalogue. The mean of the magnitude differences is computed and this information provides a measure of the sky transparency. Each data point in the magnitude attenuation plots is the mean of the magnitude differences of a single image.

Currently, the error in the magnitude attenuation is at ~2% level, and is succeptible to systematic errors caused by the sample of observed stars. As a result, it is sometimes hard to distinguish between actual attenuation due to clouds and other effects. For this reason, we provide here some references on how to interpret the data.

Interpreting The Plots

The following picture shows an example of a photometric night as seen with SkyProbe. Ideally, on a photometric night, the magnitude attenuation will center around zero with an error that is characteristic to our system. At this moment, the error in our measurement is at the 2% level, i.e., deviation from the baseline will in general be within 0.02 magnitudes.

Unfortunately, our measurement of the magnitude attenuation is sensitive to bias, as discussed above. This leads to distinct segments seen in the plot. In general, abrupt changes like this are most probably artifical. Within each segment, the deviation from the mean is quite small. This lends further support to the shift being caused by biased measurement rather than real attenuation. Notice the slight swelling around t = 7. At this moment, the analysis procedure is not accurate enough to tell us conclusively whether it is due to clouds drifting over or some other artificial effects in the analysis. The bottom panel in the plot is the standard deviation of the mag attenuation in an 1 hr window.

Below we show an example of a non-photometric night. See the archive for more example nights.