Using the TAOS telescopes, we observed a transit of a known exo-planet, XO-2b, at Feb. 9th, 2008. Two consecutive high speed zipper mode data runs, lasting a total of nearly 3 hours, were collected during the observation, which is enough to cover the duration of the transit event. (Two data runs were performed because the first run terminated early due to an unknown glitch in the scheduling software.) Due to the low signal-to-noise ratio of TAOS light-curves (mainly attributed to the high sampling rate of 5Hz) we first binned the light-curves of XO-2b with 1,000 data points and then removed trends using Trend Filtering Algorithm (Kovacs et al. 2002). To remove trends, we used several hundred field stars extracted from the same zipper mode data runs.
Figure 1 shows the raw lightcurves before binning and detrending and Figure 2 shows detrended lightcurves. In both figures, the top panels show the lightcurves of XO-2b and the bottom panels show the lightcurves of a comparison star. As the figures show, the transit signals are clearly visible after data processing. The green line in the top panel of Figure 2 is the theoretical transit lightcurve of XO-2b (see Mandel & Agol 2002 for details about the lightcurve modeling for transits).