  We present z band photometry of three consecutive transits of the exoplanet
TrES-1, with an accuracy of 0.15% and a cadence of 40 seconds. We improve upon
estimates of the system parameters, finding in particular that the planetary
radius is 1.081 +/- 0.029 Jupiter radii, and the stellar radius is 0.811 +/-
0.020 solar radii. The uncertainties include both the statistical error and the
systematic error arising from the uncertainty in the stellar mass. The transit
times are determined to within about 15 seconds, and allow us to refine the
estimate of the mean orbital period: P = 3.0300737 +/- 0.0000026 days. We find
no evidence for star spots or other irregularities that have been previously
reported.
