  The Kepler Mission relies on precise differential photometry to detect the 80
parts per million (ppm) signal from an Earth-Sun equivalent transit. Such
precision requires superb instrument stability on time scales up to ~2 days and
systematic error removal to better than 20 ppm. To this end, the spacecraft and
photometer underwent 67 days of commissioning, which included several data sets
taken to characterize the photometer performance. Because Kepler has no
shutter, we took a series of dark images prior to the dust cover ejection, from
which we measured the bias levels, dark current, and read noise. These basic
detector properties are essentially unchanged from ground-based tests,
indicating that the photometer is working as expected. Several image artifacts
have proven more complex than when observed during ground testing, as a result
of their interactions with starlight and the greater thermal stability in
flight, which causes the temperature-dependent artifact variations to be on the
timescales of transits. Because of Kepler's unprecedented sensitivity and
stability, we have also seen several unexpected systematics that affect
photometric precision. We are using the first 43 days of science data to
characterize these effects and to develop detection and mitigation methods that
will be implemented in the calibration pipeline. Based on early testing, we
expect to attain Kepler's planned photometric precision over 80%-90% of the
field of view.
