Instrument Performance in Kepler's First Months

TL;DR

This paper evaluates Kepler's instrument performance during its initial commissioning, identifying systematics and confirming detector stability, to ensure the mission's goal of detecting Earth-like transits with high precision.

Contribution

It provides an early assessment of Kepler's instrument stability, identifies complex artifacts and systematics, and develops mitigation methods for high-precision photometry.

Findings

Detector properties match ground tests, indicating stability.

Complex image artifacts interact with starlight and thermal effects.

Expected to achieve planned photometric precision over most of the field.

Abstract

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.