Photometer and Spacecraft
The Kepler photometer is a simple single purpose instrument. It is basically a Schmidt telescope design with a 0.95-meter aperture and a 105 deg2 (about 12 degree diameter) field-of-view (FOV). It is pointed at and records data from just a single group of stars for the four year duration of the mission.
The photometer is composed of just one "instrument," which is, an array of 42 CCDs (charge coupled devices). Each 50x25 mm CCD has 2200x1024 pixels. The CCDs are read out every 6 seconds to prevent saturation. Only the information from the CCD pixels where there are stars brighter than mv=14 is recorded. (The CCDs are not used to take pictures. The images are intentionally defocused to 10 arc seconds to improve the photometric precision.) The data are integrated for 30 minutes.
The instrument has the sensitivity to detect an Earth-size transit of an mv=12 G2V (solar-like) star at 4 sigma in 6.5 hours of integration. The instrument has a spectral bandpass from 400 nm to 850 nm. Data from the individual pixels that make up each star of the 100,000 main-sequence stars brighter than mv=14 are recorded continuously and simultaneously. The data are stored on the spacecraft and transmitted to the ground about once a month.
The spacecraft provides the power, pointing and telemetry for the photometer. Pointing at a single group of stars for the entire mission greatly increases the photometric stability and simplifies the spacecraft design. Other than the small reaction wheels used to maintain the pointing and an ejectable cover, there are no other moving or deployable parts The only liquid is a small amount for the thrusters which is kept from slosh by a pressurized membrane. This design enhances the pointing stability and the overall reliability of the spacecraft.