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
three 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 Kepler Photometer
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 week.
Photometer Mounted on the Spacecraft
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
gimbaled antenna for communications, the small gyroscopes used
to maintain the pointing and an ejectable cover, there are no
other moving or deployable parts and there are no liquids to
slosh. This enhances the pointing stability and the overall reliability
of the spacecraft.
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