Photometer and spacecraft description

The instrument is a wide-FOV differential photometer with a 100 square degree FOV that continuously and simultaneously monitors the brightness of 100,000 main-sequence stars with sufficient precision to detect transits by Earth-size planets orbiting G2 dwarfs.

38 W. J. Borucki et al.: The Kepler mission

Sunshade

Schmidt Corrector 0.95 m dia

Schmidt Corrector 0.95 m dia

Schmidt Corrector

Figure 1. Isometric view of the Kepler photometer.

Graphite Cyan ate Metering Structure

Figure 1. Isometric view of the Kepler photometer.

The brightness range of target stars is from visual magnitude 9 through 15. The photometer is based on a modified Schmidt telescope design that includes field-flattener lenses near the focal plane. Figure 1 is an isometric view of the photometer. The corrector has an aperture of 0.95 m with a 1.4 m diameter F/1 primary. This aperture is sufficient to reduce the Poisson noise to the level required to obtain a 4a detection for a single transit from an Earth-size planet transiting a 12th magnitude G2 dwarf with a 6.5 hour transit. The focal plane is composed of 42 1024 x 2200 backside-illuminated CCDs with 27 pm pixels.

The detector focal plane is at prime focus and is cooled by heat pipes that carry the heat out to a radiator in the shadow of the spacecraft. The low-level electronics are placed immediately behind the focal plane. A four-vane spider supports the focal plane and its electronics and contains the power- and signal-cables and the heat pipes.

The spacecraft bus encloses the base of the photometer and supports the arrays and the communication, navigation, and power equipment. Two antennas with different frequency coverage and gain patterns are available for uplink commanding and for data downlink. A steerable high-gain antenna operating at Ka band is used for high-speed data transfer to the Deep Space Network (DSN). It is the only articulated component other than the ejectable cover. Approximately one GByte/day of data are recorded and then transferred to the ground every few days when contact is made with the DSN. The spacecraft provides very stable pointing using four fine guidance sensors mounted in the photometer focal plane. Small thrusters are used to desaturate the momentum wheels. Sufficient expendables are carried to extend the mission to six years.

Both the photometer and the spacecraft are being built by the Ball Aerospace and Technology Corporation (BATC) in Boulder, Colorado. NASA Ames manages the photometer development, mission and operations, and scientific analysis. JPL manages space-

Photometer
Figure 2. Integrated spacecraft and photometer.

craft and mission development. A more comprehensive discussion of the mission design is given in Koch et al., found in the poster book for this symposium.

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