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Figure 1 Click on image for larger resolution | Figure 2
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This image from NASA's Kepler mission shows the telescope's full field of
view — an expansive star-rich patch of sky in the constellations
Cygnus and Lyra stretching across 100 square degrees, or the equivalent of
two side-by-side dips of the Big Dipper.
A cluster of stars, called NGC 6791, and a star with a known planet,
called TrES-2, are outlined. The cluster is eight billion years old, and
located 13,000 light-years from Earth. It is called an open cluster
because its stars are loosely bound and have started to spread out.
TrES-2 is a hot Jupiter-like planet known to cross in front of, or
transit, its star every 2.5 days. Kepler will hunt for transiting planets
that are as small as Earth.
Kepler was designed to hunt for planets like Earth. Of the approximately
4.5 million stars in the region pictured here, more than 100,000 were
selected as candidates for Kepler's search. The mission will spend the
next three-and-a-half years staring at these target stars, looking for
periodic dips in brightness. Such dips occur when planets cross in front
of their stars from our point of view in the galaxy, partially blocking
the starlight.
The area in the lower right of the image is brighter because it is closer
to the plane of our galaxy and is jam-packed with stars. The area in upper
left is farther from the galactic plane and contains fewer stars.
The image has been color-coded so that brighter stars appear white, and
fainter stars, red. It is a 60-second exposure, taken on April 8, 2009,
one day after the spacecraft's dust cover was jettisoned.
To achieve the level of precision needed to spot planets as small as
Earth, Kepler's images are intentionally blurred slightly. This minimizes
the number of saturated stars. Saturation, or "blooming," occurs when the
brightest stars overload the individual pixels in the detectors, causing
the signal to spill out into nearby pixels. These spills can be seen in
the image as fine white lines extending above and below some of the
brightest stars. Blooming is an expected side effect of Kepler's
ultra-sensitive camera. Some of the lightly saturated stars are candidates
for planet searches, while those that are heavily saturated are not.
The grid lines across the picture show how the focal plane is laid out on
Kepler's camera —the largest ever launched in space at 95
megapixels. There are 42 charge-coupled devices (CCDs), paired into
square-shaped modules, whose outline can be seen in the image. A thin
black line in each module shows adjacent pairs of CCDs. The thicker black
lines that cross through the image are from structures holding the modules
together, and were purposely oriented to block out the very brightest
stars in Kepler's field of view.
The four black corners of the image show where the fine-guidance sensors
reside on the focal plane. These sensors are used to hold the telescope's
gaze steady by measuring its position on the sky 10 times every second,
and by feeding this information to the spacecraft's attitude control
system.
Ghost images also appear in the image, which are reflections off the
lenses above the CCDs. These expected artifacts were mapped out during
ground testing for Kepler, and will not affect science observations
because they will be removed as the data are processed.