The Kepler Field of View
Since transits only last a fraction of a day, Kepler must monitor all target stars continuously. Their brightnesses must be measured at least once every few hours. (We sum the light accumulated in this time to obtain a statistically significant measurement). The ability to continuously view the stars being monitored dictated that the field of view (FOV) must never be blocked at any time during the year. So, Kepler orbits the Sun (heliocentric orbit), not the Earth. Further, to avoid the Sun, the FOV must be out of the ecliptic plane. Additionally, the payload envelope of the launch vehicle limited the sunshade size. As a result, the target field is >55º from the ecliptic plane. (The ecliptic plane is the plane of the Earth's orbit around the Sun. During the year, the Sun's apparent path through the sky lies on the ecliptic plane.)
The secondary requirement is that the FOV have the a large number of stars, but be off the galactic plane to reduce field confusion. This lead to the selection of a region along the Cygnus arm of our Galaxy as shown.
A region of the extended solar neighborhood in the Cygnus region along the Orion arm centered on galactic coordinates (76.32º,+13.5º) or RA=19h 22m 40s, Dec=+44º 30' 00' has been chosen. The star field is far enough from the ecliptic plane so that the Sun does not shine into the telescope at anytime during the year. In addition, this FOV virtually eliminates any confusion resulting from occultations by asteroids and Kuiper-belt objects, which commonly orbit near the ecliptic plane. Comet-size objects in the Oort cloud subtend too small an angular size and move too rapidly to be a problem.
Data from the US Naval Observatory digitization of the Palomar Observatory Sky Survey (USNO-A1.0) (Dave Monet, 1996), complete to mv=18, was used to determine that the actual number of stars with mv<14 of all spectral types and luminosity classes in the 105 deg2 FOV to be 223,000. About 61%, i.e., 136,000, are estimated to be main-sequence stars. Prior to launch high-resolution spectroscopy was performed to identify and eliminate the giant stars in the FOV. During the first year of the mission, the 25% most active of the dwarf stars were eliminated reducing the number to 100,000 useful target stars.
The squares show the FOV of each of the 21 CCD modules (each with two 2200x1024 pixel CCDs). Each module is 5 sq deg. Note that the gaps between the CCD modules are aligned so that about half of the 15 stars in the FOV brighter than mv=6 fall in these gaps.
Download (PDF, 2.02 MB) a PDF copy of this map.