Since transits only last a fraction of a day, all the stars must be monitored continuously, that is, their brightnesses must be measured at least once every few hours. (We must sum the light accumulated in this time to obtain a statistically significant measurement). The ability to continuously view the stars being monitored dictates that the field of view (FOV) must never be blocked at any time during the year. Therefore, to avoid the Sun the FOV must be out of the ecliptic plane. The payload envelope of the launch vehicle limits the sunshade size and hence the target field to be >55° from the ecliptic plane.
The secondary requirement is that the FOV have the largest possible number of stars. This leads to the selection of a region along the Cygnus arm of our Galaxy as shown. |
Extended Solar Neighborhood
The stars sampled are similar to the
immediate solar neighborhood.
Young stellar clusters,
ionized HII regions and the
neutral hydrogen, HI, distribution
define the arms of the Galaxy. |
To meet the goals of making statistically
meaningful conclusions, the mission design should be such at
least 45 terrestrial planets (R<1.3 Re) are expected,
requiring many thousands of stars to be observed simultaneously
in one FOV. (Continuously re-orienting the photometer to view
fewer bright stars in many different fields-of-view (FOV) increases
the mission complexity and cost and is less efficient than using
a single FOV.)
A region of the extended solar neighborhood
in the Cygnus region along the Orion arm centered on galactic
coordinates (76.32°,+13.5°) or RA=19h 22m 40s, Dec=+44°
30' 00' has been chosen. The star field is far enough from the ecliptic
plane so as not to be obscured by the Sun at any time of the
year. This field also virtually eliminates any confusion resulting
from occultations by asteroids and Kuiper-belt objects. 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 is 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 are eliminated reducing the number
to 100,000 useful target stars.
Location of the
Kepler Mission FOV on the Sky
The squares show
the FOV of each of the 21 CCD modules. Each 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 a pdf copy of this map.
See also
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