Updated 10/06/04
Target Field of View
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 for the lifetime of the mission. 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 Orion 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-Lyra region along the Orion arm centered on galactic coordinates (76.5°,+13.3°) or RA=19h 22m40s , Dec=+44° 30' has been chosen. (The location was moved in Oct 2004 to reduce the effects of false positives due to background eclipsing binary stars.) 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 multi-band photometry 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.
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