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I suppose that the Kepler field was well chosen. It is rich in stars and has minimal background light since it is pointing out from the disc.

But other things could have been considered. For example, a field which contains the most stars which are near enough to be followed up by ground based telescopes. And why wasn't Kepler designed to change its pointing so it could cover several fields, such as all of the above? Funny enough, it does so now as K2 after two of its reaction wheels have broken, so studying only one field doesn't seem to be a technical issue, but something which astronomers prefer. Why?

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Changing fields breaks the timeline of the observations. If you have variation with a period similar to when you switch, you might completely miss it. Even if the periods are different, it degrades your observations and makes it harder to draw conclusions. The more uninterrupted viewing you get, the better.

This mission document covers some of the reasons for the choice of the field.

  • Continuously viewable during the mission (you can't pick something where the earth, sun or moon constrains your viewing).
  • Rich in stars. Observe as many as possible.
  • The size of the sunshade limits the field to be more than 55 degrees from the ecliptic.

Although it can image lots of stars, the actual field isn't all that large. Attempting to pick a single field with some fraction nearby would be difficult, and was not Kepler's mission. In fact the resource of follow-up observations from ground telescopes was mentioned as one reason a northern field was chosen over a southern one.

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  • $\begingroup$ I just heard that Kepler was pointed to a field of old stars. Young stars are noisy and harder to find planets at. About 44 minutes into this stressful lecture by David Soderblom, "CfA Colloquium: Stars of an Uncertain Age: The Problem of Determining Stellar Ages". $\endgroup$
    – LocalFluff
    Aug 20, 2015 at 19:29
  • $\begingroup$ Nice find. I wish there were some documents I could find to corroborate it. In fact one NASA page in describing the field says it consists of young star clusters. kepler.nasa.gov/Science/about/targetFieldOfView I wonder if this was an unofficial constraint? $\endgroup$
    – BowlOfRed
    Aug 20, 2015 at 19:48
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The prime objective of the Kepler mission was to attempt to find "Earth-like" planets using the transit technique.

To establish that you definitely have a transiting planet requires, at a minimum, that you see three regularly spaced transits.

The Kepler mission (originally) was planned for 4 years. Thus to ensure the detection of 3 transits for planets in a 1-year orbit really requires that you observe a large set of stars continuously for that period (since the transits are quite brief).

In order to do this, you need to observe a field in which neither the Sun nor the Earth get in the way during the year. This requires you to look away from the ecliptic plane.

Then, to get a large number of stars in the fixed field of view, a direction was chosen that was close, but not in the Galactic plane and viewing along a spiral arm. I believe this was done to maximise the number of stars with $V<16$ for which Kepler would supply good photometry. Closer to the plane would have given even more stars, but many would have been faint and one runs into more difficulties in terms of resolving which star is actually the variable when there is too much confusion with many sources.

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