Here Be Dragons

HERE BE DRAGONS
5.2 Interferometry using a reference star. Two beams of light from the reference star, captured by
the two telescopes, are combined to produce interference fringes. To do the same for the star of
interest, whose light waves are approaching the telescopes at a slightly different angle, the length
of the optical delay line has to be adjusted slightly. The amount of this adjustment gives the angular
separation between the two stars.
The Kepler mission is competing (unsuccessfully, so far) for a slot
in the so-called Discovery Program, a series of NASA missions characterized
by the slogan "Faster, cheaper, better." These missions are
supposed to cost less than $300 million and to take no more than
3 years to complete. The Discovery Program is a response to budgetary
constraints and to the megaprojects of the past, some of which became
so overloaded with bells and whistles that their failure was all
but assured.
There is another planet-finding technique, however, into which
NASA is planning to invest most of its efforts, and that is optical interferometry,
which combines the light of two or more telescopes that are
located at some distance from each other, thereby gaining the resolution
of a single telescope with a diameter equal to the distance between
the two. The method will be used not only to detect the side-toside
wobble of a star that is orbited by planets, but eventually to image
those planets directly and even to analyze their atmospheres.
The tricky part is combining the light from the two telescopes. This
must be done in such a way as to preserve information about the
phase relationships (the relative positions of the individual wave
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