Here Be Dragons

THE PLANET FINDERS
fronts) of the light reaching them. Thus, it is necessary to measure the
length of the paths taken by the starlight entering the two telescopes,
with a level of precision down in the billionths of a meter.
To explore and develop the techniques of optical interferometry,
NASA has constructed the Palomar Testbed Interferometer (PTI) (see
Color Plate 6). 13 PTI is run by a large group of JPL scientists, headed by
Mike Shao; but when, sometime after midnight, we climb down from
the catwalk of the Hale telescope and tap on the door of the PTI'S
"beam-combining building," we are greeted by a lowly Caltech undergraduate,
Ben Lane. Though he appears altogether too young to be entrusted
with such an expensive piece of hardware, Ben is intimately familiar
with all its mysteries, because he helped assemble and test it.
He is soon snowing us with an avalanche of techno-speak—
"dichroics," "delay lines," "tilt tables," "heterodyne lasers," "singlemode
fibers," "fringe trackers," and so forth. Luckily one of us
(Koerner) uses interferometry (at radio wavelengths) in his own research,
so at least the principle of the thing is clear.
Basically, two telescopes (the "north" and "south" telescopes, on
this particular night) look at the same small region of sky, which is
chosen to contain at least two stars: the star of interest and a nearby
bright reference star. The reference star is taken as a fixed landmark;
the star of interest, on the other hand, will be changing its position
slightly from night to night, if it possesses planets. Thus, the aim is to
determine as precisely as possible the position of the star of interest
with respect to the reference star. (The reference star may have some
"wobble" of its own, of course. Therefore, in practice it's necessary to
use more than one reference star, in order to determine which motions
are intrinsic to the star of interest.)
Because the reference star and the star of interest are at slightly
different positions in the sky, the wave fronts of the light coming from
them approach the telescopes at slightly different angles. For example,
if the peaks of the waves from the reference star happen to strike the
two telescopes synchronously (in phase), then the wave fronts from
the star of interest will strike the two telescopes slightly out of phase—
the peaks may reach one telescope at the same time as the slopes or
the troughs of the waves reach the other telescope. The interferometer
has to measure the phase difference between waves from the two
stars: this is a measure of the angular separation between them, as
measured along the axis running through the two telescopes.
To accomplish this, beam splitters inside each of the telescopes sep-
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