Interferometry
The sharper the image desired, the bigger the telescope
required.
Interferometry takes this to its logical conclusion. By combining light
from two or more widely separated
telescopes (tens of meters), it is possible to obtain imaging
resolution equivalent to that of a telescope the size of the
separation between the telescopes. Although interferometry has been
standard practice at radio wavelengths for decades, it has only
recently seen widespread and successful application at optical and
infra-red wavelengths. This is due mostly to the stringent
requirements on pathlength control (better than a small fraction of
the wavelength of light, or a few nanometers). However, with the
advent of high-speed computer control, many of the practical
difficulties associated with optical/IR interferometry have been
overcome.
An optical/IR interferometer provides extreme (by contemporary
astronomical standards) angular resolution. The Palomar Testbed
Interferometer (PTI) , where I do most of my research (I am a
memeber of the PTI
Collaboration ), operates at wavelengths between 1.6 and 2.4
microns (H,K bands) and has a "baseline" (aperture separation) of 110
meters. It is capable of resolving objects with angular sizes of 1-4
milli-arcseconds (5 nano-radians, or equivalent to a man standing on
the Moon, seen from Earth). By contrast standard
astronomical telesopes produce images with resolutions around 1
arcsecond (which works out to a few tenths of a mile on the moon).
However, I should point out that interferometers are very limited in
what they can observe - they require bright, high-contrast sources
(after all, you don't have the same mirror area as a large telescope),
and hence you couldn't use an interferometer to actually image a man
standing on the Moon...
I have used optical interferometry to study a number of interesting
astronomical topics, including:
Interferometry is quite useful for
providing direct measurements
of stellar diameters. Such measurements provide constraints for the
various theoretical models used to understand stellar structure. One
class of stars that hasn't been well measured yet is low-mass stars,
specifically main sequence dwarfs with masses between 0.8 and 0.2
solar masses. I have an ongoing research program to measure the
apparent diameters of a handfull of such low-mass stars, with the goal
of a measurement precision of 3%.
My first results were recently published as an
Astrophysical Journal Letter .
Another use of interferometry is in resolving the orbits of
spectroscopic binary stars. Since the interferometer measures the
visual orbit of the components in a binary system, it - when combined
with radial velocity data - allows one to solve for all the orbital
parameters of the system, including orbital inclination and hence
component masses. The precision achievable can be as high as 1%,
providing valuable constraints on stellar models.