Discussion
Optical interferometers are extremely complex, unfilled aperture photon-collecting telescopes in the visual (sometimes the near infrared, too), which produce synthesized images and fringe data "on the fly" (unlike radio interferometers which are privileged to record the data for later synthesis), essentially by taking an inverse Fourier transform of the incoming data. Astrometry is understood by precisely measuring delay line additions while fringing, to match the light path differences from baseline ends. Using essentially trigonometry the angle and position of where the array is 'pointed' can be determined, thus inferring a precise position on the sphere of the sky.
Only a few exist that can be considered operational. To date NPOI has produced the highest resolution optical images of any astronomical instrument, though this may change when the CHARA array and Magdalena Ridge Observatory Interferometer begin optical-band operations. The first astronomical object imaged (resolved) by NPOI was Mizar, and since, a significant amount of astrometry, reference tie frame, rapid rotator star, and Be stellar disk study has been performed. NPOI is capable of determining positions of celestial objects to a few milli-arcsecond, in part due to the optical anchoring of its components using a complex metrology array of lasers that connect main optical elements to each other and to bedrock.
Many specialized lasers are also used to align the long train of optics. The current NPOI siderostat array remains the world's only long-baseline (437-meter) optical interferometer that can simultaneously co-phase six elements. NPOI is expected to grow significantly in capability with the pending addition of four 1.8-meter aperture IR/Optical telescopes into the current array. The enhanced array will also employ adaptive optics techniques. This layout and increased sparse aperture will permit significant improvements to the science capability, from a tenfold increase in measuring ever-fainter wide-angle astrometry targets, to improved positional determination for numerous binary and flare stars. When the 1.8m telescope addition are complete, NPOI also will undertake additional studies of dust and proto-planetary disks, and planetary systems and their formation.
Read more about this topic: Navy Optical Interferometer
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