Военно-морской оптический интерферометр высокой точности США

US Navy astronomical interferometer

The Navy Precision Optical Interferometer (NPOI) is an American astronomical interferometer, with the world's largest baselines, operated by the Naval Observatory Flagstaff Station (NOFS) in collaboration with the Naval Research Laboratory (NRL) and Lowell Observatory. The NPOI primarily produces space imagery and astrometry, the latter a major component required for the safe position and navigation of all manner of vehicles for the DoD. The facility is located at Lowell's Anderson Mesa Station on Anderson Mesa about southeast of Flagstaff, Arizona (US). Until November 2011, the facility was known as the Navy Prototype Optical Interferometer (NPOI). Subsequently, the instrument was temporarily renamed the Navy Optical Interferometer, and now permanently, the Kenneth J. Johnston Navy Precision Optical Interferometer (NPOI) – reflecting both the operational maturity of the facility, and paying tribute to its principal driver and retired founder, Kenneth J. Johnston. The NPOI project was initiated by the United States Naval Observatory (USNO) in 1987. and at the NPOI site.

The NPOI is an astronomical interferometer laid out in a three arm "Y" configuration, with each equally spaced arm measuring long. There are two types of stations that can be used in the NPOI. Astrometric stations, used to measure the positions of celestial objects very accurately, are fixed units placed apart, with one on each arm and one at the center. Imaging stations can be moved to one of nine positions on each arm, and up to six can be used at one time to perform standard observing. Light from either type of station is first directed into the feed system, which consists of long pipes which have been evacuated of all air. They lead to a switchyard of mirrors, where the light is directed into the six Long Delay Lines, which is another set of long pipes that compensate for the different distances to each station. The light is then sent into the Beam Combining Facility, where it enters the Fast Delay Lines. This third set of evacuated pipes contains mechanisms that move mirrors back and forth with a very high degree of accuracy. These compensate for the movement of the mirrors as they track an object across the sky, and for other effects. Finally, the light leaves the pipes inside the BCF and goes to the Beam Combining Table, where the light is combined in a way that allows images to be formed. and assigned to the Naval Observatory Flagstaff Station. as noted, in collaboration with Lowell Observatory and the Naval Research Laboratory at Anderson Mesa. NOFS funds all principal operations, and from this contracts Lowell Observatory to maintain the Anderson Mesa facility and make the observations for NOFS to conduct the primary astrometry. The Naval Research Laboratory (NRL) also provides funds to contract Lowell Observatory's and NRL's implementation of additional, long baseline siderostat stations, facilitating NRL's primary scientific work, synthetic imaging (both celestial and of orbital satellites). When complete by 2013, NPOI will run the longest baseline interferometer in the world. The three institutions – USNO, NRL, and Lowell – each provide an executive to sit on an Operational Advisory Panel (OAP), which collectively guides the science and operations of the interferometer. The OAP commissioned the chief scientist and director of the NPOI to effect the science and operations for the Panel; this manager is a senior member of the NOFS staff and reports to the NOFS Director. NPOI is an example of the Michelson Interferometer design, with the principal science managed by NOFS. Lowell Observatory and NRL join in the scientific efforts through their fractions of time to use the interferometer; science time is 85% Navy (NOFS and NRL); and 15% Lowell. NPOI is one of the few major instruments globally which can conduct optical interferometry. See an illustration of its layout, at bottom. NOFS has used NPOI to conduct a wide and diverse series of scientific studies, beyond just the study of absolute astrometric positions of stars,; additional NOFS science at NPOI includes the study of binary stars, Be Stars, Oblate stars, rapidly rotating stars, those with starspots, and the imaging of stellar disks (the first in history) and flare stars. In 2007–2008, NRL with NOFS used NPOI to obtain first ever closure phase image precursors of satellites orbiting in geostationary orbit. Installation plans for a 1m Array have been developed by NRL and Lowell Observatory, based on the funded science performed.

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.