Description
The NPOI is an astronomical interferometer laid out in a three-arm "Y" configuration, with each equally-spaced arm measuring 250 meters (820 ft) long. There are two types of stations that can be used in the NPOI. Astrometric stations, used to measure the positions of stars very accurately, are fixed units placed 21 meters (69 ft) 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.
Both types of station have three elements: a siderostat, a Wide Angle Star Acquisition (WASA) camera, and a Narrow Angle Tracking (NAT) camera. The first is a precisely-ground flat mirror 50 cm (20 in) in diameter. The WASA cameras control the aiming of the mirror at the celestial target. The reflected light from the siderostat is directed through a telescope which narrows the beam down to the diameter of the pipes, which is 12 cm (4.7 in). The light then hits the mirror of the NAT, which compensates for atmospheric effects and directs the light into the feed system.
In 2009 NOFS began final plans for NPOI to incorporate four 1.8 m (71 in) aperture optical-infrared telescopes into the array, which were accepted by the Navy in 2010, and assigned to the Naval Observatory Flagstaff Station. They were originally intended to be "outrigger" telescopes for the W. M. Keck Observatory in Hawaii, but were never installed and incorporated into Keck's interferometer. Three telescopes are being prepared for near-immediate installation, while the fourth is currently at Mount Stromlo Observatory in Australia and will be incorporated at some point in the future. The new telescopes will help with faint object imaging and improved absolute astrometry, due to their greater light-gathering abilities than the existing siderostats.
NOFS operates and leads the science for the Navy Precision Optical Interferometer, as noted, in collaboration with Lowell Observatory and the Naval Research Laboratory at Anderson Mesa. NOFS funds all principle 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 a successful example of the venerable Michelson Interferometer design, with the principle 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 the 1.8m Array were developed in 2010-2012 by the scientific and engineering staffs at NOFS, based on the funded science performed by NOFS and NRL. In 2012 NOFS, with support from USNO, CNMOC and the Chief of Naval Operations' Oceanographer staff, began developing funding and programming plans in order to install the array. NOFS endeavors to facilitate construction starts in the 2012-2015 timeframe.
Read more about this topic: Navy Optical Interferometer
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