Many interesting things are afoot at the Navy Precision Optical Interferometer
ABSTRACT
The Navy Precision Optical Interferometer (NPOI) is currently undergoing a fundamental renaissance in its functionality and capabilities. Operationally, its fast delay line (FDL) infrastructure is completing its upgrade from a VME/VxWorks foundation to a modern PC/RTLinux core. The Classic beam combiner is being upgraded with the New Classic FPGA-based backend, and the VISION beam combiner has been upgraded over this past summer with low-noise EMCCD cameras, resulting in substantial gains in sensitivity. Building on those infrastructure improvements, substantial upgrades are also in progress. Three 1-meter PlaneWave CDK1000 telescopes are being delivered to the site, along with their relocatable enclosure-transporters, and stations are being commissioned for those telescopes with baselines ranging from 8 meters to 432 meters. Baseline-wavelength bootstrapping will be implemented on the facility back-end with a near-infrared beam combiner under development. Collectively, these improvements mark substantial progress in taking the facility towards realizing its full intrinsic potential.
INTRODUCTION
The Navy Precision Optical Interferometer (NPOI) is an astronomical long-baseline optical interferometer located outside Flagstaff, Arizona, on Anderson Mesa. NPOI collects and combines light from up to six apertures simultaneously, to form a high-spatial-resolution synthetic aperture. The array has been in operation since the mid-1990’s as previously described by Armstrong et al.1 and updated recently by them.2 Three institutions are involved in the operation and ongoing development of the NPOI. The US Naval Observatory (USNO) oversees operations and has a primary interest in astrometric observing; the US Naval Research Laboratory (NRL) provides engineering support and has a primary interest in interferometric imaging; the Lowell Observatory provides infrastructure support and observing staff under contract with the Navy, and is an active science partner; and amongst these integrated partners there is quite a bit of overlap in these particular roles.
2. GENERAL DESCRIPTION OF THE CURRENT ARRAY
A brief description of the current configuration of the array can be broken up in the basic subsystem tasks: light collection, beam transport & array layout, beam delay, and beam combination.
Light collection. Light is collected at half-meter siderostat flats, which track objects in the sky for observation. This light is reflected upwards towards 12 cm fast tip-tilt narrow-angle tracking (NAT) mirrors, which can be used to rapidly follow angular errors in light caused by atmospheric turbulence and/or siderostat tracking errors; the 12 cm NAT mirror size is the effective limit for on-sky aperture for a siderostat station. The size of the NAT mirrors is the effective limiting aperture size of each array feed upon the sky. A wide-angle star acquisition (WASA) imaging system can be inserted in between the siderostats and the NAT mirrors via a translation stage. The WASA system is co-axial with the beam off the NAT towards the siderostat, and views the sky off the siderostat; the WASA system field-of-view is large (30 arcmin) and can be used for initial image capture when going on sky.
Six siderostats are ‘imaging’ type feeds for the array, consisting simply of the siderostat flats, and WASA & NAT subsystems. These feeds can be moved to different parts of the array, as noted below. Four of the siderostats are more elaborate ‘astrometric’ type feeds. These feeds are fixed in location near the center of the array, with the outer three forming a roughly ∼37 meter equilateral triangle, with the forth at the center of that triangle. These feeds are additionally equipped with laser metrology hardware which monitors the position of the siderostat position relative to the mesa bedrock; this position information, along with other pathlength monitoring hardware of the array, enables wide-angle all-sky astrometric measurements to be made. As of April 2018 this portion of the NPOI facility is currently mothballed; details on the astrometric experiment of NPOI are described by Benson.
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