We describe the Navy Prototype Optical Interferometer (NPOI), a joint project of the Naval Research Laboratory (NRL) and the US Naval Observatory (USNO) in cooperation with Lowell Observatory. The NPOI has recently begun operations at the Lowell Observatory site near Flagsta †, Arizona, obtaining its Ðrst images, of a binary star, in 1996 May and June and its Ðrst limb-darkening observations during 1996 November to 1997 February. This paper gives an overview of the NPOI, including the characteristics of optical interferometry that a †ect its design.The NPOI includes subarrays for imaging and for astrometry. The imaging subarray consists of six moveable 50 cm siderostats feeding 12 cm apertures, with baseline lengths from 2.0 to 437 m. The astrometric subarray consists of four Ðxed 50 cm siderostats feeding 12 cm apertures (35 cm apertures to be installed in 1998), with baseline lengths from 19 m to 38 m. The shared back end covers 450È850 nm in 32 channels. The NPOI features vacuum feed and delay systems, active group-delay fringe tracking, and a high degree of automation. The astrometric subarray also includes an extensive site laser metrology system to measure the motions of the siderostats with respect to one another and to the bedrock.For imaging stellar surfaces, arrays with equal spacing between elements are superior to arrays that have been laid out to optimize (u, v) coverage and that therefore have unequal spacing. The imaging subarray of the NPOI provides a number of equally spaced conÐgurations with linear scales at ratios of B1.64. Unequally spaced conÐgurations are available for a variety of other imaging programs. Coherence across either type of imaging conÐguration is maintained by "" phase bootstrapping ÏÏ : the phases on the longest baselines, on which fringes may be too weak to track, are stabilized by tracking fringes on the shortest baselines.In principle, the four elements of the astrometric subarray provide enough independent baselines to solve for stellar positions and the array geometry simultaneously while observing each of 11 stars only once.The anticipated magnitude limit is 7 mag or better with 12 cm apertures and average seeing ; with 35 cm apertures, we expect the limit to be one or more magnitudes fainter. The anticipated wide-angle astrometric precision of the NPOI is B2 mas. The best angular resolution of the imaging subarray will be B200 kas. Our experience with the Mark III interferometer suggests that we will be able to measure stellar diameters as small as 200 kas with 1% precision and binary star separations as small as o B 65 kas (for *m B 0 mag) or o B 200 kas (for *m B 3È4 mag). With its large bandwidth and phase bootstrapping, the imaging subarray should be able to make images resolution elements across the disks Z10 of nearby late-type stars.

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The first station of the Long Wavelength Array (LWA1) was completed in April 2011 and is currently performing observations resulting from its first call for proposals in addition to a continuing program of commissioning and characterization observations. The instrument consists of 258 dual-polarization dipoles, which are digitized and combined into beams. Four independently-steerable dual-polarization beams are available, each with two tunings of 16 MHz bandwidth that can be independently tuned to any frequency between 10 MHz and 88 MHz. The system equivalent flux density for zenith pointing is ∼3 kJy and is approximately independent of frequency; this corresponds to a sensitivity of ∼5 Jy/beam (5σ, 1 s); making it one of the most sensitive meter-wavelength radio telescopes. LWA1 also has two "transient buffer" modes which allow coherent recording from all dipoles simultaneously, providing instantaneous all-sky field of view. LWA1 provides versatile and unique new capabilities for Galactic science, pulsar science, solar and planetary science, space weather, cosmology, and searches for astrophysical transients.Results from LWA1 will detect or tightly constrain the presence of hot Jupiters within 50 parsecs of Earth. LWA1 will provide excellent resolution in frequency and in time to examine phenomena such as solar bursts, and pulsars over a 4:1 frequency range that includes the poorly understood turnover and steep-spectrum regimes. Observations to date have proven LWA1's potential for pulsar observing, and just a few seconds with the completed 256-dipole LWA1 provide the most sensitive images of the sky at 23 MHz obtained yet. We are operating LWA1 as an open skies radio observatory, offering ∼2000 beamhours per year to the general community. At the same time, we are operating a backend for all-sky imaging and total-power transient detection, approximately 6840 hours per year (∼78% duty cycle).

Abstract-LWA1 is a new radio telescope operating in the frequency range 10-88 MHz, located in central New Mexico. The telescope consists of 258 pairs of dipole-type antennas whose outputs are individually digitized and formed into beams. Simultaneously, signals from all dipoles can be recorded using one of the instrument's "all dipoles" modes, facilitating all-sky imaging. Notable features of the instrument include high intrinsic sensitivity (≈ 6 kJy zenith system equivalent flux density), large instantaneous bandwidth (up to 78 MHz), and 4 independentlysteerable beams utilizing digital "true time delay" beamforming. This paper summarizes the design of LWA1 and its performance as determined in commissioning experiments. We describe the method currently in use for array calibration, and report on measurements of sensitivity and beamwidth.

We report on the first successful simultaneous combination of six independent optical telescopes in an interferometric array. This is double the number of independent telescopes, and 5 times the number of independent baselines, heretofore combined simultaneously. This was accomplished with the Navy Prototype Optical Interferometer at Lowell Observatory, near Flagstaff, Arizona. We describe the main technologies demonstrated, including hybrid six-way beam combination, nonredundant multiple optical path modulation for fringe separation, and the fringe detection electronics. To test the array's suitability for highresolution stellar imaging, we observed the hierarchical triple star Virginis, and we present the first images resolving all three components. The orbital motions of these stars were followed during winter and spring of 2002. Preliminary, astrometrically determined orbits of the two components in the close pair by reference to the tertiary were derived. This enabled the estimation of the mass ratio (1.27) of the components in the close pair. We also determined the relative orbital inclination to be 31. Future work needed to improve the calibration of the data is discussed.