R. B. Hindsley

Vega is a Rapidly Rotating Star

Vega, the second brightest star in the northern hemisphere, serves as a primary spectral type standard. Although its spectrum is dominated by broad hydrogen lines, the narrower lines of the heavy elements suggested slow to moderate rotation, giving confidence that the ground-based calibration of its visible spectrum could be safely extrapolated into the ultraviolet and near-infrared (through atmosphere models), where it also serves as the primary photometric calibrator. But there have been problems: the star is too bright compared to its peers and it has unusually shaped absorption line profiles, leading some to suggest that it is a distorted, rapidly rotating star seen pole-on. Here we report optical interferometric observations that show that Vega has the asymmetric brightness distribution of the bright, slightly offset polar axis of a star rotating at 93 per cent of its breakup speed. In addition to explaining the unusual brightness and line shape peculiarities, this result leads to the prediction of an excess of near-infrared emission compared to the visible, in agreement with observations. The large temperature differences predicted across its surface call into question composition determinations, adding uncertainty to Vegas age and opening the possibility that its debris disk could be substantially older than previously thought.

First Observations with a Co-phased Six-Station Optical Long-Baseline Array: Application to the Triple Star ? Virginis

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 high

Navy Prototype Optical Interferometer Imaging of Line Emission Regions of beta Lyrae Using Differential Phase Referencing

We present the results of an experiment to image the interacting binary star β Lyrae with data from the Navy Prototype Optical Interferometer using a differential phase technique to correct for the effects of the instrument and atmosphere on the interferometer phases. We take advantage of the fact that the visual primary of β Lyrae and the visibility calibrator we used are both nearly unresolved and nearly centrally symmetric, and consequently have interferometric phases near zero. We used this property to correct for the effects of the instrument and atmosphere on the phases of β Lyrae and to obtain differential phases in the channel containing the Hα emission line. Combining the Hα-channel phases with information about the line strength, we recovered complex visibilities and imaged the Hα emission using standard radio interferometry methods. Our images show the position of the Hα-emitting regions relative to the continuum photocenter as a function of orbital phase, indicating a major axis line of nodes along Ω = 249° ± 4°. The orbit is smaller than previously predicted, a discrepancy that can be alleviated if we assume that the system is at a larger distance, or if the stellar continuum contribution to the Hα channel was underestimated. We do not detect a jet in the Hα images, which may be due to the limited resolution of the observations along the direction perpendicular to the orbital plane. We find that the differential phase results are consistent with those obtained from a more standard analysis using squared visibilities (V 2s) and closure phases, which also indicate an Hα disk radius of 0.6 ± 0.1 mas, and ΔV = 1.30 ± 0.1 and ΔR = 1.20 ± 0.1 mag for the magnitude difference between the stars.

Improved Coherent Integration through Fringe Model Fitting

Maximizing the signal-to-noise ratio (SNR) of interferometric measurements of faint or low-visibility sources requires coherent integration. Coherent integration requires the use of a fringe-tracking method for determining the appropriate quantities to use for cophasing a large number of short low-SNR measurements. We present a method for tracking fringes during postprocessing. The method is based on fitting a time-dependent model of the fringe pattern to a sequence of data frames in order to determine the phase parameters for the central frame. This approach was chosen in accordance with the philosophy that optimal parameter estimation depends on a accurate model of the data. The model contains two parameters which must be chosen: the order of the time variation, and the number of simultaneous data frames to fit to. We show that this procedure results in a better SNR than a group-delay approach, particularly for low-NV2 data. In one case, the fringe-modeling approach improved coherent SNR by a factor of 2.6 over the group-delay approach. The fringe-modeling algorithm is tested on observations of the star ξ Bootis obtained from the Navy Prototype Optical Interferometer. We also offer suggestions for future improvements to the fringe-tracking model.

Coherent integration results from the NPOI

In this paper we will discuss the current status of coherent integration with the Navy Prototype Optical Interferometer (NPOI). Coherent integration relies on being able to phase reference interferometric measurements, which in turn relies on making measurements at multiple wavelengths. We first discuss the generalized group-delay approach, then the meaning of the resulting complex visibilities and then demonstrate how coherent integration can be used to perform very precision measurement of stellar diameters. The phase of the complex visibility is particularly attractive as a data product because it is not biased in the same way as visibility amplitudes. We discuss the relative SNR of triple-product phases and single-baseline phases. We then demonstrate how singlebaseline phases can be used to make accurate measurements of magnitude differences and separations of binary stars.

An imaging interferometer for compact sources

This paper presents the results of a study designed to test the feasibility of imaging satellites in geostationary orbit from the ground. We argue that the instrument should be an interferometer consisting of > 30 telescopes mounted on a common, steerable boom. Light from the telescopes is fed to the beam combiner with optical fibers. The delays are equalized by steering the boom and stretching the fibers. The feed system and delay lines are replaced with single mode fibers. This system should be better throughput than the optical interferometers in use today and should be able to reach the sensitivity needed to image these targets with meter-scale telescopes. Calculations supporting this claim and a system design are presented.

A Catalog of Faint Reference Stars in 398 Fields of Extragalactic Radio Reference Frame Sources

Positions of 89,422 stars in 398 fields of extragalactic reference frame sources have been determined using the Hamburg Zone Astrograph (northern hemisphere) and the US Naval Observatory Twin Astrograph, then stationed at the Black Birch Astrometric Observatory, New Zealand (southern hemisphere). Most stars are in the magnitude range 12 ≤ V ≤ 14, and the positions are accurate to ≈50 mas per coordinate at the epoch of observation, which ranges from the beginning of 1976 to the end of 1991. The catalog (ERLcat) is available on-line from USNO.

A radio/optical reference frame. 5: Additional source positions in the mid-latitude southern hemisphere

We report new accurate radio position measurements for 30 sources, preliminary positions for two sources, improved radio postions for nine additional sources which had limited previous observations, and optical positions and optical-radio differences for six of the radio sources. The Very Long Baseline Interferometry (VLBI) observations are part of the continuing effort to establish a global radio reference frame of about 400 compact, flat spectrum sources, which are evenly distributed across the sky. The observations were made using Mark III data format in four separate sessions in 1988-89 with radio telescopes at Tidbinbilla, Australia, Kauai, USA, and Kashima, Japan. We observed a total of 54 sources, including ten calibrators and three which were undetected. The 32 new source positions bring the total number in the radio reference frame catalog to 319 (172 northern and 147 southern) and fill in the zone -25 deg greater than delta greater than -45 deg which, prior to this list, had the lowest source density. The VLBI positions have an average formal precision of less than 1 mas, although unknown radio structure effects of about 1-2 mas may be present. The six new optical postion measurements are part of the program to obtain positions of the optical counterparts of the radio reference frame source and to map accurately the optical on to the radio reference frames. The optical measurements were obtained from United States Naval Observatory (USNO) Black Birch astrograph plates and source plates from the AAT, and Kitt Peak National Observatory (KPNO) 4 m, and the European Southern Observatory (ESO) Schmidt. The optical positions have an average precision of 0.07 sec, mostly due to the zero point error when adjusted to the FK5 optical frame using the IRS catalog. To date we have measured optical positions for 46 sources.

The new classic instrument for the navy precision optical interferometer

The New Classic instrument was built as a electronics and computer upgrade to the existing Classic beam combiner at the Navy Precision Optical Interferometer (NPOI). The classic beam combiner is able to record 32 of 96 available channels and has a data throughput limitation which results in a low duty cycle. Additionally the computing power of the Classic system limited the amount of fringe tracking that was possible. The New Classic system implements a high-throughput data acquisition system which is capable of recording all 96 channels continuously. It also has a modern high-speed computer for data management and data processing. The computer is sufficiently powerful to implement more sophisticated fringe-tracking algorithms than the Classic system, including multi-baseline bootstrapping. In this paper we described the New Classic hardware and software, including the fringe-tracking algorithm, performance, and the user interface. We also show some initial results from the first 5-station, 4-baseline bootstrapping carried out in January 2015.

Interferometric imaging of geostationary satellites

Even the longest geosatellite, at 40 m, subtends only 0.2 arcsec (1 microradian). Determining structure and orientation with 10 cm resolution requires a 90 m telescope at visual wavelengths, or an interferometer. We de- scribe the application of optical interferometry to observations of complex extended targets such as geosatellites, and discuss some of its challenges. We brie y describe our Navy Optical Interferometer (NOI) groups eorts toward interferometric observations of geosatellites, including the rst interferometric detection of a geosatellite. The NOI observes in 16 spectral channels (550{850 nm) using up to six 12-cm apertures, with baselines (separa- tions between apertures) of 16 to 79 m. We detected the geosatellite DirecTV-9S during glint seasons in March 2008 and March 2009, using a single 16 m baseline (resolution 1:6 m). Fringes on a longer baseline were too weak because the large-scale structure was over-resolved. The fringe strengths are consistent with a combination of two size scales, 1:3 m and & 3:5 m. Our near term NOI work is directed toward observing geosatellites with three or more 10 to 15 m baselines, using closure phase measurements to remove atmospheric turbulence eects and coherent data averaging to increase the SNR. Beyond the two- to three-year time frame, we plan to install larger apertures (1.4 and 1.8 m), allowing observations outside glint season, and to develop baseline bootstrap- ping, building long baselines from chains of short baselines, to avoid over-resolution while increasing maximum resolution. Our ultimate goal is to develop the design parameters for dedicated satellite imaging interferometry.