We recently used archival and newly obtained data from the Navy Precision Optical Interferometer to measure the fundamental properties of 87 stars. The sample consisted of 5 dwarfs, 3 subgiants, 69 giants, 3 bright giants, and 7 supergiants, and spanned a wide range of spectral classes from B to M. We combined our angular diameters with photometric and distance information from the literature to determine each star’s physical radius, effective temperature, bolometric flux, luminosity, mass, and age. Several dozen of the stars have visibility curves sampled down to the first null, where the visibilities drop to zero. Here we present preliminary results showing limb-darkening fits for the five zero crossing stars that have the best coverage of the second lobe.
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.
We describe the current status of the Navy Precision Optical Interferometer (NPOI), including developments since the last SPIE meeting. The NPOI group has added stations as far as 250m from the array center and added numerous infrastructure improvements. Science programs include stellar diameters and limb darkening, binary orbits, Be star disks, exoplanet host stars, and progress toward high-resolution stellar surface imaging. Technical and infrastructure projects include on-sky demonstrations of baseline bootstrapping with six array elements and of the VISION beam combiner, control system updates, integration of the long delay lines, and updated firmware for the Classic beam combiner. Our plans to add up to four 1.8 m telescopes are no longer viable, but we have recently acquired separate funding for adding three 1 m AO-equipped telescopes and an infrared beam combiner to the array.
We plan to measure the angular diameters of a sample of Penn State-Torun Planet Search (PTPS) giant exoplanet host star candidates using the Navy Precision Optical Interferometer. The radii of evolved giant stars obtained using spectroscopy are usually ill-defined because of the method’s indirect nature and evolutionary model dependency. The star’s radius is a critical parameter used to calculate luminosity and mass, which are often not well known for giant stars. Therefore, this problem also affects the orbital period, mass, and surface temperature of the planet. Our interferometric observations will significantly decrease the errors for these parameters. We present preliminary results from NPOI observations of six stars in the PTPS sample.
We describe multi-baseline observations of a geostationary satellite using the Navy Precision Optical Interferometer (NPOI) during the glint season of March 2015. We succeeded in detecting DirecTV-7S with an interferometer baseline length of 8.8 m on two nights, with a brief simultaneous detection at 9.8 m baseline length on the second night. These baseline lengths correspond to a resolution of ~4 m at geostationary altitude.
This is the first multiple-baseline interferometric detection of a satellite.
The Navy Precision Optical Interferometer is an astronomical optical interferometer operating near Flagstaff, Arizona. A
joint program between the United States Naval Observatory, the Naval Research Laboratory and Lowell Observatory, it
has historically been involved in space imagery and astrometry. More recent work has pushed for the addition of more
baselines. It is currently capable of co-phasing 6 elements, so the commissioning of additional baselines requires ease of
use and reconfigurability. At the time of this publication, a seventh station has been added and the final commissioning
work on an eighth and ninth station are being completed. These last two stations will increase the longest baseline to 435
meters. This paper discusses the work to date on adding these stations and provides details on increased capabilities.
We observed 85 stars using the Navy Precision Optical Interferometer in order to determine their angular diameters. Here we present preliminary uniform disk fits for the stars. Many of the targets have measurements through the first zero crossing and onto the second lobe of the visibility curve. We will use these observations to test limb darkening laws, namely the effectiveness of plane parallel versus spherically symmetric models. These results have important implications for the accuracy with which we can determine the limb darkening of stars used as calibrators on long baselines being implemented in the near future on the NPOI, which will almost certainly have to be at least semi-resolved. The validation or exposure of systematics in the limb darkening laws can also be applied to any number of stars observed interferometrically.
The Navy Precision Optical Interferometer (NPOI) was designed from the beginning to support baseline boot- strapping with equally-spaced array elements. The motivation was the desire to image the surfaces of resolved stars with the maximum resolution possible with a six-element array. Bootstrapping two baselines together to track fringes on a third baseline has been used at the NPOI for many years, but the capabilities of the fringe tracking software did not permit us to bootstrap three or more baselines together. Recently, both a new backend (VISION; Tennessee State Univ.) and new hardware and firmware (AZ Embedded Systems and New Mexico Tech, respectively) for the current hybrid backend have made multi-baseline bootstrapping possible.
VISION is the next generation science camera for the Navy Optical Interferometer (NOI). In comparison to
the current beam combiner of NOI, VISION will deliver higher precision data products and better
exibility by
incorporating single mode bers for spatial ltering and by using low-noise detectors. VISION can coherently
combine up to six telescope beams using an image-plane combination scheme. This results in simultaneous
measurement of 15 visibility amplitudes and 10 independent closure phases that can be used to reconstruct
multipixel images of stars.
We report a tentative interferometric detection of an earth-orbiting artificial satellite using optical interferometry. We
targeted four geosynchronous communications satellites with the Navy Prototype Optical Interferometer (NPOI) near
Flagstaff, AZ, and obtained interferometric fringes on one of them, DIRECTV-9S. We used an east-west 15.9-meter
baseline of the NPOI and took data in 16 spectral channels covering the 500-850 nm wavelength range. Observations
took place during the "glint season" of 28 February to 3 March 2008, when the geometry of the solar panel arrays and the
Sun's position creates glints as bright as 2nd magnitude of a few minutes' duration each night. We detected fringes on
the satellite at approximately the 2 σ level on 1 March at magnitude 4.5. Subsequent analysis shows that the fringe
amplitudes are consistent with a size scale of 2 meters (50 nanoradians at geosynchronous orbit) in an east-west
direction. This detection shows that interferometric detection of satellites at visual wavelengths is possible, and suggests
that a multi-baseline interferometer array tailored to the angular size and brightness of geosynchronous satellites could
lead to images of these satellites.
We have detected a satellite via optical interferometry for the first time, using a 16 m baseline of the Navy
Prototype Optical Interferometer (NPOI) to observe the geostationary communications satellite DirecTV-9S
during the "glint" seasons of February-March 2008 and 2009 when the sun-satellite-NPOI geometry was favorable
for causing specular reflections from geostationary satellites. We used the US Naval Observatory Flagstaff
Station 1 m telescope to generate accurate positions for steering the NPOI. Stars are the easiest targets for
optical/infrared interferometers because of their high surface brightness. Low surface brightness targets are
more difficult: if they are small enough not to be resolved out by typical baselines, they are likely to be too faint
to produce detectable fringes in an atmospheric coherence time. The 16 m NPOI baseline, the shortest available
at the time of our observations, resolves out structures larger than ~ 1.5 m at the geostationary distance, while
a typical size for the solar panel arrays is 2 m x 30 m. Our detection indicates that a small fraction of the
satellite glinted, not surprising given that the solar panels are not accurately flat. Our fringe data are consistent
with a two-component image consisting of a 1 to 1.3 m higher surface brightness component and a significantly
larger lower surface brightness component. The brightness of the glints (2.m 4 and ~ 1.m 5 for the two detections in March 2009) and the size scale suggest that the compact component has an albedo of 0.06 to 0.13, while the
larger-scale component is much darker, if circular geometry is assumed.
We report progress on the United States Naval Observatory, Navy Prototype Optical Interferometer, Astrometric Catalog
(UNAC). This catalog uses observations from eight astrometric observation runs (Jan. 2005 - Nov. 2009) at the Navy
Prototype Optical Interferometer (NPOI). The goal of the first release of the UNAC is to provide an astrometric catalog
of at least 100 bright (V < 6) stars with precise positions accurate to < 16 milliarcseconds. In this paper we report on
some of the data processing methods used to obtain absolute astrometric positions from optical interferometer data. We
also discuss plans for assessing the accuracy of our interferometrically derived absolute astrometric positions.
We report an interferometric detection of an earth-orbiting artificial satellite using optical interferometry. We targeted
four geosynchronous communications satellites with the Navy Prototype Optical Interferometer (NPOI) near Flagstaff,
AZ, and obtained interferometric fringes on one of them, DIRECTV-9S. We used an east-west 15.9-meter baseline of
the NPOI and took data in 16 spectral channels covering the 500-850 nm wavelength range. Observations took place
during the "glint season" of 28 February to 3 March 2008, and then again in February - March 2009, when the geometry
of the solar panel arrays and the Sun's position creates glints as bright as 2nd magnitude of a few minutes' duration each
night. We detected fringes on the satellite at approximately the 2 sigma level on 1 March at magnitude 4.5. Subsequent
analysis shows that the fringe amplitudes are consistent with a size scale of 2 meters (50 nanoradians at GEO) in an east-west
direction. This detection shows that interferometric detection of satellites at visual wavelengths is possible, and
suggests that a multi-baseline interferometer array tailored to the angular size and brightness of geosynchronous
satellites could lead to images of these satellites.
The instrumental status of the Navy Prototype Optical Interferometer (NPOI) since the last SPIE meeting in 2006 is
summarized, along with the results of the current science programs. The commissioning of new stations and plans for
greatly increased telescope apertures are discussed, along with other instrumentation upgrades. Recent results in the
areas of wide-angle astrometry, binary stars, physical modeling of the circumstellar disks of early-type stars,
improvements in coherent averaging, and phase-reference imaging are also reviewed.
Recovering images from optical interferometric observations is one of the major challenges in the field. Unlike
the case of observations at radio wavelengths, in the optical the atmospheric turbulence changes the phases on
a very short time scale, which results in corrupted phase measurements. In order to overcome these limitations,
several groups developed image reconstruction techniques based only on squared visibility and closure phase
information, which are unaffected by atmospheric turbulence. We present the results of two techniques used by
our group, which employed coherently integrated data from the Navy Prototype Optical Interferometer. Based
on these techniques we were able to recover complex visibilities for several sources and image them using standard
radio imaging software. We describe these techniques, the corrections applied to the data, present the images of
a few sources, and discuss the implications of these results.
We have developed an approach for systematically investigating the optical throughput performance of the different segments of a Michelson stellar interferometer, and applied it to the characterization of the Navy Prototype Optical Interferometer (NPOI). We report the results of the first phase of throughput measurements on NPOI, as well as some of the lessons learned.
Since the current generation of ground-based optical interferometers all suffers from varying degree of throughput degradation while the dominant causes for throughput loss are expected to vary for each individual instrument, the methodologies and approaches developed here could be of general use for the quantitative characterization of the throughput performance of the different optical interferometers, a prerequisite for its ultimate improvement.
We demonstrate a new calibration technique that can be applied to multi-spectral interferometric observations.
The technique measures a fixed-pattern in squared visibility measurements across the spectral channels of each
baseline. Because the fixed-pattern appears to be stable on time scales longer than one night, nightly or weekly
averages can be calculated based on observations of calibrator stars. The averaged fixed-pattern values can then
be removed from data of target stars. We demonstrate the performance of the calibration technique on actual
observations obtained with the Navy Prototype Optical Interferometer and show that the fixed-pattern effects
can be suppressed by up to an order of magnitude.
We present the results of differential phase experiments done with data from the Navy Prototype Optical Interferometer (NPOI). We take advantage of the fact that this instrument simultaneously records 16 spectral channels in the wavelength range 550-850nm, for multiple baselines. We discuss the corrections applied to the data, and show the results obtained for Vega and the Be star β Lyrae.
We report on experiments in multi-wavelength phase referencing using the Navy Prototype Optical Interferometer (NPOI). In these experiments we use the unique capability of the NPOI to simultaneously observe 16 spectral channels covering 512-850 nm on multiple baselines simultaneously. We present observations of the well-known Be star ζ Tauri using custom filters which allow us to isolate the Hα line in a single spectral channel while the other channels observe the stellar continuum. Since the central star is unresolved, we can use the data in the continuum channels to calibrate the spectral line data. Using the phase information recovered in this way, it is possible for the first time to use standard techniques to construct simple images of the line-emitting region around the star.
The technical status of the Navy Prototype Optical Interferometer (NPOI) since the last
SPIE meeting is summarized along with the current science programs. The instrument is
operated in an automatic observational mode, obtaining over 10,000 stellar observations
in the period, June 2004 through March 2006. The scientific program has been directed
at astrometry, TPF candidate stars, binary stars and other interesting targets such as Be
stars. A significant database of NPOI observations obtained in 1997-2004 is being
analyzed for binaries and single stars such as rapid rotating stars: Altair and Vega.
We present preliminary results from an ongoing survey for multiplicity among the bright stars using the Navy Prototype Optical Interferometer (NPOI). While the NPOI has previously concentrated on producing "visual" orbits of known close speckle and spectroscopic binaries, we have now embarked on a broader survey to detect new binary/multiple systems. We first present a summary of previous NPOI observations of known binary and multiple systems to illustrate the instrument's detection sensitivity for binaries at large magnitude differences over the range of angular separation detectable by the NPOI (currently 3 - 300 mas). We then discuss early results of the survey of bright stars north of declination -20°. This survey, which compliments previous surveys of the bright stars by speckle interferometry, initially emphasizes stars in a proposed Terrestrial Planet Finder (TPF) target list. To date, 29 of the 60 brightest TPF candidate stars (V ≤ 4.3) have each been observed on multiple nights. Preliminary analysis of these data indicates the possible detection of stellar companions to several of these stars.
We describe recent science projects that the Navy Prototype Optical Interferometer (NPOI) scientific staff and collaborators are pursuing. Recent results from the wide angle astrometric program and imaging programs (rapid rotators, binaries and Be stars) will be summarized. We discuss some of the technology that enables the NPOI to operate routinely as an observatory astronomical instrument.
We review the theory of rotating stars, first developed 80 years ago. Predictions include a specific relation between shape and angular velocity and between surface location and effective temperature and effective gravity. Seen at arbitrary orientation rapidly rotating stars will display ellipsoidal shapes and possibly quite asymmetric intensity distributions. The flattening due to rotation has recently been detected at PTI and VLTI. With the increasing baselines available in the visible and the implementation of closure phase measurements at the NPOI it is now possible to search for the surface brightness effects of rotation. Roche theory predicts only large scale deviations from the usual centro-symmetric limb-darkened models, ideal when the stellar disks are only coarsely imaged as now. We report here observations of Altair and Vega with the NPOI using baselines that detect fringes beyond the first Airy zero in both objects. Asymmetric, non-classical intensity distributions are detected. Both objects appear to be rotating at a large fraction of their breakup velocity. Vega is nearly pole on, accounting for its low apparent rotational velocity. Altair's inclination is intermediate, allowing high S/N detection of all the predicted features of a Roche spheroid. We describe how these objects will test this fundamental theory and how Vega's role as a standard will need reinterpretation.
KEYWORDS: Data modeling, Calibration, Visibility, Stars, Interferometers, Data integration, Signal to noise ratio, Spectrometers, Observatories, Phase measurement
We applied an algorithm for the coherent integration of visibility data of the Navy Prototype Optical Interferometer in the reduction of observations of Altair. This algorithm was first presented at the SPIE meeting in Kona in 2002 and is based on the principle of phase bootstrapping a long baseline using the fringe delays and phases measured on the two shorter baselines with which it forms a triangle in a three-station array. We show that the SNR of the visibility amplitudes and closure phases is significantly increased compared to the standard incoherent integration, also enabling us to use all 28 wavelength channels (instead of 20) afforded by the NPOI spectrometers. The recovery of the data at the blue end is important for constraining any models of this star.
The Navy Prototype Optical Interferometer has recently been equipped with specially-designed filters that pass Hα emission in a 2.5 nm band, suppress the continuum 50 nm to either side, and pass the continuum further from the Hα line. These filters allow fringe tracking on continuum light while taking data at Hα. Five- and six-aperture NPOI configurations have also been implemented recently. The improvement in U-V coverage with these configurations promises greater image fidelity in multi-spectral imaging as well as in specific lines, such as the very interesting Hα line. Using an array simulator operating in the AIPS++ environment, we simulate observations of Hα emission, assuming approximate source structure taken from earlier work in the literature. These simulations demonstrate the increased imaging capability of multi-aperture arrays and help define optimum Hα observation strategies.
This paper describes the current status of the Navy Prototype Optical Interferometer with emphasis on imaging and on changes since the last review in this forum.
KEYWORDS: Visibility, Signal to noise ratio, Algorithm development, Atmospheric modeling, Observatories, Data integration, Detection and tracking algorithms, Data modeling, Interferometers, Stars
We have developed an algorithm to determine precise fringe phases
in the presence of atmospheric turbulence. We use phase bootstrapping
to improve parameter estimates of weak fringes observed on long
baselines through coherent integration. With data from the Navy
Prototype Optical Interferometer (NPOI) on γ Sagittae, we demonstrate the importance of this method for the study of limb-darkening of stars.
In January of 2002, we began routine observations using the Navy Prototype Optical Interferometer with six siderostats operating simultaneously. We present recent images obtained with this 6-beam operations mode of the NPOI. We report on the implemented system enhancements that make this possible, such as the beam combiner, fringe detection electronics, fringe tracking algorithm and the control system. We discuss the present state of the NPOI data analysis, including amplitude and phase bias corrections, cross talk considerations, and calibration issues.
A review of operational procedures and requirements evolving at the Navy Prototype Optical Interferometer (NPOI) provides some useful insights for the automation, maintenance and operation of large optical interferometers even as construction and instrument development continues. Automation is essential for efficient, single operator observing. It is important to integrate ease of operation and maintenance into the instrument design from the start. In final form, the Navy Prototype Optical Interferometer, NPOI, will use six portable siderostats for imaging stars and narrow angle astrometry of multiple stars as well as four fixed siderostats designed for all sky astrometry. Currently all four astrometric siderostats and two transportable siderostats are operational. All six beams from the siderostats now in use have been combined coherently to form images of multiple stars at milliseconds of arc resolution.
The Navy Prototype Optical Lnterferometer, NPOI, is routinely used to measure visibility amplitudes and closure phase for stellar objects at optical wavelengths (e.g. , Benson et al. ,' Hajian et al.2) . In this poster we describe the fringe data collection aspects and the real time algorithm that enables us to actively track fringes with the
instrument. For a detailed description of the overall instrument see Armstrong et al. .
We present results of an on-going program to measure limb- darkened angular diameters of late-type giant stars using the Navy Prototype Optical Interferometer (NPOI). Observing with three elements of the NPOI and using twenty spectral channels covering the wavelength range from 520 nm to 850 nm we are able to extend the spatial frequency coverage beyond the first null in the stellar visibility function for a number of K giants. These observations make use of the technique of phase bootstrapping in which the shorter baselines with high visibilities are used to keep the longer baselines in phase. The data are inconsistent with a uniform brightness stellar disk. Adopting a particular limb- darkening model enables us to derive an angular diameter with high precision. The total uncertainty is dominated by our knowledge of the wavelength scale. These observations also include measurements of the closure phase, which shows a jump of 180 degrees at the position of the first null in the visibility amplitude. This is the first time a non-zero closure phase has been measured on a single star with a separate element optical interferometer.
The Infrared Optical Telescope Array (IOTA) is an interferometric facility currently observing in the near infrared bands at the Smithsonian Institution's Fred Lawrence Whipple Observatory in Arizona. The 45 cm siderostats can be moved on an L-shaped track allowing discrete bases ranging between 5 and 38 m. The capability to combine beams with fiber optics in the K band (2 micrometers <EQ (lambda) <EQ 2.4 micrometers ) has been demonstrated on the Fiber Link Unit for Optical Recombination (FLUOR) at Kitt Peak National Observatory, in which two existing 0.8 m telescopes have been coherently coupled by means of optical fibers. FLUOR is now set as a focal instrument of IOTA. It uses single-mode fluoride glass waveguides and couplers as a substitute for mirrors and beamsplitters to perform beam transportation and recombination. Processing the light in single-mode waveguides offers the possibility to self-calibrate each interferogram against the loss of fringe visibility induced by atmospheric turbulence, thus improving the accuracy of the fringe visibility measurements. The FLUOR unit can be operated as a Mach-Zehnder interferometer to produce zero-baseline spectra used in double-Fourier interferometry to obtain the visibility as a function of wavelength. In the current status, a N-S baseline of 21.2 m is used to observe late-type starts and derive their angular diameters.
The first two telescopes of the Infrared-Optical Telescope Array (IOTA) project are now in place and yielding data at the Smithsonian Institution's F. L. Whipple Observatory on Mt. Hopkins, near Tucson, Arizona. The IOTA collectors are 45 cm in diameter, and may be moved to various stations in an L-shaped configuration with a maximum baseline of 38 m. A third collector will be added as soon as funding permits. Each light-collector assembly consists of a siderostat feeding a stationary afocal Cassegrain telescope that produces a 10-X reduced parallel beam, which is in turn directed vertically downward by a piezo-driven active mirror that stabilizes the ultimate image position. The reduced beams enter an evacuated envelope and proceed to the corner of the array, where they are turned back along one arm for path compensation. The delay line, in one beam, consists of two parts: one dihedral reflector positioned in a slew-and-clamp mode to give the major part of the desired delay; and a second dihedral mounted on an air-bearing carriage to provide the variable delay that is needed. After delay, the beams exit from the vacuum and are directed by dichroic mirrors into the infrared beam-combination and detection system. The visible light passes on to another area, to the image-tracker detectors and the visible-light combination and detection system. The beams are combined in pupil-plane mode on beam splitters. The combined IR beams are conveyed to two cooled single-element InSb detectors. The combined visible-light beams are focussed by lenslet arrays onto multimode optical fibers that lead to the slit of a specially-designed prism spectrometer. For the visible mode, the delay line is run at several wavelengths on one side of the zero- path point, so that several cycles of interference occur across the spectrum. First results were obtained with the IR system, giving visibilities for several K and M stars, using 2.2 micrometers radiation on a N-S baseline of 21.2 m. From these measurements we obtained preliminary estimates of effective stellar diameters in the K band.
Construction of a two-telescope Michelson spatial inteferometer to be operated at a nominal wavelength of 2.2 microns in the near-IR began in May 1987. Nearly all of the mechanical, electronic, and optical hardware of the Infrared Michelson Array (IRNMA) is currently in place and has been tested. Proof-of-concept has been demonstrated, and efforts are currently underway to improve the system operation to produce reliable, calibrated fringe visibilities.
In this second paper on IRMA (Infrared Michelson Array) the control and data acquisition system for the instrument are described. This paper will concentrate on a description of the computer-siderostat control system. Three personal computer based systems perform the high level control such as star catalog maintenance, current coordinate computations and siderostat/delay line position and rate computations, control of the analog to digital conversion, data storage and quick look data reduction. Low level control, such as generating the pulses that are sent to various drive micro-steppers are performed by seperate Motorola 68000 based microprocessor boards. Command words are passed between the control computers and the various microprocessor boards through a multichannel parallel-to-serial interface. These hardware systems and the software responsible for pointing and tracking with the siderostats, will be described in the text of this paper.
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