Prototype Optical Interferometer Research Articles

ζ Orionis A is a Double Star

A close, 4th magnitude companion to ζ Orionis A has been resolved with the Navy Prototype Optical Interferometer at Lowell Observatory. This confirms an indication of multiplicity from observations with the stellar intensity interferometer at Narrabri 26 years ago. The new component in the multiple system ADS 4263, ζ Orionis Ab, is 2 mag fainter than Aa, which is a supergiant of type O9.5. During 1998 February and March, the pair had a mean separation of 42 mas. Orbital motion was subsequently detected, but the corresponding arc allows only a preliminary ephemeris.

The Accuracy of Ground-Based Optical Interferometry Observations

AbstractThe problem of wide-angle astrometry via interferometry is to recover the two coordinates for each star from the observed delays. In the absence of atmospheric turbulence, the geometrical delay can be defined as: dG, ij(t) ≡ dj(t) ‒ di(t) = Bij(t)·ŝ0 ‒ Cij. Here the geometrical delay is the difference between the delay line lengths di and dj that is required to equalize the effective optical paths from the star to the point of beam combination via each of two apertures i and j. Bij(t) is the baseline between the apertures, ŝ0 is the star position, and Cij is the difference between the ‘fixed’ internal optical path lengths Ci and Cj within the instrument. In principle, sufficient delay measurements would allow solution for the baseline vectors and the delay constants, as well as the positions of the stars. However, the actual situation is greatly complicated by the presence of the atmosphere and the fact that neither the delay ‘constants’ Cij nor the baseline vectors are stable over time. The design of, and the analysis of the data from, any ground-based optical interferometer must overcome all three of these effects. The design, operation, and the analysis of data from the Navy Prototype Optical Interferometer (NPOI) are presented here as examples of how to overcome the effects of the atmosphere and the instrumental instabilities in order to achieve accurate wide-angle astrometry. The status of the implementation of these techniques at the NPOI is presented.

Cepheid Diameters from Optical Interferometry: The NPOI Survey

AbstractWe present linear radii for four Cepheid variable stars, spanning a pulsation period range from 3 to 11 d, measured with the Navy Prototype Optical Interferometer (NPOI). We compare these radii to those found using traditional indirect methods, and to various period–radius relations found in the literature.

Stellar Angular Diameters of Late-Type Giants and Supergiants Measured with the Navy Prototype Optical Interferometer

We have measured the angular diameters of 50 F, G, K, and M giant and supergiant stars using the Navy Prototype Optical Interferometer at wavelengths between 649 and 850 nm and using three baselines with lengths up to 37.5 m. Uniform-disk diameters, obtained from fits to the visibility amplitude, were transformed to limb-darkened diameters through the use of limb-darkening coefficients for plane-parallel stellar atmosphere models. These limb-darkened diameters are compared with those measured with the Mark III optical interferometer and with those computed by the infrared flux method. Sources of random and systematic error in the observations are discussed.

Imaging and Modeling of Double Stars with the Navy Prototype Optical Interferometer: a Continuation of the Mark III Double Star Program

AbstractWe present results from the new Navy Prototype Optical Interferometer (NPOI) – the orbit of the double star Mizar A and measurements of Matar – which are compared to an orbit determined with the predecessor of NPOI, the Mark III Interferometer. Uncertainties of the orbital inclination are between 0.1 and 0.3 degrees; those of the semi-major axis are less than 0.5 percent. We determine the component masses and other parameters of the double star system, including relative photometry, directly through a fit to a combination of interferometric and spectroscopic data. Algorithms were developed to handle any hierarchical stellar system.We describe plans for the study of spectroscopic double stars with NPOI, which draw from our experience with the Mark III binary program. This program yielded orbits of 26 stars, of which 17 were published with mass determinations of 24 components using spectroscopy. The accuracy of the physical parameters were often limited by the spectroscopy. We show that in order to benefit from the high precision of the interferometric observations, new high-precision spectroscopic observations, combined with improved algorithms for the detection of the secondaries, are required.

A New Multichannel Fourier Transform Spectrometer

AbstractStellar spectra have been obtained using a multichannel Fourier Transform Spectrometer (FTS) which incorporates components of the Navy Prototype Optical Interferometer. It is well known that a FTS can provide superior wavelength stability as compared to traditional spectrometers. Unfortunately the FTS traditionally suffers from exceptionally poor sensitivity, which until now has limited its uses to sources with high fluxes and/or those with narrow band emission (e.g. the Sun, nebulae, and laboratory samples). We present stellar observations using a new FTS design which overcomes this sensitivity limitation by using a conventional multichannel spectrometer in conjunction with the FTS system. The signal-to-noise ratio of spectra from our test-bed observations are consistent with the theoretical prediction and show that for N channels the sensitivity scales like N, while the signal-to-noise ratio scales like . With this type of an instrument on a 3-m telescope and 9 000 channels we expect to be able to detect and measure such exciting astrophysical phenomenon as gravitational redshifts from single, main sequence stars and extrasolar planets of terrestrial mass.

Navy Prototype Optical Interferometer Observations of the Double Stars Mizar A and Matar

We present a detailed analysis of two spectroscopic binaries based on new observations obtained with the Navy Prototype Optical Interferometer (NPOI). While the data calibration needs refinement, first results show the impressive potential of NPOI, both in terms of speed and precision, for imaging and modeling the orbits of spectroscopic binaries. We determine the orbital parameters of Mizar A (ζ1 Ursae Majoris) and Matar (η Pegasi), and derive masses and luminosities using published radial velocities and Hipparcos trigonometric parallaxes. The results on Mizar A are compared to earlier work done with the Mark III interferometer, while data from this instrument were combined with NPOI data in the Matar analysis.

The Navy Prototype Optical Interferometer

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 Flagstaff, Arizona, obtaining its first images, of a binary star, in 1996 May and June and its first limb-darkening observations during 1996 November to 1997 February. This paper gives an overview of the NPOI, including the characteristics of optical interferometry that affect 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 fixed 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 450-850 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 configurations with linear scales at ratios of ≈ 1.64. Unequally spaced configurations are available for a variety of other imaging programs. Coherence across either type of imaging configuration is maintained by bootstrapping: 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 ≈ 2 mas. The best angular resolution of the imaging subarray will be ≈ 200 μas. Our experience with the Mark III interferometer suggests that we will be able to measure stellar diameters as small as 200 μas with 1% precision and binary star separations as small as ρ ≈ 65 μas (for Δm ≈ 0 mag) or ρ ≈ 200 μas (for Δm ≈ 3-4 mag). With its large bandwidth and phase bootstrapping, the imaging subarray should be able to make images 10 resolution elements across the disks of nearby late-type stars.

Direct Confirmation of Stellar Limb Darkening with the Navy Prototype Optical Interferometer

Using three elements of the Navy Prototype Optical Interferometer and observing in 20 spectral channels covering 520-850 nm, we have implemented a phase bootstrapping technique in which short baselines with high visibilities are used to keep the longer baselines with low visibilities in phase. Using this method, we have been able to extend the spatial frequency coverage beyond the first zero of the stellar visibility function for two K giants α Arietis, and α Cassiopeiae. The data are inconsistent with a uniform-disk model and confirm the presence of limb-darkened radial profiles. Adopting a particular limb-darkening law enables us to determine the diameter with small formal errors (one part in 1000). In addition, we have measured closure phases for both stars. The closure phases show a jump of 180° at the first zero in the visibility amplitude, which was expected.

Seeing Tests at Four Sites in Support of the NPOI Project

view Abstract Citations (5) References (12) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Seeing Tests at Four Sites in Support of the NPOI Project Hutter, D. J. ; Elias, N. M., II ; Peterson, E. R. ; Weaver, W. B. ; Weaver, G. ; Mozurkewich, D. ; Vrba, F. J. ; Simon, R. S. ; Buscher, D. F. ; Hummel, C. A. Abstract This paper reports astronomical seeing measurements made at four sites in the western United States during 1991-92. This study was conducted as part of the site selection process for the Navy Prototype Optical Interferometer. An extensive series of observations were made at Chews Ridge and Anderson Ridge, CA (≍36 000 equivalent FWHM seeing estimates). Observations made from Chews Ridge over 49 nights yield an average FWHM seeing value of 1."1, while those made from Anderson Ridge over 34 nights yield an average FWHM seeing of ≍1:4. However, observations made from Anderson Ridge at a location away from any nearby trees indicate that more open sites in that area may enjoy seeing comparable to that measured at Chews Ridge. Simultaneous free-air, ground-level observations at the U.S. Naval Observatory's Flagstaff Station and at Anderson Mesa, both near Flagstaff, AZ, yield similar seeing values to each other (≥0".1 difference). Publication: The Astronomical Journal Pub Date: December 1997 DOI: 10.1086/118690 Bibcode: 1997AJ....114.2822H Keywords: SITE TESTING full text sources ADS | data products CDS (1)

Multichannel optical aperture synthesis imaging of lambda1 URSAE majoris with the Navy prototype optical interferometer.

We have used the Navy Prototype Optical Interferometer (NPOI) to obtain the first multichannel optical aperture synthesis images of a star. We observed the spectroscopic binary ~ 1 Ursae Majoris at 6 to 10 milliarcseconds separation during seven nights, using three interferometric baselines and 19 spectral channels (A.A. 520-850 nm) of the NPOI. After editing, a typical90 sec scan yielded fringe visibilities at 50 spatial frequencies and closure phases at 15 wavelengths. Three to five scans were obtained each night. The separations and position angles are in good agreement with the visual orbit obtained with the Mark III interferometer (Hummel et al. 1995, AJ, 110, 376) but show small systematic difference that can be used to improve the orbit. The closure phase data provide a sensitive measure of the magnitude difference between the components. These results demonstrate the power of broad-band interferometric observations for fast imaging and the utility of vacuum delay lines for simultaneous observations over a wide band. These observations are the first to produce simultaneous visibilities and closure phases with a separate-aperture optical interferometer, and the second to produce closure phase images, following the results from COAST reported by Baldwin et al. (1996, A&A, 306, L13). The angular resolution here is the highest ever achieved at visual wavelengths, exceeding by an order of magnitude the best thus far achieved by any single-aperture optical telescope. We generated complex visibilities and closure phases (the data types commonly used in radio interferometry) from the optical data and used standard radio interferometry techniques to produce these images. However, the fundamental observables of optical interferometry, the squared visibility amplitude and the closure phase, require the development of new analysis techniques. © 1997 American Astronomical Society. [S0004-6256(97)01709-3]

The USNO Astrometric Interferometer

The U.S. Naval Observatory Astrometric Interferometer (USNOAI) will be the dedicated astrometric sub-array of the new Navy Prototype Optical Interferometer (NPOI) at Lowell Observatory, which is being built in collaboration with the Naval Research Laboratory. The USNOAI will be in operation on Anderson Mesa, near Flagstaff, Arizona, in late 1993. The instrument will incorporate four siderostats which will be located in a Y-shaped configuration, and will feature state-of-the-art delay lines, a real-time zeroth-order fringe tracking algorithm, a full-array laser metrology system to monitor baseline motion, and a limiting magnitude of 10. The instrument will be capable of simultaneous observations on six baselines, and will produce star positions accurate to a few mas. With it, we will be able to maintain the HIPPARCOS reference frame with matching accuracy over the next few decades.

The Baseline Metrology System of the USNO Astrometric Interferometer

The USNO Astrometric Interferometer (USNOAI; a subarray of the Navy Prototype Optical Interferometer at Lowell Observatory) is presently under construction and expected to begin limited operations within a year. The main goal of the USNOAI observations is to provide a northern hemisphere catalog of several thousand stars with positions known to a few mas. In order to meet this requirement, a baseline laser metrology system must be employed to measure the three-dimensional motions of the baselines with an accuracy better than ~ 0.1 μm. The metrology scheme, as presently conceived, represents the largest and most complex high-resolution laser metrology system ever attempted.