J. T. Armstrong

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 phase 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.

THEORETICAL LIMB DARKENING FOR CLASSICAL CEPHEIDS. II. CORRECTIONS FOR THE GEOMETRIC BAADE-WESSELINK METHOD

The geometric Baade-Wesselink method is one of the most promising techniques for obtaining a better calibration of the Cepheid period-luminosity relation by means of interferometric measurements of accurate diameters. In this paper we present new wavelength- and phase-dependent limb-darkening (LD) corrections based on our time-dependent hydrodynamic models of the classical Cepheid ζ Gem. We show that a model simulation of a Cepheid atmosphere, taking into account the hydrodynamic effects associated with the pulsation, shows strong departures from the LD otherwise predicted by a static model. For most of its pulsational cycle the hydrodynamic model predicts a larger LD than the equivalent static model. The hydrodynamics affects the LD mainly at UV and optical wavelengths. Most of these effects evolve slowly as the star pulsates, but there are phases, associated with shocks propagating into the photosphere, in which significant changes in the LD take place on timescales of the order of less than a day. We assess the implication of our model LD corrections fitting the geometric Baade-Wesselink distance of ζ Gem for the available near-IR PTI data. We discuss the effects of our model LD on the best-fit result and analyze the requirements needed to test the time dependence of the LD with future interferometric measurements.

Small glints as an aid for imaging geosats using an optical Michelson interferometer

Abstract A Michelson optical interferometer, such as an upgraded version of the Navy Precision Optical Interferometer, could image geosynchronous satellites (geosats) with resolution of roughly 1 m. Baselines that sample features as small as 0.2 m can be built, however, the fringes would be swamped by the resolved component. Recent observations have shown that small glints known as “glintchen,” aside from being a nuisance, serve to isolate and highlight the signal from these structures. Imaging of geosats during glintchen events can determine the dimensions of these structures and can also play a critical role in determining if these glintchen are due to a previously undetected companion satellite. An approach for performing this glint-aided imaging of geosats and the wealth of detail it would yield, is discussed.

H-alpha observations using closure phases at the NPOI

We have enhanced the spectral resolution of the Navy Prototype Optical Interferometer (NPOI) at the H-alpha line to 3 nm (FWHM). We use customized filters that suppresses light in the ~600-725 nm window except for light at the H-alpha wavelength (656.3 nm). The bands shortward of 600 nm and longward of 725 nm are used for fringe tracking and for calibrating the system fringe visibility. We have used these filters to observe H-alpha emission from circumstellar material around Be stars. Closure phases from our initial observations of the Be star zeta Tau with three array elements suggest that the H-alpha emission is not centered on the star. We will show these three-element results, as well as recently-acquired data from the NPOI using 4, 5, and 6 stations.

Precision narrow-angle astrometry of binary stars with the Navy Prototype Optical Interferometer

The Navy Prototype Optical Interferometry (NPOI) group has started an astrometric search for planets in binary star systems based on the idea of using the binary components as position references for one another and looking for deviations from Keplerian motion. Our search will complement the radial velocity (vR) searches in three ways. We will observe stars of all spectral types; vR searches are limited to the FGKM range, where stars exhibit narrow spectral lines. We will search for planets in relatively large orbits (more than about 4 AU) where our method is most sensitive; vR searches are most sensitive to close-in planets. Finally, we will examine binary star systems, which with a few exceptions have been excluded from vR surveys. Our targets are binaries with both components in the interferometric field of view, producing a periodic variation in the fringe visibility (V2) across the (u,v) plane. Past NPOI results from closer binaries (separations in the tens of mas) show residuals of tens of microarcseconds about the best-fit orbits. The larger separations we are observing produce more V2 oscillations across the (u,v) plane, offering the possibility of higher precision. We discuss the level of precision in test observations and the steps that will be needed to convert precision into accuracy.

Optical drift test of the long delay line stations on the Navy Prototype Optical Interferometer

At the Navy Prototype Optical Interferometer (NPOI), during stellar fringe acquisition and tracking, optical stations along the NPOI vacuum line array remain in passive mode. Optical drift amplitude and rate must remain below certain limits lest stellar acquisition and fringe tracking become unachievable. Subsequent to each observation, relay mirrors are reconfigured within the long delay line stations to provide appropriate constant delays. The placement of these mirrors must be reliable and repeatable within certain tolerances. We describe the results of drift tests conducted on the current long delay line stations.

Estimation of Fringe Parameters

In this report we explore replacing the widely used optimal V2 estimator with a model-fitting approach. We show that it is possible to fit the fringe power spectra with a physically reasonable model. This approach eliminates the biggest problem with the standard squared visibility estimator - determining the additive, dector-noise bias. We examine the dependence of the bias on count rate for consistency betwee on- and off-fringe measurements. The change of bias with fringe frequency provides additional information about the performance of the detectors. We have also applied a similar approach to the bias correction for the triple product, with comparable results.

Alignment of Vacuum Feed Stations on the Navy Prototype Optical Interferometer

At the Navy Prototype Optical Interferometer (NPOI) we have developed a two-stage method for preparation and installation of the optical feed relay stations (elevators). This method reduces contamination, increases consistency, and allows greater management in testing and upgrades. In stage one, we prepare a pre-alignment facility in a laboratory. Using this facility we accurately position the feed stations, internal optics and detector optics relative to the NPOI array line-of-sight. The feed station is cleaned, assembled, internally aligned, tested and placed in its vacuum canister. It is stored under vacuum until transported to the array. In stage two, we align the station on the array by global five-axis adjustments of the vacuum canister. No further independent internal alignments are necessary. The canister is continuously under vacuum during global alignments. We describe the methodology and techniques for installing the optical feed stations.