In this paper, we describe a practical implementation of an image reconstruction method designed to generate
a map of the brightness distribution from data consisting of squared visibilities and complex closure amplitudes
resulting from observations of an astronomical target with a broadband, multichannel, spatial optical interferometer.
Given the data, the method estimates the true brightness distribution with a model sampled on a
rectangular grid of discrete positions on the sky with the assumption that the model intensities in the region
not defined by the discrete positions being described by bilinear interpolation of the discrete intensities. The developed
image reconstruction method has been applied to real observational data obtained from existing optical
Optical long-baseline interferometric data is commonly calibrated with respect to an external calibrator, which is either an unresolved source or a star with a known angular diameter. A typical observational strategy involves acquiring data in a sequence of calibrator-target pairs, where the observation of each source is obtained separately. Therefore, the atmospheric variations that have time scales shorter than the cadence between the target-calibrator pairs are not always fully removed from the data even after calibration. This results in calibrated observations of a target star that contain unknown quantities of residual atmospheric variations. We describe how Monte Carlo simulations can be used to assess quantitatively the impact of atmospheric variations on fitted model parameters, such as angular diameters of uniform-disk models representing semi- and fully-resolved single stars.
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.
In this paper we use coherently integrated visibilities (see separate paper in these proceedings<sup>1</sup>) to measure the
properties of binary stars. We use only the phase of the complex visibility and not the amplitude. The reason
for this is that amplitudes suffer from the calibration effect (the same for coherent and incoherent averages) and
thus effectively provide lower accuracy measurements. We demonstrate that the baseline phase alone can be used
to measure the separation, orientation and brightness ratio of a binary star, as a function of wavelength.
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 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.