Visibility measurements with Michelson interferometers, particularly the measurement of fringe contrast, are
affected by various atmospheric and instrumental effects, all of which reduce the measured contrast. To compensate
for this, stars with known or predictable diameters (calibrators) are observed so that the overall reduction
in the visibility can be measured. Objects with the smallest possible diameters are preferred as calibrators, since
the predicted visibilities become less sensitive to any uncertainties. Therefore, unreddened, early type stars are
usually chosen if they are available because they are relatively bright for a given angular diameter. However,
early type stars bring additional complications. Rapid rotation, common with these stars can cause variations
in the visibility amplitudes due to oblateness and surface brightness asymmetries that are larger than implied by
the usual error estimates. In addition, rotation can introduce significant phase offsets. Using Roche models, von
Zeipel theory, and the observed constraints of V, B-V, and v sin i, it is possible to put limits on the size of these
effects and even estimate the distribution of possible visibilities. To make this easily available to the community,
we are in the process of creating a catalog of possible calibrators, including histograms of the visibilities, calculated
for configurations used at a number of observatories. We show the examples of several early type stars
which are potential calibrators using parameters appropriate for the Navy Prototype Optical Interferometer.
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.
Avalanche photodiodes offer many advantages for photon counting in the visible and near IR. However, as with all pulse counting systems, finite response times result in missed pulses as signal levels are increased. Further, APDs build up a pulse by accelerating electrons through large potential drops before being quenched which can result in significant heating with increasing signal levels and subsequent loss of quantum efficiency. Both these effects, heating and deadtime, result in a significantly non-linear response at high signal levels. We report here a combination of simple models of the thermal behavior of the detectors and the finite nature of counting electronics that allows us to account for these efects. We also demonstrate a simple method for measuring off line the parameters of this model. With a relatively few free parameters we are able to restore linearity very close to detector saturation. A silver lining is that the combined loss of quantum efficiency (heating) and detected pulses (deadtime) provides a factor of two gain in incoming signal levels before saturation.
Atmospheric turbulence is a major impediment to ground-based optical interferometry. It causes fringes to move
on ms time-scales, forcing very short exposures. Because of the semi-random phase shifts, the traditional approach
averages exposure power spectra to build signal-to-noise ratio (SNR). This incoherent average has two problems:
(1) A bias of correlated noise is introduced which must be subtracted. The smaller the visibility/the fainter the
target star, the more diffcult bias subtraction becomes. SNR builds only slowly in this case. Unfortunately, these
most difficult small visibility baselines contain most of the image information. (2) Baseline phase information is
discarded. These are serious challenges to imaging with ground based optical interferometers. But if we were able
to determine fringe phase, we could shift and integrate all the short exposures. We would then eliminate the bias
problem, improve the SNR, and we would have preserved most of the phase information. This coherent averaging
becomes possible with multi-spectral measurements. The group delay presents one option for determining phase.
A more accurate approach is to use a time-dependent model of the fringe. For the most interesting low-visibility
baselines, the atmospheric phase information can be bootstrapped from phase determinations on high-visibility
baselines using the closure relation. The NPOI, with 32 spectral channels and a bootstrapping configuration,
is well-suited for these approaches. We will illustrate how the fringe modeling approach works, compare it to
the group-delay approach, and show how these approaches can be used to derive bias-free visibility amplitude
and phase information. Coherent integration provides the highest signal-to-noise (SNR) improvement precisely
in the situations where SNR builds most slowly using incoherent averaging. Coherent integration also produces
high-SNR phase measurements which are calibration-free and thus have high real uncertainties as well. In this
paper we will show how to coherently integration on NPOI data, and how to use baseline visibilities and calibrate coherently integrated visibility amplitudes.
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.
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.
We have developed a new algorithm for simulating atmospheric phase
screens that is extremely efficient and flexible. It retains the N log N efficiency of algorithms based on the FFT and is able to take
advantage of the small amount of atmosphere actually sampled by highly dilute arrays such as NPOI. Additionally, there is flexibility in the technique to relax (or adopt) the traditional "frozen screen" approximation. We describe the basis for the algorithms and how the code has been structured. Timing estimates are developed and we show preliminary results from the code which exhibit the correct behavior of phase difference power spectra with baseline.
In 1991 the Astrophysics Division of NASA's Office of Space Science convened the Space Interferometry Science Working Group to consider in more detail the science goals of a space interferometer mission to do wide-angle astrometry at optical wavelengths. In addition, the working group considered the merits of alternative mission concepts for achieving those goals. We describe the current state of the adopted mission concept, and review the candidate astrometric science program. In addition to the main goal of precision astrometry, the concept interferometer has a limited capability for high- resolution imaging using rotational aperture synthesis. A phase A start on this mission has been made in 1996, an launch is planned for 2003.
In the two years since the last SPIE meeting on this topic there has been much activity in both ground and space based interferometry. I review those developments, I also summarize the Strawman Science Proposal prepared by the Space Interferometry Science Working Group as a gauge for evaluating the AIM instrument proposals. I then review the recent discovery of the disk structure in M106 using radio interferometry. As an example of where we want to go with optical interferometry, the M106 case argues for infrared capabilities, significant fields of view, and the availability of auxiliary instruments, e.g. spectrographs, in the imaging focal plane.