Some of the Galilean satellites show evidence of two distinct surface types, a high albedo frost and a dark, presumably silicate material. We are beginning a program to map the spatial distribution of these components with a Michel son spatial interferometer. This will be done both in and next to the 2.0µm frost absorption band. While the mapping program is just beginning, this investigation has already yielded new information on the wavelength dependence of seeing in the infrared.

Heterodyne spatial interferometry at a wavelength of 11 microns has been used to examine properties of circumstellar dust shells. Among the objects which have been observed are several M-type supergiants and Mira variables as well as several peculiar infrared stars. These measurements provide information on the temperature and spatial distribution of dust grains. Possible future developments in heterodyne interferometry include longer baselines for higher resolution and the use of larger telescopes for greater sensitivity.

Speckle interferometry techniques have improved appreciably during the past few years. Current progress to be reviewed includes: steps towards a fully digital analysis in the photon counting mode allowing for high resolution on faint astrophysical objects (i.e. quasars), practical prospects for phase recovery procedure, and the design of compact, self-contained interferometers for on-line results at remote sites.

One tries to define an optimal method for the measurement of small apparent stellar motions. The available or proposed techniques and atmospheric disturbances are studied first. A special purpose interferometer is then devised; it is found far more complex and not in any way better than a previously described direct imaging system.

The two meter baseline interferometer of the Royal Observatory, Edinburgh, will be described. This instrument was designed specifically for the observation of binary stars which have angular separations intermediate between the classical visual binaries and spectroscopic systems. Observations on bright stars suggest that it should be possible to measure the fringe visibility with an accuracy of a few percent.

Recent theoretical studies of the signal to noise ratio (SNR) of photon limited speckle (image plane) interferometry are reviewed. The SNR of an estimate of the object power spectrum is evaluated for both the single and double aperture cases, for arbitrary light levels. The SNR for the auto-correlation function method of analysis is also given for the low light level case and applied to the special case of binary star observations. The SNRs for the power spectrum and autocorrelation function analyses are compared and a comparison is also made between speckle (image plane) and amplitude (pupil or aperture plane) interferometry. Limiting observable magnitudes are estimated for some relevant cases.

A coordinated program of instrumental development and astronomical observation in Amplitude Interferometry has been conducted at the University of Maryland for the past ten years. The objective of this overall program will be presented. The method of operation and the fringe detection technique of the University of Maryland Amplitude Interferometer will be described and reviewed from both the theoretical and the instrumental point of view. The relationship between the detection technique of Amplitude Interferometry and the various parameters of the atmospheric model will be described and the measured Modulation Transfer Function of the telescope/atmosphere will be illustrated and compared with the MTF obtained in other techniques. Samples of the data obtained during the Amplitude Interferometry observing program of the past six years will be presented. This has operated to sixth magnitude, and has reproducibility resolved objects to 0.011 arc seconds with precision of better than 0.002 arc seconds. The reproducibility and stability of repeated measurements of the stellar diameters will be presented. The various uses of this data in astronomy will be briefly described.

The future application of Amplitude Interferometry will be described, particularly a system to obtain high sensitivity and very high resolution with the same detection system and same signal processing as the current instrument. However, for the immediate future, the two instrumental questions are being addressed separately in the form of the Multi-Aperture Amplitude Interferometer, which operates on a conventional telescope and has an increased data rate (by a factor of 8,000) and the Very Long Baseline Amplitude Interferometer with the increased resolution. These instruments and their current status will be described as well as their relationship to the earlier program and their eventual use in a Multi-Aperture Very Long Baseline Amplitude Interferometer system. Finally the possible applications of these techniques Interferometry to a satellite system will be briefly mentioned.

An incoherent spatial interferometer has been used to measure the shapes of circumsteliar dust envelopes at infrared wavelengths. The objects IRC +10216, VY CMa, and NML Cyg exhibit a large variation of fringe visibility with position angle Their circumstellar envelopes are probably flattened disks. For IRC +10216, visibility measurements from 5.0 to 20µm are discussed in detail to illustrate the physical information available from interferometric work. In VY CMa and NML Cyg, disk orientation is strongly correlated with polarimetric position angles and with the distribution of OH maser sources. Interferometric measurements are shown to be relevant to studies of star/planet formation and to studies of mass loss in long-period variables

We analyze the behaviour of an amplitude interferometer with aperture diameters of the order of r0. A simple optical correlator can be used to measure the source coherence function, and the limiting magnitude is found to depend principally on the optical bandwidth. The basic instrument appears to have a limiting magnitude m = +8, but this can be improved by more elaborate correlators. The effects of atmospheric turbulence on such an interferometer can be substantially reduced by an active optical system which compensates for the wavefront tilts in the incident light, and it appears that the limiting magnitude of the interferometer, when atmospheric effects are taken into account, is around m = 7 - 8.

A brief review of existing angular diameter measurements of single objects is given. Potential astronomical programmes for high angular resolution stellar interferometry in the visual region of the spectrum are discussed with particular regard to the sensitivity, resolution, and accuracy required for significant astrophysical results to be obtained. This discussion is used as a basis for suggesting the minimum requirements for future high angular resolution stellar interferometers.

The angular resolution of conventional astrophotography is limited to about 1 sec of arc due to image degradation by the atmosphere and telescope aberrations. Higher resolution can be achieved by Labeyrie's speckle interferometry, which yields the autocorrelation of astronomical objects with diffraction-limited resolution, e.g. 0.02 sec of arc with 5m telescope aperture.

We have investigated modifications of speckle interferometry, which yield diffraction-limited images. We investigated (1) “speckle holography” for image reconstruction of objects near an unresolvable point source (reference star), (2) “speckle masking” for image reconstruction of binary stars (without a reference star), and (3) “phase flipping” for image reconstruction of centro-symmetric objects. Together these three image reconstruction methods represent a set of speckle evaluation procedures, which can yield high resolution images of most astronomical objects that are bright enough for speckle interferometry. Speckle holography was already applied to astronomical objects. We reconstructed real images of the binary star Zeta Cancri A-B and the triple star ADS 3358 A-B-C ( 0.07 sec of arc resolution with 1.8m telescope aperture).

The effects of atmospheric turbulence on the image structure in a ground based Michelson Stellar Interferometer are analyzed. A complete system requires compensation both for the wavefront distortions over the collecting apertures and for path length differences between them. Wavefront sensing and compensation have been previously described and analyzed. In this paper, a method of path length compensation and estimation of the mutual coherence function of the source is developed using matched filters in multiple spectral bands. A practical implementation of the resulting system is described and estimates of its performance given.

The possible use of the Very Long Amplitude Interferometer in several programs of astrometric measurements and observations will be described. These programs include short-arc or differential astrometry, large-arc astrometry for zonal corrections in the FK-4 datalog and absolute astrometry primarily for the study of earth rotation. The basic techniques for these different applications, the relevant measurement precision and accuracy, and the related signal-to-noise ratios will be discussed. The role of the atmosphere and the critical atmospheric parameters, as they relate to the astrometric measurements, will be discussed including the atmospheric delay correction by the use of two separate wavelengths. The role of various types of ground motion i.e. seismic disturbance for the different types of astrometry will be discussed and data presented as to the possible effects. Measurements on the stability of equipment will also be included. In addition the use of a two color refractometer for site surveys and the evaluation of gravity waves in the atmosphere will be considered. The procedure for references to the vertical (which is required for earth rotation) will be considered.

The 12 to 20 metre baseline interferometer at C.E.R.G.A. built by A. Labeyrie has now been operational for two years. The purpose of this instrument was initially to be a prototype showing the feasibility of long baseline coherent arrays and for testing the techniques needed for them. Yet, pending completion of large interferometers, this small 2 x 26 cm instrument has proven able to yield astrophysical data: a dozen stars up to third magnitude have been observed with a 3 x 10-3 arc second resolution on diameters and a 5 x 10-4 arc second accuracy for binary separation measurements in the case of α Aur.

What has been observed both from the technological and the astrophysical point of view will be discussed.

A review will be presented of the status of the fundamental reference system. Mention will be made of the problems limiting the accuracy of transit circles and photographic astrometry and attempts at possible improvments in the near future. The progress in the preparation of the FK5 will be briefly mentioned, as will be the various methods used to approximate an inertial reference frame. Needs for higher accuracy in Astronomy fall in the area of better modeling of the Earth's motion (rotation, nutation, precession, the ecliptic, etc.) as well as a parallax and proper motion determination for galactic structure studies, stellar dynamics and stellar evolution. It will be assumed that interferometric binary star studies will be discussed elsewhere.

Holograms of incoherent objects can be recorded with a rotation-shearing interferometer allowing corrections of wavefront errors. Holographic images taken in our laboratory, either through aberrated optics or atmospheric turbulence, have been successfully restored. A stellar hologram is presented.

An Amplitude Interferometer based on the design of an earlier interferometer but with two independent telescopes has been built at Maryland, The design and the current status of the instrument will be discussed.

This interferometer has been built with automatic guiding in each arm and is intended for use with baselines of 50 meters but can be used with longer baselines at restricted declinations.

The visibility detection will be exactly the same as that used in the cassegrain model of the interferometer which has operated successfully for several years on the 100 and 200 inch Hale telescopes. The visibility will be measured simultaneously in three different paths with slightly different delays which will allow for automatic location and tracking of the white light fringe envelope.

Over the last two decades, an extensive body of knowledge has been developed concerning the effects of atmospheric turbulence on optical propagation. Much of this is directly relevant to astronomical imaging, and with proper interpretation, to the type of pseudo-imagery that is of concern to us at this conference. This paper will provide an overview of this matter, hopefully with sufficient insight developed that the reader will be able to quickly estimate the nature and magnitude of the turbulence effects to be expected in a pseudo-imagery process. The paper starts with a review of turbulence effects on conventional imagery, reviewing the “nondimensional” nature of the turbulence statistics, presenting the local measure of the optical strength of turbulence, , and developing the resolution scale, r0. It presents a statistical view of the nature of the wavefront distortion geometry, indicating the dominance of the random wavefront tilt component. The MTF for conventional imagery and for speckle interferometry (Labeyrie) is presented with comments concerning their relationship. Following that, the foundation of the speckle imagery concept (Knox-Thompson) is presented. Results are then set forth for the allowable spectral bandwidth in speckle techniques, as well as results defining the allowable field-of-view size (isoplanatism) and the allowable exposure time for speckle techniques. Taken all together, these results provide a basis for estimating most of the significant effects of atmospheric turbulence in speckle interferometry and speckle imagery.

The use of Stellar Speckle Interferometry has been extended to the near infrared using a single element detector. Preliminary measurements indicate that diffraction limited information is available in this wavelength region.