This paper describes implementation of algorithms as they are related to real time phase measurements and control for a synthetic aperture optical system. Simulation and experimental data are presented which show the differences when these algorithms are implemented on different types of machines.

Intensity interferometry measures the square of the absolute value of the normalized coherence function. The phase of the coherence function is lost. The possibility of deriving the phase function by using higher-order intensity correlations from the signal collected by a large array of mirrors is discussed. The array is not phased, and each mirror is considered to be of the intensity interferometer type, i.e., it only concentrates light onto a detector without regard to the imaging quality. Knowledge of the coherence function yields the two-dimensional intensity distribution of self-luminous or intrinsically luminous objects.

Coherent superposition (phasing) in optical arrays can only be achieved when the individual beams maintain their initial coherence throughout the system. Reflections on metallic surfaces cause phase shifts of less than 180 degrees resulting in elliptical polarization which depends both on the plane and angle of incidence. Since the use of relay mirrors is essential in phased optical arrays , these phase shifts have an effect on the mutual coherence of the beams associated with the individual array elements. This paper presents the results of an investigation to estimate polarization effects for optical phased array systems, and the resulting loss of mutual coherence. Of specific interest were cases where sets of relay mirrors define different planes of incidence within the optical array. It was found that the resulting losses in coherence are a strong function of the angle of incidence at each mirror in the relay systems. The resulting implications for the design of optical array systems are discussed. It is also shown how the results can be applied to broadband, partially coherent radiation.

This paper describes a new technique to align and phase the multiple apertures in phased array optical systems. A variety of potential applications exists for this approach, including optical metrology, interferometry, and control systems for next generation astronomical telescopes. The approach as described here uses broadband radiation to accurately measure large optical pathlength differences, and to monitor the performance of a laboratory microprocessor-controlled phasing system. This paper presents theoretical analyses along with results obtained during experimental investigations.

A crucial parameter for an optical phased array is the sensitivity of the system performance to alignment and phasing errors among the telescopes. For an array of telescopes to be coherently phased, the focal planes of each telescope must be exactly superposed. This paper presents mathematical derivations of all the conditions needed to achieve the required degree of superposition. These conditions are derived by analytically computing the point spread function for a combined aperture system (i.e., all apertures are included). The conditions for ideal superposition cause this function to be constant over an extended field-of-view. The sensitivity of a phased array system to single-or multiple-telescope deviations can thus be quantitatively evaluated, for monochromatic or broadband light.

The various manufacturing approaches used for generating, grinding, figuring and lightweighting aspheric optical elements are discussed. Measurement of these elements during manufacture, using direct contact profilometry is also described.

More than 40 reflectance spectra in the range from 20 to 500 pm have been obtained of a variety of coatings, binders, and additives to identify promising components of an infrared-opaque coating for the Space Infrared Telescope Facility. Certain combinations of materials showed a specular reflectance below 0.1 throughout the spectral range measured.

The bidirectional scattering distribution functions (BSDF's) of zinc selenide are reported. The scatter in both reflection and transmission modes was measured at 0.6328, 3.39, and 10.6 μm and for several angles of incidence.

This paper describes the optical design of an integrating sphere - Fourier Transform Spectrophotometer (FT S) instrument for measuring diffuse IR reflectance as a function of angle, temperature and wavelength. The integrating sphere is 5 inches in diameter with a center mounted sample stage permitting beam incidence angles of 10 to 70 degrees. Samples can be mounted back-to-back for relative measurements and ports are included for specular subtraction of the reflected beam at 20 and 60 degree incidence angles. A heater capable of producing temperatures over 150°C h'as been included in a sample mount on the wall of the sphere. In addition, the sphere can be rotated about the beam port, permitting operation in both the center and wall mounted modes. Two detectors are planned for the sphere: a 16 mm² square cooled HgCdTe detector and an uncooled 3 mm diameter DTGS detector which is coupled to the sphere with a nonimaging compound elliptic concentrator (CEC). The CEC restricts the detector's field-of-view (FOV) to uniform contrast areas on the sphere wall with essentially no change in detector flux. The sphere's coating consists of a 0.5 micron thick gold film on a aluminum substrate, with a mean feature size of approximately 20 μm; a similar coating with a roughness average of approximately 10 pm was also considered. Measurements of the 10 pm coating's total spectral reflectance from 0.3 - 20.0 pm and the bi-directional reflectance distribution function (BRDF) at 3.8, 10.6, and 20.0 μm are presented. The BRDF results show a Lambertian character in fixed azimuthal planes and no specular peaks until the wavelength equals 20 μm and the incident angle is 80 degrees.

A new class of two-stage optical systems are proposed for the measurement of diffuse reflectance. The proposed systems consist of a single ellipsoidal or dual paraboloidal primary mirror and a nonimaging compound parabolic concentrator (CPC) as the secondary mirror. When operated in the direct mode, the CPC's larger aperture is occupied by the reflectometer's detector, the CPC's smaller aperture resides at the focus of the primary mirror, and the CPC half-angle is chosen so as to minimize the angular dependence of the detector's response. The resultant uniformity of the detector/CPC response over the hemisphere overcomes a major disadvantage of using ellipsoid and paraboloid-type reflectometers in the direct mode, at the expense of a slightly larger detector area. In the reciprocal mode, the CPC's larger aperture is occupied by a high temperature source, with the CPC's field of view chosen so as limit the variation in the source's directional emissivity to less than one percent. This source design offers less stray radiation than a cavity-type source with comparable Lambertian properties. The emissivity and wavelength dependence of the CPC source is determined by the high temperature surface. An analysis of the design tradeoffs for CPC/conic section reflectometers is given including ray tracing.

The infrared emittance of surfaces is of interest to various disciplines which include: thermal imaging, remote sensing, solar energy, insulation, radiative transfer in space, etc. A brief review is given of how the measurements are made. The emittance or reflectance of a variety of samples of interest to different applications are shown in the 2 to 20 micrometer (5000 to 500 wavenumber) spectral region.

Zone plate optical-spectral properties are described and key parameters are tabulated. Several arrangements are given for zone plate optical sensors, and the advantages of zone plate optical elements are listed. The axial detector (AX-D) is described as a new detector configuration that is planned to take maximum advantage of the optical-spectral properties of zone plates.

A high resolution, serial scan, thermal imager which is based upon a new compact video optical scanner and the SPRITE (Signal Processing In The Element) detector is described. The scanning principle is briefly reviewed, and the 8-12 μm SPRITE detector configuration characteristics are summarized. An electronic system is used to control the scan drives and to process the video for RS 170 compatibility. Finally, the performance of the system is defined by such figures of merit as NEAT and MRAT.

The design and optical performance of a reimaging telescope with a 25 degree FOV that was constructed with off-the-shelf optical components is described. The wide field-of-view dictated the use of a field lens to prevent vignetting at the erector lens. Packaging restrictions on telescope length forced the use of low f-number lenses. Constraints on the selection of the erecting lens and the effects on optical performance of lens shape and position of a single element field lens are presented. The optical performance is shown to be limited by existing eyepiece designs.

Basic principles of operation and design considerations are described for phased array optical systems intended to operate at visible or infrared wavelengths. General descriptions of array design considerations as determined by functional requirements are presented.

A new type of optical/infrared telescope is suggested, based on a combination of the principles of radio astronomical interferometric image synthesis and computational phase retrieval. Physical configurations, design principles, necessary technical developments, possible modes of operation and important possible uses of the telescope are discussed. The design philosophy is that it should not be necessary to maintain tighter mechanical tolerances than those required conventionally for radio telescopes. Because it is intended to realize milli-arc-second resolution, implying a telescope aperture of the order of 100 m, it is recognized that it is impractical to achieve optical tolerances, so that the adopted design principles should not rely on them. The design goals can be attained by relying on heterodyne detection followed by purely digital processing, thereby permitting useful signal-to-noise ratios to be conveniently obtained by multiplexing parallel channels with the aid of large-scale-integration techniques. Besides offering highly resolved images of many of the more important astrophysical objects, the telescope could be used for imaging certain types of space vehicles. The kind of telescope proposed herein could be usefully constructed either on the surface of the earth or outside the Earth's atmosphere.

Large optical apertures can be synthesized by utilizing a phased array of smaller elements. The phasing is realized by ensuring that the individual elements are made from the same lens or mirror and properly positioned. The synthesis of MTF for various geometrical arrays (Cross, Covington-Drane, Golay Six, Thin Annulus, etc.) of such partially filled apertures has been demonstrated experimentally with incoherent illumination. Post processing techniques were used to remove the deleterious effect in the image due to auto-correlation of the individual elements. The results demonstrated that the resolution of the equivalent full aperture is achievable by the MTF synthesis which arises from cross correlation of the various elements. The limitations due to misalignment of the various elements in six degrees of freedom were investigated theoretically, and experimentally verified for both refractive and reflective elements. The experimental verification of these tolerances required interferometric measurements in the focal plane. Systems Analysis showed that the aperture must be designed so that the synthesized MTF has a modulation higher than the detector noise threshold at all spatial frequencies (AIM curve). In general, the technique does not work unless the radiation is spatially incoherent. To illustrate this, experiments were performed with laser illumination. The aperture did not synthesize unless the laser was rendered incoherent by utilizing moving diffusers or scanning techniques. Synthesis with laser illumination was achieved in a non-partially filled mode by phasing with holographic arrays. Finally, some experimental results on super-resolving pupils are shown and discussed.

In a study being conducted for the NASA Marshall Space Flight Center Perkin-Elmer is evaluating several advanced optical space telescope concepts for achieving both higher sensitivity and higher angular resolution than the Hubble Space Telescope. These concepts include one-dimensional and two-dimensional coherent arrays of mirrors in both focal and afocal configurations. We will discuss some of the configurations that have been proposed for achieving a major advance in optical astronomy by means of arrays that might be launched and assembled in space about the year 2005. A fundamental trade-off concerns synthetic aperture concepts that provide very high angular resolution and designs that provide sensitivity for faint extended source imaging. We will review some of the advantages and disadvantages of several possible concepts with an emphasis on the technology development plans required to implement them.

The interest of a ground based slit aperture telescope (SAT) which operates with a long thin pupil has been underlined for applications in the field of high angular resolution observations by speckle-interferometric techniques. The SAT lies halfway between the Michelson stellar interferometer and the standard telescope with circular aperture. We present here the project of a space-SAT for astronomical image reconstruction. A full coverage of the two dimensional Fourier plane can be obtained by rotating the SAT around its optical axis. The method of image reconstruction is analogous to those used in computerized tomography. The interest of the use of a rotating space-SAT among synthetic apertures is its capability of giving a reconstructed image of the astronomical object simultaneously in several light wavelengths, each colored image beeing comparable with the others for fruitful astrophysical applications.

Synthetic aperture telescopes fall into two generic types; those consisting of a segmented primary mirror (perhaps substantially thinned or diluted) with a common secondary mirror, and those made up of an array of independent telescopes. Advantages and disadvantages of each type will be discussed. The diffraction-limited optical performance of several subaperture configurations (both redundant and non-redundant) will be presented and compared in terms of point spread function (PSF) characteristics and encircled energy plots. In practice, this ideal optical performance is degraded by various design, manufacturing, and operational errors. On-axis optical performance degradation due to rms phase errors, rms pointing errors, and rms focus errors between the subapertures making up the synthesized aperture will then be discussed. Coherent imaging with phased arrays of independent afocal telescopes essentially break down into two operations; 1) a pupil mapping operation in which the width-to-separation of the reduced beams entering the beam combining telescope must replicate that of the collecting subapertures, and 2) a Fourier Transform operation which forms the coherent image from these combined beams or subapertures. A quantitative analysis of the off-axis optical performance degradation due to pupil mapping errors will be presented, as will the field dependent effects of residual design aberrations of the independent telescopes. These inherent limitations will be discussed with respect to the field-of-view requirements of various applications of scenarios.

An algorithm for the accurate control of piston phase error and the equalization of path length differences of a multiline synthetic aperture transmitter is investigated. This algorithm uses the focal plane interference of two multiline rectangular beams. A parameter space is established which provides an unambiguous hill-climbing control system by generating a function of the piston phase error that increases monotonically and smoothly with pathelength error as the error decreases to zero. Both theoretical and experimental results are provided. The error incurred due to local subaperture tilt is also found.

The purpose of this work is to develop relationships between the normalized far-field peak intensity (Strehl ratio or SR) and system jitter in a phased array device. The results are valid for arbitrary shaped apertures as well as arbitrary array patterns. For identical apertures and uncorrelated jitter among the apertures, the SR is independent of the number of apertures and their arrangement. On the other hand for correlated jitter, the far field degradation is sensitive to the array pattern and is least sensitive for a close-packed array.

The Air Force Weapons Laboratory phased array experiment, PHASAR, uses a multiline argon laser to phase together the output of three beam expanding telescopes. A two beam interference technique is used to compare the optical path differences (OPD) between the three telescope's beam paths. Generally, one telescope is designated as a reference and the other two telescopes are phased to it. One limitation of the system is its inability to positively identify the zero OPD point when scanning over a large range of OPDs. Due to the argon laser's limited spectral content, there are OPDs other than zero OPD where the various wavelengths are nearly back in phase, resulting in positions which are essentially indistinguishable from zero OPD. A simple procedure for determining OPDs that will be difficult to discriminate from zero OPD is described. This procedure provides a convenient means for evaluating sources for use in OPD sensors based on the two beam interference technique.

After several years of technology development and concept evaluation, the "four-barrel" Multiple Mirror Telescope concept has been selected for the 15-M National New Technology Telescope. Performance goals are presented. The principal optical configurations are described with emphasis on trade-offs under current study. The method planned for aligning and phasing the optics is outlined.