Brian E. Catanzaro

Comparison of full-aperture interferometry to sub-aperture stitched interferometry for a large diameter fast mirror

The Herschel Space Observatory (formerly known as FIRST) consists of a 3.5 m space telescope. Stitching sub aperture interferograms may offer considerable cost savings during testing of the flight telescope as compared to other techniques. A comparative demonstration is presented of interferogram stitching techniques that enable a composite map of a 3-D surface to be assembled from a sequence of sub-aperture measurements. This paper describes the fundamental procedures for stitching together component data sets and demonstrates such techniques with real data sets. A set of 14 sub-aperture measurements was made of a 2 m diameter all-composite mirror developed as part of the Herschel Space Observatory program and two different stitching software packages were employed to stitch together the sub-aperture surface maps. The software packages differ fundamentally in the way the sub-aperture maps are three-dimensionally stitched, one employing a local technique and the other using a global technique. The processed results from both algorithms are compared with each other and with a full-aperture reference measurement made of the same test optic. A summary of the results is presented and potential modifications and enhancements to the stitching techniques are discussed.

Optical metrology for testing an all-composite 2-meter diameter mirror

The Herschel Space Observatory (formerly known as FIRST) consists of a 3.5 m space telescope designed for use in the long IR and sub-millimeter wavebands. To demonstrate the viability of a carbon fiber composite telescope for this application, Composite Optics Incorporated (COI) manufactured a fast (F/1), large (2 m), lightweight (10.1 kg/m2) demonstration mirror. A key challenge in demonstrating the performance of this novel mirror was to characterize the surface accuracy at cryogenic (70 K) temperatures. A wide variety of optical metrology techniques were investigated and a brief survey of empirical test results and limitations of the various techniques will be presented in this paper. Two complementary infrared (IR) techniques operating at a wavelength of 10.6 microns were chosen for further development: (1) IR Twyman-Green Phase Shifting Interferometry (IR PSI) and (2) IR Shack-Hartmann (IR SH) Wavefront Sensing. Innovative design modifications made to an existing IR PSI to achieve high-resolution, scannable, infrared measurements of the composite mirror are described. The modified interferometer was capable of measuring surface gradients larger than 350 microradians. The design and results of measurements made with a custom-built IR SH Wavefront Sensor operating at 10.6 microns are also presented. A compact experimental setup permitting simultaneous operation of both the IR PSI and IR SH tools is shown. The advantages and the limitations of the two key IR metrology tools are discussed.

Photorefractive holographic lenses and applications for dynamic focusing and dynamic image shifting

Wavelength-multiplexed reflective holographic lenses were recorded in a 1-mm-thick Fe-doped LiNbO3 crystal. A simple theoretical model based on Kogelnik’s coupled-wave theory and Kuktarev’s photorefractive-material equations was used to characterize the photorefractive lenses in terms of Bragg-limited field of view, optimum space-bandwidth-product, and wavelength selectivity. Experimental measurement verified the theoretical calculations. An f/3 lens was recorded, and nearly aperture-limited diffraction performance was observed. Applications of wavelength-multiplexed photorefractive lenses were shown by demonstration of dynamic focusing and lateral image shifting.

Cryogenic (70K) measurement of an all-composite 2-meter diameter mirror

The Herschel Space Observatory (formerly known as FIRST) consists of a 3.5 m space telescope. As part of a JPL- funded effort to develop lightweight telescope technology suitable for this mission, COI designed and fabricated a spherical, F/1, 2 m aperture prototype primary mirror using solely carbon fiber reinforced polymer (CFRP) materials. To assess the performance of this technology, optical metrology of the mirror surface was performed from ambient to an intended operational temperature for IR-telescopes of 70K. Testing was performed horizontally in a cryogenic vacuum chamber at Arnold Engineering Development Center (AEDC), Tennessee. The test incorporated a custom thermal shroud, a characterization and monitoring of the dynamic environment, and a stress free mirror mount. An IR-wavelength phase shifting interferometer (IR PSI) was the primary instrument used to measure the mirror surface. From an initial surface figure of 2.1 microns RMS at ambient, a modest 3.9 microns of additional RMS surface error was induced at 70K. The thermally induced error was dominated by low-order deformations, of the type that could easily be corrected with secondary or tertiary optics. In addition to exceptional thermal stability, the mirror exhibited no significant change in the figure upon returning to room temperature.

Design considerations and algorithms for partitioning optoelectronic multichip modules

There is considerable interest in the development of optical interconnects for multichip modules (MCMs) to improve their performance. For effective utilization of the optical and electronic technologies, a methodology for partitioning the system is required. The key question to be answered is which technology should be used for each interconnect in a given netlist: optical or electronic. We introduce the computer-aided design approach for partitioning optoelectronic systems into optoelectronic MCMs. We first discuss the design trade-off issues in an optoelectronic system design, including speed, power dissipation, area, and diffraction limits for free-space optics. We then define a formulation for optoelectronic MCM partitioning and describe new algorithms for optimizing this partitioning based on the minimization of the power dissipation. The models for the algorithms are discussed in detail, and an example of a multistage interconnect network is given. Different results, with the number and size of chips being variable, are presented in which improvement for the system packaging has been observed when the partitioning algorithms are applied.

Architecture of an integrated computer-aided design system for optoelectronics

To design and package optoelectronic (OE) systems more effectively, computer-aided design (CAD) systems are needed in the engineering environment. The use of CAD systems for designing electronic computing systems is well accepted by the electronic community. CAD systems for engineering optical design are also widely used in the optics community. However, design automation of OE engineering requires more than the existing tools for system component design. Also required is research on OE design methodology and a complete set of tools. We explore a new area that will be important to OE system design: OE system CAD. We present an integrated CAD system for free-space optical interconnected optoelectronics that leverages on existing CAD technologies. The details of the system and the guidelines for developing compatible tools are described. Case studies of two OE CAD tools developed using these guidelines are also presented.

The ESA Herschel Telescope Tiger Team metrology review: modeling

ESA commissioned a Tiger Team to review the discrepancy between the prediction and measurement of the telescope back focal length. A team of 16 engineers and scientists collocated at ESAs Estec facility to review the finite element models, optical models, and supporting data to validate the methodology of prediction and verify the results. The team used several modeling techniques including: paraxial models, first order thermal expansion models, full system and metrology raytracing, deterministic and stochastic finite element models. The techniques, assumptions, and results will be discussed.

The effects of aberrations (low order and quilting) on the performance of the all-composite design for the Herschel Space Observatory

The effects of specific aberrations on the optical performance of the all-composite design for the Herschel Space Observatory are examined. A review of the all-composite design for the large aperture (3.5 m) telescope that satisfies the target specifications is presented. Cyrogenic experiments with a carbon fiber reinforced polymer (CFRP) 2 m demonstration mirror have yielded empirical bounds on the high- and low-order spatial frequency aberrations that will be anticipated in the full 3.5 m Ritchey-Chretien telescope design. Detailed analysis is presented on the effect of the low order aberrations of the primary mirror on the system wavefront error and encircled energy. Predictable limits of correction via low order shaping of the secondary mirror are described. The impact of higher order surface errors on the encircled energy and the stray light will also be presented. Comments are made regarding the impact of the optical prescription and CRFP design on flight telescope testing.

The ESA Herschel Telescope Tiger Team Metrology Review: Test Results

ESA commissioned a Tiger Team to review the discrepancy between the prediction and measurement of the telescope back focal length. A team of 16 engineers and scientists collocated at ESAs Estec facility to review the test results in the context of the mission requirements and predictions for behavior of the telescope. Extensive analysis was performed on the random and systematic errors in the test results. Both room temperature and cryogenic test data was scrutinized. Error budgets, test results, and conclusions from the Tiger Team will be discussed.

Herschel Space Telescope: Optical test and model correlation

Subsequent to the cyrogenic optical testing of the 3.5 m silicon carbide ESA Herschel Space Telescope, a discrepancy between the prediction and measurement of the telescope back focal length was identified. Efforts to resolve this discrepancy involved both improvements to modeling and more thorough examination of the optical test techniques. The most significant input to modeling was the use of extremely precise cryogenic strain measurements. The optical test methods were extensively analyzed for accuracy, sensitivity, and systematic errors. Model results, error budgets, test results, and conclusions will be presented and discussed.