The design and manufacturing of high efficiency and reliable volume phase holographic optical elements require photosensitive material where it is possible to finely control the refractive index modulation. Bayfol HX photopolymers show this feature together with other interesting advantages, in particular the self-developing and the large refractive index modulation. In this paper, the design of Volume Phase Holographic Gratings (VPHGs) is reported underlying the relationship of gratings’ performances with the refractive index modulation. The trend of this property with the change of the laser power density and the ratio of the two writing beams is shown. Based on these results, VPHGs for astronomical instrumentation have been designed and manufactured.

Photopolymeric films with highly tunable refractive index modulation for high precision diffractive optics

Field-dependent aberrations for misaligned reflective optical systems

Equipment and techniques were developed at the University of Arizona for fabricating large computergenerated holograms (CGH’s) for measuring aspheric telescope mirrors. A large laser writing machine was built to fabricate binary zone plates onto spherical surfaces up to 1.8 meters in diameter and with focal ratios as fast as ƒ/1. This machine writes 6-μm to 150-μm features with radial position accuracy better than 1 μm rms over the full diameter. The problems of applying and processing photoresist are avoided by writing the patterns using a simple thermochemical technique. Numerous holograms up to 1.2 m across have been successfully written and tested. Convex secondary mirrors for University of Arizona telescopes are interferometrically measured using concave spherical test plates with circular CGH’s fabricated onto the curved surfaces. Diffraction from the holograms compensates for the aspheric departure of the secondary mirrors. The accuracy of the measurements relies on the ability to fabricate binary circular patterns with thousands of rings to micron accuracy. The development of the equipment and techniques for making these patterns was motivated by the requirement to measure these optical surfaces. Circular patterns are optimally fabricated using polar coordinate machines that expose rings by rotating the substrate under a fixed writing beam. The hologram accuracy depends on the quality of the rotation bearing, the ability to control the radial position of the writing beam, and the ability to locate the center of rotation. This geometry has been used by several groups for writing accurate zone plates onto small, flat substrates. Also, diffractive optics for infrared applications have been turned using precision lathes with diamond tools.

Fabrication of large circular diffractive optics

We investigate the possibility to produce photochromic CGHs with maskless lithography methods. For this
purpose, optical properties and requirements of photochromic materials will be shown. A diarylethene-based
polyurethane is developed and characterized. The resolution limit and the in
uence of the writing parameters
on the produced patterns, namely speed rate and light power, have been determined. After the optimization of
the writing process, gratings and Fresnel Zone Plates are produced on the photochromic layer and diraction
eciencies are measured. Improvements and perspectives will be discussed.

Characterization of photochromic computer-generated holograms for optical testing

The advantages and disadvantages of the configurations for high resolution solar telescopes are discussed within two broad groups: those with steerable mountings and those with fixed mountings. We then consider simple optical tests, stabilization of the internal optical path, windows, vibration, guiding and alignment systems, improving the observations, and solutions for large-aperture telescopes for Stokes polarimetry observations. This review does not address all the problems. It is not a compendium of solar telescopes, nor does it include any discussion of focal-plane instrumentation.

High resolution solar telescopes

The misalignment of the secondary and primary mirrors, in a full aperture Cassegrain test, introduces coma and astigmatism into the wavefront. Equations were derived which relate the magnitude and orientations of these two aberrations to the tilt and decenter of one mirror with respect to the other when tested in an autocollimating mode. By determining the coma and astigmatism, introduced into the wavefront, using Zernike circle polynomial fitting techniques, the tilt and decenter correction can be computed which would optically align the secondary to primary. These alignment equations were verified by using a Space Telescope optical' formula, misaligning the secondary mirror, and calculating the resulting coma and astigmatism in the wavefront. The tilt and decenter, necessary to realign the two mirrors of the Space Telescope were calculated from the derived equations, and found to agree with the perturbed secondary mirror misalignment. An autocollimation test procedure will be described for obtaining the full aperture wavefront of a Cassegrain-type system, both on-axis and in the field.

Alignment Of A Full Aperture System Test Of A Cassegrain Telescope

A method of interferometrically measuring large convex aspheres using test plates with computer generated holograms was developed at the University of Arizona. We present the results from a set of experiments that demonstrate the accuracy, flexibility, and the simplicity of performing the holographic test. A low-cost stand-alone setup as built for implementing this test on a 38-cm convex hyperboloid. A direct comparison of the CGH measurement with results from a classical Hindle test shows excellent agreement. We also demonstrate the unique attribute of this test to measure bare glass surfaces and highly reflective surfaces without making any modifications to the test equipment.

/pdf/demonstration-of-accuracy-and-flexibility-of-using-cgh-test-1ad4s7azv4.pdf

Demonstration of accuracy and flexibility of using CGH test plates for measuring aspheric surfaces

Optical metrology was developed to sense the wavefront of a mosaic optical system to correctly position and bend an active optical element. The optical system being examined is a telescope, with a segmented primary mirror, tested in auto-collimation. Located in the focal plane are two interferometers, a simultaneous phase shifting interferometer which measures the system wavefront and a lateral shearing interferometer which measures piston. The resultant wavefront and piston data are passed to a master computer which computes the forces and vectors necessary to position and bend the active element. This paper describes these interferometers, their evolution, requirements, calibration, and data evaluation techniques employed. Results from actual system testing are presented.

Optical metrology used to sense and control a segmented optical system

Localized wavefront performance analysis (LWPA) is a system that allows the full utilization of the system optical transfer function (OTF) for the specification and acceptance of hybrid imaging systems. We show that LWPA dictates the correction of wavefront errors with the greatest impact on critical imaging spatial frequencies. This is accomplished by the generation of an imaging performance map-analogous to a map of the optic pupil error-using a local OTF. The resulting performance map a function of transfer function spatial frequency is directly relatable to the primary viewing condition of the end-user. In addition to optimizing quality for the viewer it will be seen that the system has the potential for an improved matching of the optical and electronic bandpass of the imager and for the development of more realistic acceptance specifications. 1. LOCAL WAVEFRONT PERFORMANCE ANALYSIS The LWPA system generates a local optical quality factor (LOQF) in the form of a map analogous to that used for the presentation and evaluation of wavefront errors. In conjunction with the local phase transfer function (LPTF) it can be used for maximally efficient specification and correction of imaging system pupil errors. The LOQF and LPTF are respectively equivalent to the global modulation transfer function (MTF) and phase transfer function (PTF) parts of the OTF. The LPTF is related to difference of the average of the errors in separated regions of the pupil. Figure

Use of localized performance-based functions for the specification and correction of hybrid imaging systems

An approach, which utilizes Modulation Transfer Function (MTF), is used to develop tolerances for manufacture, assembly, and use of an optical system, and to predict the overall system MTF. This is accomplished by determining the MTF sensitivities for radius of curvature, thickness, and index of refraction for each optical surface. Each of these optical elements can then be toleranced for manufacturing and assembly based upon MTF sensitivity. In addition, the MTF effect due to tilt and decenter of each optical component can also be determined; this provides a bases for system alignment and focus requirements. The manufacturing effect for each optical component is converted to an MTF equivalent. The overall system MTF is then predicted by cascading the design MTF, focus MTF, alignment MTF, and manufacturing MTF.© (1981) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Michael J. Fehniger

Papers

Alignment Of A Full Aperture System Test Of A Cassegrain Telescope

Demonstration of accuracy and flexibility of using CGH test plates for measuring aspheric surfaces

Optical metrology used to sense and control a segmented optical system

Use of localized performance-based functions for the specification and correction of hybrid imaging systems

Optical criteria for the prediction of optical system modulation transfer function (MTF)