Dual subaperture stitching for large flat mirror testing.

The Ritchey-Common test is a widely used method for testing flat mirrors with a larger reference spherical mirror. However, with the increase in the size of the flat mirror, the fabrication of the spherical mirror becomes more time consuming and expensive. In this study, we developed a novel technique to test a large flat mirror with a smaller reference sphere using a dual subaperture stitching (DSS) method. One part of the DSS technique is a modified Ritchey-Common test, which uses a reference sphere smaller than the flat mirror. The other part involves scanning along the centerline of the flat mirror. The former can be used to determine the surface form error (SFE), except for rotationally symmetric components, such as power and spherical aberrations, which can be measured by the latter. To perform the stitching process using a smaller reference sphere, the flat mirror was repeatedly rotated by a fixed angular step. One of the advantages of the rotation of the flat mirror is that it can be used to identify the errors resulting from the reference sphere, which do not vary during the rotation of the flat mirror. We verified the DSS method using a 152 mm diameter optical flat mirror and determined the root-mean-square (rms) measurement error to be only 0.2 nm, which was comparable to the error of the full-aperture interferometric measurement. In addition, we tested a 1.2 m diameter flat mirror with a reference sphere with an aperture of 0.8 m and measured its SFE to be 11.9±0.5nm rms.

CVD SiC deformable mirror with monolithic cooling channels.

We propose a novel deformable mirror (DM) for adaptive optics in high power laser applications. The mirror is made of a Silicon carbide (SiC) faceplate, and cooling channels are embedded monolithically inside the faceplate with the chemical vapor desposition (CVD) method. The faceplate is 200 mm in diameter and 3 mm in thickness, and is actuated by 137 stack-type piezoelectric transducers arranged in a square grid. We also propose a new actuator influence function optimized for modelling our DM, which has a relatively stiffer faceplate and a higher coupling ratio compared with other DMs having thin faceplates. The cooling capability and optical performance of the DM are verified by simulations and actual experiments with a heat source. The DM is proved to operate at 1 kHz without the coolant flow and 100 Hz with the coolant flow, and the residual errors after compensation are less than 30 nm rms (root-mean-square). This paper presents the design, fabrication, and optical performance of the CVD SiC DM.

Adjustable bipod flexures for mounting mirrors in a space telescope.

A new mirror mounting technique applicable to the primary mirror in a space telescope is presented. This mounting technique replaces conventional bipod flexures with flexures having mechanical shims so that adjustments can be made to counter the effects of gravitational distortion of the mirror surface while being tested in the horizontal position. Astigmatic aberration due to the gravitational changes is effectively reduced by adjusting the shim thickness, and the relation between the astigmatism and the shim thickness is investigated. We tested the mirror interferometrically at the center of curvature using a null lens. Then we repeated the test after rotating the mirror about its optical axis by 180° in the horizontal setup, and searched for the minimum system error. With the proposed flexure mount, the gravitational stress at the adhesive coupling between the mirror and the mount is reduced by half that of a conventional bipod flexure for better mechanical safety under launch loads. Analytical results using finite element methods are compared with experimental results from the optical interferometer. Vibration tests verified the mechanical safety and optical stability, and qualified their use in space applications.

Three-shell-based lens barrel for the effective athermalization of an IR optical system.

We have developed a new IR optical system that consists of three mirrors and four lenses, and that operates in the temperature range 8°C-32°C. This temperature range can induce thermoelastic deformation in the lenses and their mounting subassembly, leading to a large defocus error associated with the displacement of the lenses inside the barrel. We suggest using a new three-shell-based athermalization structure composed of two materials with different coefficients of thermal expansion (Invar and aluminum). A finite element analysis and the experiment data were used to confirm that this new athermalization barrel had a defocus error sensitivity of 11.6 nm/°C; this is an improvement on the widely used conventional single-shell titanium barrel model, which has a defocus error sensitivity of 29.8 nm/°C. This paper provides the technical details of the new athermalization design, and its computational and experimental performance results.

Bipod flexure for 1-m primary mirror system.

We present an analytical formulation of the bipod flexure for mounting the 1-m primary mirror in a space telescope. Compliance and stiffness matrices of the bipod flexure are derived to estimate theoretical performance and to make initial design guidelines. We use finite element analysis to optimize the bipod design satisfying the application requirements. Experimental verification is achieved by vibration test with a dummy mirror system.

Fiber-diffraction interferometer for vibration desensitization.

We describe a new interferometer designed to obtain high immunity to vibration by generating the reference wave by means of fiber diffraction so as to share a common path with the measurement wave. A spatial phase shifter is added to secure vibration desensitization by capturing four phase-shifted interferograms simultaneously without time delay. Experimental results prove that the proposed interferometer is capable of providing stable measurements with a level of fringe stabilization of less than 1 nm in a typical workshop environment equipped with no excessive ground isolation for antivibration.

Nonparaxial free-space diffraction from oblique end faces of single-mode optical fibers.

We investigate free-space diffraction of light that emanates from obliquely cleaved end faces of single-mode fibers. Emphasis is placed on precise prediction of the wave-front sphericity of fiber-generating waves in the nonparaxial Fresnel propagation region spanning an entire hemispherical observation surface. Rayleigh-Sommerfeld scalar diffraction theory is used to produce an analytic closed-form solution with a nonparaxial approximation. The result allows the wave front's sphericity and the amplitude distribution of fiber-generating waves to be evaluated precisely with less computation than for existing numerical or infinite-series solutions.