The James Webb Space Telescope (JWST) Optical Telescope Element (OTE) consists of a 6.6 meter clear aperture, all-reflective, three-mirror anastigmat. The 18-segment primary mirror (PM) presents unique and challenging assembly, integration, alignment and testing requirements. A full aperture center of curvature optical test is performed in cryogenic vacuum conditions at the integrated observatory level to verify PM performance requirements. Two wavefront calibration tests are utilized to verify the low and mid/high spatial frequency performance of the test system. In this paper the methods and results of the wavefront calibration tests are presented.
Three auto-collimating flats (ACFs) of 1.5 meter clear aperture are being manufactured for use in the JSC Cryo-Optical
Metrology test of the James Webb Space Telescope. In-process interferometric testing of the ACFs is used to guide their
surface-figure processing. The surface measurement is performed in a vacuum chamber at both room (+20 °C) and
cryogenic (-240 °C) temperatures. With a 12-inch beam diameter FizCam interferometer, sub-aperture measurements are
taken across the ACF diameter at multiple rotations. These measurements are stitched together to compute the surface
figure. The figure change between room-temperature and cryogenic temperature is measured and used to enable cryo-figuring
based on room-temperature measurements. The data analysis is calibrated to account for gravity sag on test-set
optics and surface aberrations caused by vacuum pressure and temperature gradients on vacuum-chamber windows. The
first ACF is complete and meets specification with surface error of less than 75 nm RMS.
The James Webb Space Telescope (JWST) integration includes a center of curvature test on its 18 primary mirror
segment assemblies (PMSAs). This important test is the only ground test that will demonstrate the ability to align all 18
PMSAs. Using a multi-wavelength interferometer (MWIF) integrated to the test bed telescope (TBT), a one-sixth scale
model of the JWST, we verify our ability to align and phase the 18 PMSAs. In this paper we will discuss data analysis
and test results when using the MWIF to align the segments of the TBT in preparation for alignment of the JWST.
The Optical Telescope Element (OTE) consists of a 6.6 m, all-reflective, three-mirror anastigmat. The 18-segment
primary mirror (PM) presents unique and challenging assembly, integration and alignment verification requirements.
Each mirror segment is mechanically integrated with the Primary Mirror Backplane Support Structure (PMBSS) using
compound angle shims to compensate for global alignment and local co-planarity errors. The processes used to
determine the mechanical shim prescription, primary mirror alignment and integration, and placement verification are
discussed. In an effort to reduce process uncertainty and program risk, the JWST program recently conducted a PMSA
Integration Demonstration at ITT. Through this activity, full scale demonstrations of the Ground Support Equipment
(GSE) and critical integration processes were successfully completed. The results of these demonstrations indicate that
the equipment, processes, and procedures developed by ITT meet the critical requirements for PMSA placement.
A large aperture dynamic wavefront sensor (WFS) was tested and qualified for use against its design requirements. The WFS was designed to measure the relative slope of dynamic wavefronts; therefore, the test system created dynamic wavefronts, moving at 35 Hz to 315 Hz, with slopes on the order of 50 nanoradians (nR). The essential test system was an f/2.3 parabolic mirror with a laser source at the focal point, offset laterally by a fold mirror. The reflected light was nominally collimated and incident on the WFS at zero degrees. The source hardware was mounted on two crossed-translation stages that could drive a 540 μm, 1/2 Hz trapezoidal motion, inducing tilt in the collimated beam. This 100 microradians (μR) wavefront modulation calibrated the WFS. The fold mirror was mounted on a PZT, which oscillated the fold mirror from 35 Hz to 315 Hz, at tilt angles near 10 μR. This tilt moved the virtual source point, inducing wavefront tilts in the collimated output beam on the order of 100 nR. These fast, very small wavefront tilts were used to test the WFS performance. The test system, procedure, and calibration procedures are described.
Progressive addition lenses (PALs) are vision correction lenses with a continuous change in power, used to treat the physical condition presbyopia. These lenses are currently fabricated using non-rotationally symmetric surfaces to achieve the focal power transition and aberration control. In this research, we consider the use of Gradient-Index (GRIN) designs for providing both power progression and aberration control. The use of B-Spline curves for GRIN representation is explained. Design methods and simulation results for GRIN PALs are presented. Possible uses for the design methods with other lenses, such as unifocal lenses and axicons, are also discussed.
One approach for obtaining a surface representation is to fit Zernike polynomials (in a least squares sense) to discrete data points in the full aperture. The mathematics for this have been described using both matrix and vector notation. Additionally, vector notation has been used to describe how to obtain a surface representation from orthogonal (x, y) slope data. The result of that paper was a matrix operator for linearly combining the first eight Zernike polynomial coefficients fit to x- and y-slope data to produce a Zernike polynomial surface representation. This paper extends that process by presenting a systematic approach for obtaining the linear relationship between slope and surface using the first 49 Zernike polynomials.
Ronchi interferometry is an optical testing technique similar to shearing interferometry. A coherent wavefront is interfered with a sheared form of itself by placing a periodic grating at, or near, the focus of an optical system. The resultant interference pattern contains information about the wavefront's slope in a direction perpendicular to the grating structure. The wavefront can be reconstructed from two orthogonal slope data sets via the process of sampling, ordering, and fitting. This paper develops a linear-algebra vector notation model of the interferogram sampling and fitting process.
Interferometry is an optical testing technique based on the interference of light. Fringes are formed when the optical path difference (OPD) between a reference beam and an object beam is an integral multiple of the illuminating wavelength. This OPD is extracted through the process of sampling, ordering, and interpolating. This paper develops a linear-algebra vector notation model of the interferogram sampling and interpolation process.
Knowledge of the scatter characteristics of candidate infrared sensor dome materials is necessary for the evaluation of
image quality and susceptibility to bright off-axis sources. For polycrystalline materials in particular, the scattering levels
are high enough to warrant concern. To evaluate the effects of scatter on image quality, estimates of the window Point
Spread Function (PSF), or its transform, the Optical Transfer Function (OTF) are required. Additionally, estimates of the
material scatter cross-section per unit volume are essential for determining flare susceptibility. Experimental procedures and
models in use at JHU/APL allow the determination of each.
Measurement results are provided for samples of A1203 (ordinary ray), Y203, LaO3-doped Y203, MgAL2O4, and ALON.
Applications of these results are illustrated for planar windows having arbitrary orientations with respect to the optical axis.