Figuring large off-axis segments to the diffraction limit Hubert Martin Steward Observatory University of Arizona

Objectives Use established polishing process to make an off-axis segment of a 5.3 m f/0.7 parent paraboloid. Demonstrate and refine techniques for manufacturing off-axis aspheres. Provide a test-bed for new measurement techniques for off-axis aspheres.

Large optics experience Steward Observatory Mirror Lab has pioneered methods of making the world’s largest and most aspheric astronomical optics, including two 8.4 m f/1.14 primary mirrors for the Large Binocular Telescope.

Drive to shorter focal lengths A major thrust of the UA program is making mirrors with short focal lengths, allowing short, stiff telescope structures and small, economical enclosures. Short focal length is even more important for HEL projectors that must point and track rapidly. UA mirrors are significantly faster—f/1.1 vs f/1.8—than any other mirrors for large telescopes. They are 4 times more aspheric than other mirrors of the same size. Asphericity is the biggest impediment to making faster telescopes. –In polishing, it is difficult to keep the tool (lap) in intimate contact with the optical surface whose curvature changes with position. –In testing, it is difficult to produce an accurate template wavefront.

Overcoming limits imposed by asphericity A unique polishing system is built around a large, stiff tool that changes shape actively to match the curvature variations of the aspheric mirror. 1.2 m diameter stressed lap polishing a 6.5 m f/1.25 primary mirror. The lap consists of an aluminum plate and 18 actuators that apply bending moments to the plate. The plate is stiff enough to provide strong passive smoothing on small scales, but bends actively to follow the mirror’s curvature variations.

1.7 m off-axis segment We will use stressed-lap polishing system to make off- axis segment of 5.3 m f/0.7 parent paraboloid. Segment is 1.7 m diameter x 100 mm thick Zerodur. Aspheric departure is 2.7 mm p-v, primarily astigmatism.

Polishing system for 1.7 m mirror Computer-controlled polishing machine includes turntable, horizontal slide, 30 cm diameter stressed lap, and swing-arm profilometer accurate to 50 nm rms. Stressed lap is shown polishing a 0.9 m x 1.5 mm thick shell for a deformable secondary mirror. The lap comprises a 13 mm thick Al plate,12 actuators to control shape, and 3 actuators to control pressure and pressure gradients.

Asphericity of off-axis segment Demonstration segment is 8 times more aspheric than the most aspheric astronomical mirror in its size class. Because departure is low order, lap bending is modest, less than for astronomical mirrors. Aspheric departure of segment (mm) 2.7 mm peak-valley Bending of 30 cm stressed lap (microns) 150 microns peak-valley

1.7 m mirror and support Mirror is supported by 36 actuators, active in the telescope and passive for manufacture.

Generating the aspheric surface Mirror has been generated by Kodak. Asymmetric aspheric surface was generated by 3-D control of tool position. This operation produces the 2.7 mm p-v aspheric surface to accuracy of 40 microns rms. UA Mirror Lab has same capability, but machine is currently committed to larger mirrors.

Metrology for HEL optics using Computer Generated Holograms Jim Burge Optical Sciences Center University of Arizona

Metrology issues for HEL optics Manufacture (polishing) depends on accurate surface measurement. The optical test must constrain additional degrees of freedom for off-axis segments. The measurements must readily allow the segments to match. Complex beam trains must be accurately aligned.

Technologies being developed CGH test for segmented mirrors that uses a test plate as a reference, with CGH for aspheric correction Use of multiplexed patterns to provide simultaneous wavefront and alignment information Use of multiplexed patterns to provide reference for alignment of optical systems

What are Computer Generated Holograms? The CGH is a pattern written onto an optical surface. Diffraction from the pattern modifies the amplitude and phase of the light. Optimal patterns are designed in a computer to create desired effects. These patterns are written using methods and equipment developed for integrated circuits. UA has depth of experience applying CGH technology to technical systems. Spot diagram showing multiple diffraction orders generated by the binary hologram. Binary CGH with 1 wave Zernike spherical and 36 waves of tilt.

Testing segmented mirrors For most optics, the power in the surface does not need to be controlled accurately because a focus adjustment in the system will be made This is not possible for segments of a mirror, where the pieces must fit together. See what an interferogram of a mirror would look like if the segment ROC was not matched

CGH with test plate Uses convex reference surface with CGH to correct aspheric departure. Different mirror segments measured with the same test set would have the same radius of curvature.

Demonstration of test plate with CGH 30 cm diameter test mirror, 4-m ROC Measured using 30 cm fused silica test plate Phase shift interferometry applied by pushing mirror with PZTs 20 mm CGH Excellent visibility interferogram No special problems with setup, alignment, or spurious orders of diffraction

Results of CGH demonstration Conventional measurement rms Difference rms Measurement using Test plate with CGH rms The difference of rms is made up from rms low order, expected from alignment tolerances rms, looks like coherence noise

Use of CGH for alignment CGHs can include patterns for aligning the CGH to the incident wavefront. Using multiple patterns outside the clear aperture, many degrees of freedom can be constrained using the CGH reference.

CGH projection of alignment marks The basic idea – multiplex numerous holograms on a single substrate to provide both wavefront and alignment information. For alignment, the CGH projects bright crosshair patterns

CGH for testing off axis parabola A single substrate provides: - reference for interferometer - null lens for aspheric surface - creates 5 reference marks, 4 around edge, 1 on optical axis

CGH alignment of a 24-in off axis parabola CGH null lens incorporates alignment marks. Easily align axis to 0.5 mm by eye. /20 rms measured surface interferogram Phase map 15 m ROC, 1.5 m off axis

Projection of fiducial marks The positions of the crosshairs can be controlled to micron accuracy. The patterns are well defined and can be found using a CCD. Measured pattern at 15 m from CGH. Central lobe is only 100 µm FWHM.

Two system designs are now in production Paraboloidal mirror, showing the full f/0.9 parent 250 mm clear aperture for the off axis part Paraboloidal mirror, showing the full f/0.5 parent The off axis part is 200 mm diameter CGH Interferometer Off axis portions of steep aspheric surfaces CGHs have features designed into them that provide alignment of CGH to interferometer and CGH to aspheric surface These tests will be operational in early November 2003.

Optical test for 1.7-m off axis mirror f/0.7 parent asphere Interfer- ometer Tilted spherical mirror CGH 100 mm diameter 1.7-m off axis aperture Currently developing detailed alignment plan, including the use of CGHs to provide the references.