Optical Sciences Center and Steward Observatory University of Arizona Designs of null test optics for 8.4-m, ƒ/1.1 paraboloidal mirrors Jim Burge Optical Sciences Center and Steward Observatory University of Arizona Several null lenses are considered for measuring the primary mirrors for UA’s Large Binocular Telescope Infrared null lens using a diamond-turned asphere Giant refractive Offner-type null lens Gregorian variation of Offner reflective design Null lens using a binary computer-generated hologram

U of A is making the world’s steepest large primary mirrors

Primary mirror measurements are difficult because of the large surface departure from spherical

IR null lens using diamond turned asphere Uses 10.6 µm light from CO2 laser Similar to successful design used for 6.5-m mirrors, re-uses Ge lens DT asphere gives perfect wavefront and excellent imaging Ultimate accuracy is less important for IR than visible Calibrate with Computer Generated Hologram to 0.1 µm rms

Previous IR null lens for 6.5-m ƒ/1.25 CGH measurement of this null lens shows 0.02 l rms error Includes 0.007 l rms low order spherical aberration

Null lens evolution 8.4-m ƒ/1.14 6.5-m ƒ/1.25 3.5-m ƒ/1.75 Paraxial focus

Offner-type refractive null 390 mm diameter BK7 lens 90 mm thick R/0.74 convex sphere Paraxial interferometer focus focus 2.1 meters Similar to previous designs Design gives excellent correction Limited by glass quality in large lens Manufacture of large, fast convex surface is difficult 0.003 l rms -.02 l

Gregorian version of Offner reflective null 0.002 l rms Uses mirrors to solve index problem Gives excellent performance Has been analyzed in detail using structure functions Difficult opto-mechanical design -.02 l

CGH null lens Uses 2 CGH’s Compact design, Illumination CGH controls slope for both reference and test wavefronts Reference CGH creates reference wavefront Compact design, Can phase shift by pushing reference CGH with PZTs Needs more careful study

CGH creates reference wavefront Illumination CGH CGH to create reference wavefront 19 m to primary Point source/image Reference beam -1 order Littrow diffraction Test Beam 0 order twice through CGH

CGH design Requires ~12,000 rings, each accurately placed This CGH is easily within modern fabrication capabilities CGH fabrication errors will contribute 3 nm rms to surface error

Candidate null corrector designs for 8.4-m ƒ/1.14 primary mirrors

Null lens certification with CGH 58,000 rings 200 mm diameter Measures conic constant to accuracy of <0.0001

LBT CGH is easier than previous

CGH fabrication verified f/1.14 holograms were manufactured by group from Russian Academy of Sciences Wavefronts were measured interferometrically Figure accuracy of 5 nm rms is typical

Conclusion The jury is still out on the type of null corrector for LBT A different important issue still needs to decided -- Holographic certification has been extremely successful The holograms are intrinsically more accurate than the null correctors What about aligning the null corrector based on the hologram? This would save a lot of money and time We could use a second, independently made hologram as verification