Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Designs of null test optics for 8.4-m, ƒ/1.1 paraboloidal mirrors Jim
Transcript
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 10000.00 LBT MMT, Magellan p-v asphere (µm) 1000.00 100.00 Hale 10.00 1.00 Herschel 0.10 0.01 1700 1750 1800 1850 1900 year 1950 2000 2050 Primary mirror measurements are difficult because of the large surface departure from spherical 1600 departure from sphere (µm) LBT 8.4-m 1200 MMT 6.5-m 800 VATT 1.8-m 400 AF 3.5-m Gemini/VLT 8-m 0 -5000 -4000 -3000 -2000 -1000 0 1000 2000 position on mirror(mm) 3000 4000 5000 IR null lens using diamond turned asphere 50 mm ZnSe w/asphere TwymannGreen interferometer • • • • • 200 mm diam Ge plano-convex lens Paraxial focus (19.2 meters to primary mirror) 170 mm diam ZnSe lens 1.3 meters 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 APERTURE IR INTERFEROMETER FOLD FLAT DIVERGER 50 mm diam 12 mm thick ZnSe bi-convex (one asphere) RELAY LENS 200 mm diam 28 mm thick Germanium plano-convex CGH measurement of this null lens shows 0.02 l rms error Includes 0.007 l rms low order spherical aberration FIELD LENS 80 mm diam 14 mm thick ZnSe plano-convex TO PRIMARY MIRROR 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 focus interferometer focus 2.1 meters • • • • 0.003 l rms Similar to previous designs Design gives excellent correction Limited by glass quality in large lens Manufacture of large, fast convex surface is difficult -.02 l Gregorian version of Offner reflective null COLLIMATED INTERFEROMETER SPHERICAL PRIMARY 750 mm diam 2 METER TOTAL LENGTH MANGIN SECONDARY 50 mm diam TWO FIELD LENSES 90 mm diam TO PRIMARY MIRROR 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 Illumination CGH (etched) Reference CGH (reflects reference wavefront, transmits test wavefront) 150 mm diam Point source/ image 200 mm diam plano substrates 180 mm diam 38 mm thick lens 1.1 meters • Uses 2 CGH’s – 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 20 18 ring spacing in µm 16 14 12 10 8 6 4 2 0 0 20 40 radial position in mm 60 80 • 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 60000 LBT CGH OPD in waves • 58,000 rings • 200 mm diameter • Measures conic constant to accuracy of <0.0001 40000 MMT 20000 0 -120 -80 -40 0 40 radial position on hologram (mm) 80 120 LBT CGH is easier than previous 10 CGH for 8.4-m f/1.1 at 632.8 nm has larger features than previous, successful CGH for 6.5-m f/1.25 at 530.7 nm ring spacing (µm) 8 6 4 2 LBT MMT 0 0 20 40 60 80 radial position on hologram (mm) 100 120 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
