Designs of null test optics for 8.4-m, ƒ/1.1 paraboloidal mirrors

Jim Burge

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

Optical Sciences Center and Steward ObservatoryUniversity of Arizona

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

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LBT

Hale

MMT, Magellan

Herschel

Primary mirror measurements are difficult because

of the large surface departure from spherical

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LBT 8.4-m

M M T 6.5-m

VA TT 1.8-m G em in i/VLT8-m

AF 3.5-m

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

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2 0 0 m m d ia m G ep lano -co n ve x len s 1 7 0 m m d ia m

Z nS e le n s

P a rax ia l fo cus(1 9 .2 m e te rs to

p rim ary m irro r)

T w y m an n -G reenin te rfe ro m e te r

Previous IR null lens for 6.5-m ƒ/1.25

CGH measurement of this null lens shows 0.02 rms error

Includes 0.007 rms low order spherical aberration

Null lens evolution

Paraxial focus

3.5-m ƒ/1.75

6.5-m ƒ/1.25

8.4-m ƒ/1.14

Offner-type refractive null

• Similar to previous designs• Design gives excellent correction• Limited by glass quality in large lens• Manufacture of large, fast convex surface is difficult

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390 mm diameter BK7 lens90 mm thickR/0.74 convex sphere Paraxial

focusinterferometerfocus

Gregorian version of Offner reflective null

• Uses mirrors to solve index problem• Gives excellent performance• Has been analyzed in detail using structure functions• Difficult opto-mechanical design

SPHERICAL PRIMARY750 mm diam

MANGIN SECONDARY50 mm diam

TWO FIELD LENSES90 mm diam

2 METER TOTAL LENGTH

TO PRIMARY MIRROR

COLLIMATED INTERFEROMETER

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CGH null lens

• 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

Reference CGH (reflects reference wavefront, transmits test wavefront)150 mm diam

IlluminationCGH(etched)

Point source/image

180 mm diam38 mm thick lens

200 mm diamplano substrates

1.1 meters

CGH creates reference wavefront

19 m to primary

Point source/image

Illumination CGH CGH to create reference wavefront

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

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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

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LBT CGH is easier than previous

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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

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