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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
1700 1750 1800 1850 1900 1950 2000 2050year
0.01
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p-v
asph
ere
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LBT
Hale
MMT, Magellan
Herschel
Primary mirror measurements are difficult because
of the large surface departure from spherical
-5000 -4000 -3000 -2000 -1000 0 1000 2000 3000 4000 5000position on mirror(mm)
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artu
re f
rom
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her
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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
1 .3 m e te rs
5 0 m mZ nS ew /a sp he re
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
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
-.02
0.003 rms
2.1 meters
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
-.02
0.002 rms
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
0 20 40 60 80radial position in mm
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Null lens certification with CGH
• 58,000 rings• 200 mm diameter• Measures conic
constant to accuracy of <0.0001
-120 -80 -40 0 40 80 120radial position on hologram (m m)
0
20000
40000
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CG
H O
PD
in w
aves
LBT
M M T
LBT CGH is easier than previous
0 20 40 60 80 100 120radial position on hologram (mm)
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pac
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LBT
M M T
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