1. Optical Sciences Center and Steward Observatory University of Arizona
Designs of null test optics for 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

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

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

4. 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

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

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

7. 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

8. 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

9. 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

10. 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

11. 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

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

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

14. LBT CGH is easier than previous

15. 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

16. 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