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

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
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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
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•
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•
•
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
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•
•
•
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
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•
•
•
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 -–
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–
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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