Download A spherical mirror - College of Optical Sciences

Testing an off-axis parabolic mirror
with a CGH and a spherical mirror as
null lens
Chunyu Zhao
Rene Zehnder
Jim Burge
Buddy Martin
College of Optical Sciences, University of Arizona
College of Optical Sciences
The University of Arizona
Outline
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The mirror to be tested
Testing system design and assembly
Optical alignment
Initial testing result
Summary
College of Optical Sciences
The University of Arizona
The mirror
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Off-axis parabolic
mirror with 1.6m
diameter of clear
aperture
Parent: f/0.7
parabola with 7.7m
ROC.
Offset from the
parent vertex: 1.84m
College of Optical Sciences
The University of Arizona
Surface profile
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College of Optical Sciences
The University of Arizona
P-V: 2.767mm
RMS: 508um
RMS asti: 497um
RMS coma: 108um
RMS trifoil: 9um
RMS spherical: 5.8um
Residue: 2.5um
Surface quality spec
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Lower bending mode can be subtracted by
the following amount:
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Astigmatism: 200nm
Coma: 17nm
Trefoil: 50nm
Quatrefoil: 20nm
Spherical: 25nm
Residual:
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40nm rms
College of Optical Sciences
The University of Arizona
Requirement for testing system
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Amount of lower order mode:
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Astigmatism: 170nm
Coma: 15nm
Trefoil: 42nm
Quatrefoil: 17nm
Spherical: 20nm
Residual:
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20nm rms
College of Optical Sciences
The University of Arizona
System Configuration
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A spherical mirror
removes most of
astigmatism and
some coma –
residual 0
astigmatism 22um
and coma 46um
A CGH removes rest
of the aberrations.
College of Optical Sciences
The University of Arizona
Lens + CGH + Spherical Mirror
College of Optical Sciences
The University of Arizona
Error Budget
in
n
m
CGH
dz
tilt x
Spherical mirror
tilt y
7@
Edge
dz
tilt x
ROC
Therma
l
tilt y
Actuator
RSS
7
7@
Edge
7@
Edge
20
1deg C
Amount
Correctable
By 15 N
Forces
Needed
(N)
um
7
7@
Edge
Z5
0.2
0.0
-5.6
0.0
0.0
-37.6
0.0
0.0
38.1
346.0
1.6
Z6
133.9
-7.0
1.6
93.0
-149.5
3.1
63.0
206.5
309
346.0
13.4
Z9
-16.1
1.7
-0.3
-4.3
19.1
-0.5
-3.0
-12.2
28.4
87.0
4.9
Z10
0.0
0.0
-1.8
0.0
0.0
-15.0
0.0
0.0
15.1
87.0
2.6
Z11
5.9
1.2
0.0
1.9
-4.2
0.0
1.2
4.7
9.0
43.0
3.1
Z14
1.1
3.1
0.0
0.0
-0.6
0.0
-0.1
0.2
3.3
35.0
1.4
Z15
0.0
0.0
2.8
0.0
0.0
-0.5
0.0
0.0
2.9
35.0
1.2
RSS
College of Optical Sciences
The University of Arizona
15.0
Residual wavefront errors
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Total allowed: 40nm
Testing system budget: 20nm
Errors in lens, CGH and spherical mirror will be backed out
in nm
CGH
dz
tilt x
Spherical mirror
tilt y
dz
tilt x
ROC
Thermal
tilt y
RSS
um/md
eg
7
7
7
7
1.4
1.4
20 1degree
RMS
Fit error
2.0
2.6
1.7
1.2
2.6
2.5
0.7
3.5
6.4
Z12
-6.3
-2.0
-0.1
-0.9
4.4
-0.2
-0.6
-3.2
8.6
Z13
0.0
0.0
-0.5
0.0
0.0
3.2
0.0
0.0
3.2
RSS subtotal
Lens+CGH
8
Mirror
8
RSS Total
College of Optical Sciences
The University of Arizona
11.2
15.9
Optical Bench and Testing Tower
College of Optical Sciences
The University of Arizona
NST testing system
System assembled and
aligned in lab
College of Optical Sciences
The University of Arizona
System mounted and
aligned in test tower
(looking up)
Alignment
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Using CGH patterns to align the CGH itself
Using CGH patterns and metering rods to align
the spherical mirror
Additional CGH patterns to create cross hairs for
position the test mirror
College of Optical Sciences
The University of Arizona
The CGHs
3-segment CGH
creating crosshair
Main CGH
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Substrate
alignment CGH 
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4 CGHs creating
beams for spherical
mirror alignment
CGH creating
clocking line
College of Optical Sciences
The University of Arizona
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10 segments create 8
wavefronts.
Main CGH creates the testing
wavefront.
The ring type CGH aligns the
substrate to the
interferometer.
3 segments create a crosshair
and 1 segment creates a
clocking line to align the NST
mirror to the test optics.
4 circular CGHs send beams to
align the lateral positions of
the 4 balls mounted on the
surface of the fold sphere.
Alignment of CGH
With the reflection
fringes from the
alignment CGH
controlled to 0.5 in
power, the CGH
substrate is aligned
within 7μm.
College of Optical Sciences
The University of Arizona
Alignment of spherical mirror
Ball at mirror
CGH
Metering rod
w/ LVDTs
Lens
Ball at focus
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Spherical
mirror
Position a ball at focus of the 0th order diffraction beam after CGH, use it
as a reference to position the spherical mirror.
Put a few balls at the mirror surface, patches of CGH direct spherical
beams toward the ball and the reflection fringes are used to position the
balls accurately in lateral direction, and metering rod with LVDTs are used
to control the distance from these balls to the ball at focus. The mirror is
adjusted so that all the balls are at proper position.
College of Optical Sciences
The University of Arizona
Metering rod calibration
• The metering rods are
made of low CTE carbon
fiber tubes with invar tips
glued on both ends
• Metering rods are
calibrated between two
balls separated by
known distance.
• Since the tip of the rod
has proper curvature
slight misalignment does
not alter calibration.
College of Optical Sciences
The University of Arizona
Metering rod calibration bench
•The calibration bench serves as
master reference for the metering
rods.
• The distance between the balls was
measured by a laser tracker.
• In order to minimize measurement
errors the tracker was aligned to
direction of motion.
•The calibration bench is made of ULE
and mechanically mounted to minimize
environmental influence.
College of Optical Sciences
The University of Arizona
Alignment of spherical mirror relative to CGH
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Spherical mirror is aligned with
metering rods.
4 balls mounted on the mirror
surface. Their lateral positions
are controlled with beams from
the CGHs. Small stages position
the balls laterally to give
retroreflection.
 Initial alignment scheme
based on reflected wavefront
did not work. New scheme
based on reflected image is
being implemented.
1 ball mounted on the 0th order
beam focus. Its position is
controlled by nulling the
reflection fringes from its
surface.
Metering rods lengths are
calibrated to 3 μm.
College of Optical Sciences
The University of Arizona
Reflection fringes
From the ball @focus->
Aligning the test mirror: projected crosshair and
clocking line
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A crosshair and a clocking line are generated by CGHs to align the
Usage of crosshair and
NST mirror to the test optics.
Line
Line to get clocking
Crosshair to get x-yposition
Intensities plotted in logscale
College of Optical Sciences
The University of Arizona
Initial testing results
College of Optical Sciences
The University of Arizona
Morphed surface map
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College of Optical Sciences
The University of Arizona
P-V: 8.6 wave
RMS: 1.5 wave
Summary
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We have built a system
for interferometrically
testing an off-axis
parabolic mirror
A CGH and a spherical
mirror is used as null lens
Initial testing results are
encouraging
Experience and knowhow acquired will be
applied to testing GMT
mirrors which are 5x
scale off-axis parabolas
College of Optical Sciences
The University of Arizona
GMT Telescope