Steven C. West, Samuel H. Bailey, James H. Burge, Brian Cuerden, Jeff Hagen, Hubert M. Martin, and Michael T. Tuell, "Wavefront control of the Large Optics Test and Integration Site (LOTIS) 6.5m Collimator," Appl. Opt. 49, 3522-3537 (2010)
The LOTIS Collimator provides scene projection within a diameter collimated beam used for optical testing research in air and vacuum. Diffraction-limited performance (0.4 to wavelength) requires active wavefront control of the alignment and primary mirror shape. A hexapod corrects secondary mirror alignment using measurements from collimated sources directed into the system with nine scanning pentaprisms. The primary mirror shape is controlled with 104 adjustable force actuators based on figure measurements from a center-of-curvature test. A variation of the Hartmann test measures slopes by monitoring the reflections from 36 small mirrors bonded to the optical surface of the primary mirror. The Hartmann source and detector are located at the Cassegrain focus. Initial operation has demonstrated a closed-loop wavefront error in ambient air over the collimated beam.
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SOML horizon-pointing measurement in mirror cell using a holographic test plate and interferometer
Never
Relatively thick low-expansion ULE substrate
Secondary mirror alignment and focal plane placement
z axis, rotation about zero-coma location, rotation about secondary mirror center-of-curvature point
Six-axis hexapod positioner
Absolute: six-prism scanning pentaprism test
Invar OSS for excellent passive performance
Relative: nine-prism staring pentaprism test
Independent wavefront verification and small scale wavefront
System
Zerodur flat with positioning system to locate flat anywhere in Collimator output beam (with coarse and fine tip/tilt adjustments)
Autocollimation test with focal plane interferometer
As needed
Sections 3, 4, 5 of the text describe these components in detail.
Table 2
System Wavefront Error Budget (nm rms) Versus System Subaperture Diameter
Primary mirror
60
41
30
22
Secondary mirror
61
38
27
21
Alignment
43
15
4
0
Sum (vacuum)
96
58
41
30
Air specification ()
255
165
115
80
The specification for ambient air operation is simply scaled from the vacuum results.
Table 3
Wavefront Error Budget for Primary Mirror (nm rms) Versus System Subaperture Diameter
As-polished figure
31
24
22
18
Mirror supports
12
7
5
3
Zenith to horizon support change
34
27
17
9
Temperature gradients
20
12
8
5
Center-of-curvature test
20
12
8
5
Hartmann measurement
22
3
0
0
Sum
60
41
30
22
Table 4
Wavefront Error Budget for Manufacture and Testing of Secondary Mirror Versus System Subaperture Diameter
As-polished figure
57
36
26
20
Measurement
20
12
8
5
Support variations
6
2
1
0
Sum
61
38
27
21
Table 5
Wavefront Error (nm rms) Corresponding to Control Actions of Secondary Positioner Shown in Fig. 7
Motion
Back Focal Distance Change ()
Power WFE (nm rms)
Spherical WFE (nm rms)
Pointing Change ()
Coma WFE (nm rms)
Piston
144
23
0.23
0
0
Sensor shift
1000
161
0
0
0
M2 CoC
0
0
0
0
2.0
M2 zero coma point
0
0
0
0.11
0
Table 6
Improvement in Secondary Position of Laser Tracker Alignment Compared to That after Two Alignment Corrections Using Scanning Pentaprism System Measurements to Change Secondary Position
Secondary Mirror Alignment Term
Laser Tracker Setup
After Using Scanning Pentaprism
Primary to secondary spacing ()
870
0.1
Pitch angle ()
150
4
Yaw angle ()
290
15
Table 7
Wavefront Error Alignment Budget Including Secondary Mirror Positioner Accuracy Versus System Subaperture Diameter
Aperture Diameter
Scanning pentaprism (coma, spherical, power)
36
14
4
0
Staring pentaprism (focus)
13
2
0
0
axial resolution
12
2
0
0
lateral resolution (coma)
8
2
0
0
tilt resolution (coma)
8
3
1
0
Secondary pointing (field astigmatism)
10
2
0
0
Motion between corrections
6
1
0
0
Sum
43
15
4
0
Table 8
Summary of Analysis of Wavefront Error Contributions (nm rms) to Primary Mirror Figure and Secondary Alignment for Time after Chamber Depressurization (Compared to Thermal Wavefront Error Budget)a
Case
Secondary Alignment WFE (nm rms)
Primary Mirror WFE (nm rms)
Primary mirror support system ()
0
4.6
Scene generator radiation
0
0.2
Pentaprism scan drive power
0.29
0
Scene generator power
0
7.1
Power umbilicals
0
3.7
Chamber pump-down effects
4.2
7.5
Initial air temperature
0
6.4
Primary figure change in
0
2.6
Primary coating “e” variation
0
2.5
MLI variation
0
8.0
Other
0
3.0
Total
4.2
16.4
Error Budget
6
20
The analysis assumes hourly correction of focus, coma, and primary mirror astigmatism (not including the measurement errors, which are shown in the other wavefront error budget tables).
Table 9
Average Subaperture WFE (nm rms) with and without Measurement Uncertainties Versus Component for First LOTIS Collimator Ambient Air Wavefront Correction Tests at LMSSC Facility
WFE Component
Primary mirror horizon-pointing optimization
98
59
19
3
Secondary mirror horizon-pointing figure
48
40
24
13
Secondary mirror Al coating
1.6
1.4
0.6
0.2
PPS Collimator alignment errors (Z3, Z6, Z7)
30
14
2
0.4
Hartmann 17-mode measurement
15
7
3
0.6
Collimator wavefront constructed from measurements
102
74
43
21
PPS measurement uncertainty ()
57
24
4
0.6
Hartmann measurement uncertainty (17 modes)
8
2
0.8
0.15
CoC measurement uncertainty 17 modes
105
25
7
1
Wavefront including 1-sigma measurement uncertainties
170
81
44
21
Table 10
Commanded Secondary Motions and Astigmatic Bending of Primary Mirror Versus Wavefront Changes Measured with Both Scanning and Staring Pentaprism Systemsa
M2 Pitch
M2 Yaw
M2 Z3
M2 Z8
CCD Defocus
M1 Z 4%
M1 Z 5%
Design
2.0
2.0
23
0.23
161
100
100
Measured scan
0.20
167
Measured stare
–
–
The staring pentaprism geometry was designed to limit spherical aberration aliasing with power and, therefore, is not used to measure spherical aberration. The CCD defocus is the change in power resulting from changing the focal plane position of the detector.
Table 11
Change in Hartmann Wavefront Fit Coefficient Expressed as Percentage of Commanded Primary Mirror Astigmatism and Trefoil Bending Averaged for Many Bends Measured during Collimator Setup
Z4
Z5
Z9
Z10
Tables (11)
Table 1
Active Wavefront Control Components of LOTIS Collimatora
Component
Degree of Freedom
Control Mechanism
Feedback Mechanism
Correction Frequency
Primary mirror solid-body forces, moments, and positioning
Solid body x, y, z, , ,
x: two active lateral actuators
Hexapod platform positioner that also measures the global mirror forces and torques with load cells
SOML horizon-pointing measurement in mirror cell using a holographic test plate and interferometer
Never
Relatively thick low-expansion ULE substrate
Secondary mirror alignment and focal plane placement
z axis, rotation about zero-coma location, rotation about secondary mirror center-of-curvature point
Six-axis hexapod positioner
Absolute: six-prism scanning pentaprism test
Invar OSS for excellent passive performance
Relative: nine-prism staring pentaprism test
Independent wavefront verification and small scale wavefront
System
Zerodur flat with positioning system to locate flat anywhere in Collimator output beam (with coarse and fine tip/tilt adjustments)
Autocollimation test with focal plane interferometer
As needed
Sections 3, 4, 5 of the text describe these components in detail.
Table 2
System Wavefront Error Budget (nm rms) Versus System Subaperture Diameter
Primary mirror
60
41
30
22
Secondary mirror
61
38
27
21
Alignment
43
15
4
0
Sum (vacuum)
96
58
41
30
Air specification ()
255
165
115
80
The specification for ambient air operation is simply scaled from the vacuum results.
Table 3
Wavefront Error Budget for Primary Mirror (nm rms) Versus System Subaperture Diameter
As-polished figure
31
24
22
18
Mirror supports
12
7
5
3
Zenith to horizon support change
34
27
17
9
Temperature gradients
20
12
8
5
Center-of-curvature test
20
12
8
5
Hartmann measurement
22
3
0
0
Sum
60
41
30
22
Table 4
Wavefront Error Budget for Manufacture and Testing of Secondary Mirror Versus System Subaperture Diameter
As-polished figure
57
36
26
20
Measurement
20
12
8
5
Support variations
6
2
1
0
Sum
61
38
27
21
Table 5
Wavefront Error (nm rms) Corresponding to Control Actions of Secondary Positioner Shown in Fig. 7
Motion
Back Focal Distance Change ()
Power WFE (nm rms)
Spherical WFE (nm rms)
Pointing Change ()
Coma WFE (nm rms)
Piston
144
23
0.23
0
0
Sensor shift
1000
161
0
0
0
M2 CoC
0
0
0
0
2.0
M2 zero coma point
0
0
0
0.11
0
Table 6
Improvement in Secondary Position of Laser Tracker Alignment Compared to That after Two Alignment Corrections Using Scanning Pentaprism System Measurements to Change Secondary Position
Secondary Mirror Alignment Term
Laser Tracker Setup
After Using Scanning Pentaprism
Primary to secondary spacing ()
870
0.1
Pitch angle ()
150
4
Yaw angle ()
290
15
Table 7
Wavefront Error Alignment Budget Including Secondary Mirror Positioner Accuracy Versus System Subaperture Diameter
Aperture Diameter
Scanning pentaprism (coma, spherical, power)
36
14
4
0
Staring pentaprism (focus)
13
2
0
0
axial resolution
12
2
0
0
lateral resolution (coma)
8
2
0
0
tilt resolution (coma)
8
3
1
0
Secondary pointing (field astigmatism)
10
2
0
0
Motion between corrections
6
1
0
0
Sum
43
15
4
0
Table 8
Summary of Analysis of Wavefront Error Contributions (nm rms) to Primary Mirror Figure and Secondary Alignment for Time after Chamber Depressurization (Compared to Thermal Wavefront Error Budget)a
Case
Secondary Alignment WFE (nm rms)
Primary Mirror WFE (nm rms)
Primary mirror support system ()
0
4.6
Scene generator radiation
0
0.2
Pentaprism scan drive power
0.29
0
Scene generator power
0
7.1
Power umbilicals
0
3.7
Chamber pump-down effects
4.2
7.5
Initial air temperature
0
6.4
Primary figure change in
0
2.6
Primary coating “e” variation
0
2.5
MLI variation
0
8.0
Other
0
3.0
Total
4.2
16.4
Error Budget
6
20
The analysis assumes hourly correction of focus, coma, and primary mirror astigmatism (not including the measurement errors, which are shown in the other wavefront error budget tables).
Table 9
Average Subaperture WFE (nm rms) with and without Measurement Uncertainties Versus Component for First LOTIS Collimator Ambient Air Wavefront Correction Tests at LMSSC Facility
WFE Component
Primary mirror horizon-pointing optimization
98
59
19
3
Secondary mirror horizon-pointing figure
48
40
24
13
Secondary mirror Al coating
1.6
1.4
0.6
0.2
PPS Collimator alignment errors (Z3, Z6, Z7)
30
14
2
0.4
Hartmann 17-mode measurement
15
7
3
0.6
Collimator wavefront constructed from measurements
102
74
43
21
PPS measurement uncertainty ()
57
24
4
0.6
Hartmann measurement uncertainty (17 modes)
8
2
0.8
0.15
CoC measurement uncertainty 17 modes
105
25
7
1
Wavefront including 1-sigma measurement uncertainties
170
81
44
21
Table 10
Commanded Secondary Motions and Astigmatic Bending of Primary Mirror Versus Wavefront Changes Measured with Both Scanning and Staring Pentaprism Systemsa
M2 Pitch
M2 Yaw
M2 Z3
M2 Z8
CCD Defocus
M1 Z 4%
M1 Z 5%
Design
2.0
2.0
23
0.23
161
100
100
Measured scan
0.20
167
Measured stare
–
–
The staring pentaprism geometry was designed to limit spherical aberration aliasing with power and, therefore, is not used to measure spherical aberration. The CCD defocus is the change in power resulting from changing the focal plane position of the detector.
Table 11
Change in Hartmann Wavefront Fit Coefficient Expressed as Percentage of Commanded Primary Mirror Astigmatism and Trefoil Bending Averaged for Many Bends Measured during Collimator Setup