Transcript Alternate Surface Measurements for GMT Primary Mirror

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Scanning pentaprism measurements of off-axis aspherics
Peng Sua, James H. Burgea,b, Brian Cuerdenb, Hubert M. Martinb
aCollege
of Optical Sciences, University of Arizona, Tucson, AZ 85721, USA
bSteward Observatory, University of Arizona, Tucson, AZ 85721, USA
GMT
The pentaprism test is based on the property of a paraboloidal surface where all rays parallel to the optical axis will be reflected
Issue with testing off-axis mirror
to go through the focal point. We have developed a scanning pentaprism system that exploits this geometry to measure off-axis
paraboloidal mirrors such as those for the Giant Magellan Telescope primary mirror. Extension of the pentaprism test to offaxis mirrors requires special attention to field effects that can be ignored in the measurement of an axisymmetric mirror. The
test was demonstrated on a 1.7-m diameter off-axis mirror and matched an interferometric test of this surface to 32 nm rms.
This paper gives detailed performance results for the measurement of the 1.7-m mirror, and designs and analysis for the test of
the GMT segments.
Wave aberrations due to 0.001° field of views
Demonstration for a 1.7-m off-axis paraboloid
The 25-m f /0.7 primary mirror for the Giant Magellan
Telescope (GMT) is made of seven 8.4-m segments in a close
packed array. Each of the off-axis mirror segments has 14 mm
of aspheric departure, which makes the fabrication and testing of
the segments challenging
A scanning pentaprism system was configured for the measurement of a 1.7-m off axis
paraboloid. This off-axis parabola is the primary mirror for the New Solar Telescope at Big
Bear Solar Observatory. As an off axis part of an f/0.7 paraboloid, it is a 1/5 scale prototype for
the GMT segments.
The scanning prism is moved across
different diameters of the off axis mirror.
As a result, the scans do not have
symmetry of the parent
Image at CCD
CCD camera at
focus of paraboloid
Axis
The pentaprism test for an off-axis parabola has some special characteristics when compared with
the test for a rotationally symmetric surface. One of the four scans is in the plane containing the
optical axis of the parent. Plane symmetry is not available for scans 2, 3 and 4. Moreover, as an offaxis part of a parabolic surface, the mirror suffers field aberrations. For the case of the GMT test,
there is a 2.3:1 ratio between the image location (chief ray) shift and the coma blur in the tangential
direction. The mirror acts as if the focal length depends on position.
Because of the two features mentioned above, the in-scan and cross-scan directions of the test in the
detector plane change orientation at different pupil locations during a single scan. An intuitive way
to understand this is shown below. A cross scan error would generally shift the image to the right,
but because of the field aberration, the light from different portions of the pupil are shifted by
different amounts and they have a component in the orthogonal direction. The red dots below show
this effect for a linear scan at 45° for the off axis segment.
CCD image
pa
rent
lo
rabo
Scanning
pentaprism
The measurements were performed according to the sequence below. The data were fit to low
order aberration. Alignment aberrations were subtracted from the data leaving the figure error.
This test was performed during a phase of fabrication when the mirror had considerable error
of 106 nm rms. The scanning pentaprism data matched the data from an interferometrr with
CGH null corrector to 32 nm rms.
Scanning pentaprism system layout
of pa
Zero field
id
Fixed reference
pentaprism
with beamsplitter
The linear stage that moves the scanning prism was supported above the mirror such that it was
tilted 13.5° to align the scanning beam to the optical axis of the parent paraboloid. A CCD
camera was placed at the focal point.
If the parabolic mirror is illuminated with collimated light that is parallel to its axis, all reflected
rays go through the focal point of the parabola. If these rays are not parallel to the axis, the rays will
shift away from the focal point and they no longer intersect at a point. For a full axially symmetric
mirror, this appears as well-known coma. The off-axis segment covers part of the comatic pattern,
which appears as a combination of astigmatism and coma in the wavefront. The magnitude of this
aberration is linear with the misalignment.
ser
a
l
d
mate
Colli
parent paraboloid
Changes in field angle will linearly shift and scale the spot diagram.
The cross-scan direction depends on pupil position.
The scanning prism is translated
across the mirror diameter while the
fixed prism is held stationery.
Variations in the pitch direction are
insensitive to errors in the prism or
motion.
Off-axis mirror
The prism is actively controlled in
roll and yaw based on feedback
from an autocollimator and from
the CCD at the focus
So one component of mirror slope
errors (defined by the pitch
direction of the prism) can be
measured to < 1 µrad accuracy
Test for GMT mirror segment
Fig.9 Residuals after removing the
polynomial fit and field aberrations
The scanning pentaprism system for measuring the GMT primary mirror segments
now being built. We us a single rail supported at the center by a bearing tilted
13.5°. To perform different scans, the rail is rotated and locked at different angles.
In-scan spot displacements at CCD after
subtracting motion of the reference spot.
Flow diagram for data
collection and processing
This system will be integrated into the new large test tower at Steward
Observatory Mirror Lab. The rail assembly stows alongside the test tower to
allow access for the other optical measurements. A CCD camera is located at the
focal point, 18 meters above the mirror
The fit to the Zernike polynomials is shown.
Pentaprism rail lies in plane
perpendicular to parent axis.
Hub rotates rail to scan
different diameters.
Basic Concept
An ideal parabolic mirror will focus rays parallel to the axis to a point at the focus. One can measure errors in the
surface by sending parallel rays into the mirror and measuring where they intercept the focal plane. The scanning
pentaprism system uses a collimated light source and a pentaprism to create parallel beams that are scanned over the
surface, as shown above. For an off-axis mirror, several scans across different diameters are used to determine the loworder aberrations in the system. We used four pentaprism scans at 45° to provide sampling of low order surface errors.
Variations in tilt of the collimated source, or beam projector, also cause spot motion. We remove this error by using a
stationary pentaprism in addition to the scanning pentaprism. Common motion of both spots is due to changes in the
beam projector, while the differential motion is a measure of the slope error at the pupil position of the scanning beam.
The pentaprism test works because the deflection of the beam in the pitch direction, defined in the figure above, is
independent of rotation. The corresponding direction on the mirror surface and in the focal plane is called the in-scan
direction, and the perpendicular direction is the cross-scan direction. The scanning pentaprism system measures slope
errors in the in-scan direction with high accuracy. Rotation of the prism about its other axes (roll and yaw) causes firstorder deflection of the beam in the cross-scan direction, so we do use the cross-scan information only as a guide to
align the system, not as a measurement of surface slopes.
The only direct coupling to in-scan spot motion is a change in pitch of the beam projector, and this is removed by the
differential measurement with two pentaprisms. There are second-order effects, however, that must be considered.
Table 1 lists sources of line-of-sight error, to second order. Prism yaw will introduce quadratic motions in the in-scan
direction, and yaw of the beam projector couples with prism rotations to cause in-scan motion. Finally, there is
uncertainty in determining the in-scan direction in the focal plane. This error is called detector roll. An error of in
determining the in-scan direction will cause a coupling of cross-scan motion into the in-scan measurement.
Table 1 Contributions to line-of-sight error from prism,
beam projector, and determination of in-scan direction
Contributions to in-scan motion
Contributions to cross-scan motion
Beam projector pitch
Beam projector yaw
(Prism yaw)2
Prism yaw
(Prism yaw) x (beam projector yaw)
Prism roll
(Prism roll) x (beam projector yaw)
(Prism roll) x (beam projector pitch)
(Detector roll) x (cross-scan motion)
(Prism yaw) x (beam projector pitch)
Table 2. Surface coefficients from pentaprism test
and interferometric test for the 1.7-m NST mirror
aberration
interferometer
pentaprism
nm rms surface
nm rms surface
astigmatism 0°
8
9 ± 23
astigmatism 45°
0
-2 ± 23
coma 0°
-87
-98 ± 12
coma 90°
-4
16 ± 12
trefoil 0°
-50
-32 ± 35
trefoil 30°
9
23 ± 30
-32
-35 ± 8
spherical
Scanning pentaprism test as implemented for GMT off-axis
segments. Pentaprism rail is suspended from tower.
Table 3. Performance estimate: Monte Carlo analysis of 1 μrad rms random
error in wavefront slope for NST test as performed, NST test with 40
points/scan uniform sampling, and the GMT test sampled the same way
The scanning pentaprism test is
important for GMT because it provides
confirmation that the off-axis mirrors
are made correctly.
This demonstration at 1/5 scale is an
important milestone for GMT.
Interferometric test
106 nm rms
Pentaprism measurement
113 nm rms
nm
aberration
rms surface error (nm)
As sampled as in NST
measurement
rms surface error (nm)
NST mirror
(40 points/scan)
rms surface error (nm)
GMT segment
(40 points/scan)
Focus
15
9
44
Sine Astigmatism
23
17
84
Cosine Astigmatism
23
17
84
Sine Coma
12
6
30
Cosine Coma
12
6
30
Sine Trefoil
35
20
99
Cosine Trefoil
35
20
99
Spherical aberration
8
4
20
RSS
58
36
180