Download Novel methods used to test large flat mirrors

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10.1117/2.1200704.0696
Novel methods used to test
large flat mirrors
Matt Novak and Julius Yellowhair
The most accurate 2m-diameter flat mirror in the world was made using new surface-measurement techniques in concert with software that
simulates polishing processes.
The main issue faced when manufacturing a high-accuracy large
flat mirror is measurement accuracy. It is difficult to measure a
large flat mirror because interferometers require light reflected
from the test surface to be focused back into the instrument in
order to analyze the wavefront. Since a flat surface has no power,
it is difficult to measure unless the collimated beam from the
interferometer and the reference optic are as large as the flat.
For meter-class flat mirrors, the cost of this system quickly becomes prohibitive. Another issue that affects the production of
these large mirrors is the inability to scale up standard fabrication techniques. For example, continuous polishing (CP) machines manufacture small flat mirrors economically. However,
for meter-class mirrors, the CP process becomes less feasible because the tool diameter of a CP machine must be three times
larger than the parts being produced. Overcoming these challenges will enable the manufacture of high-precision large flat
mirrors for space-based and astronomical applications.
The most accurate technique for measuring a large flat is the
Fizeau test,1 but this is cost prohibitive for flats ≥1m. On large
flats this is usually done by testing subapertures and stitching
the data together. While this method is accurate for measuring
surface irregularities, it is not for low-order errors such as power
and astigmatism. As an alternative, the Ritchey-Common test is
sometimes used for large flats,1 however this has limited accuracy and requires a spherical reference mirror larger than the flat
being tested.
Facilities have traditionally used small tool polishing to produce flats that are too large for CP machines.2 The limiting factor in this approach is that the polishing runs are guided by test
data that, if inaccurate, make it impossible to produce a highprecision large flat mirror. In addition, the geometry of the polishing tool has a direct impact on the results. In the past, the
tool geometry was driven by experience and empirical results,
Figure 1. Scanning pentaprism measurement system. (ELCOMAT
and UDT are the companies that made the autocollimation equipment).
sometimes resulting in inconsistencies in performance. We have
addressed these issues by developing a very accurate metrology
system for measuring large flats and using software simulation
to aid in our tool design.
Our testing method for large flats employs two instruments:
we use a scanning pentaprism arrangement (see Figure 1) to
measure low order errors and we use a large-aperture Fizeau
interferometer (see Figure 2) to measure 1m subapertures in order to obtain surface irregularity data and other higher-order
errors.3, 4
The scanning pentaprism system measures low-order surface
errors such as power, astigmatism, coma, and trefoil and spherical aberration. The measurements taken with the Fizeau interferometer are stitched together to yield the higher-order surface
errors and irregularities. We combine the scanning pentaprism
data with the sub-aperture Fizeau data to provide state-of-theart surface measurement accuracy for ultra-precise large flats.
The scanning pentaprism system allows us to characterize the
residual power and lower-order errors to approximately 10nm
root-mean-square (rms) accuracy. The stitched sub-aperture
Fizeau data has uncertainty on the order of 3nm rms for a 2mclass flat.
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In conclusion, we have developed unique equipment and
capabilities that enable us to manufacture extremely accurate
2m-class flats. Future developments will include the extension
of capabilities for large computer-controlled polishing up to 4m
diameters. This large machine capacity combined with advancements in high-accuracy metrology will enable us to efficiently
produce large diameter flat mirrors for a variety of unique
systems.
The authors wish to acknowledge the contributions of Jim Burge,
Marty Valente, and the team of talented opticians and technicians who
made this work possible.
Author Information
Figure 2. Layout of large-aperture Fizeau interferometer created at our
facility. OAP: off-axis paraboloid.
Matt Novak and Julius Yellowhair
College of Optical Sciences
University of Arizona
Tucson, Arizona
Matt Novak is senior optical engineer in the Optical Engineering and Fabrication Facility at the College of Optical Sciences,
University of Arizona.
Julius Yellowhair is a PhD candidate in the College of Optical
Sciences, University of Arizona.
References
1. D. Malacara, Optical Shop Testing, Wiley, 2nd ed., 1992.
2. H. H. Karow, Fabrication Methods for Precision Optics, Wiley, 1993.
3. S. Qian, W. Jark, and P. Z. Takacs, “The penta-prism ltp: a long-trace-profiler
with stationary optical head and moving penta prism,” Revi. Sci. Instruments 66(3),
pp. 2562–2569, 1995.
4. P. Mallik, C. Zhao, and J. H. Burge, “Measurement of a 2-m flat using a pentaprism scanning system,” Proc. SPIE 5869, 2005. doi:10.1117/12.618468.
Figure 3. Plot demonstrating agreement between simulated and actual
surface removal for a particular polishing run.
Using this metrology system to guide final figuring, large flats
can be finished to a very high accuracy. At the University of Arizona, we have a significant heritage in tooling technology for
fabricating large aspheres. We have developed specialized software that accurately simulates figuring runs for a given tool geometry, pressure, and other parameters (see Figure 3). At the
College of Optical Sciences, we successfully applied this technology to simulating polishing runs on large optical flats with
small tools traditionally used on large aspheres. The simulations
and actual results repeatedly matched within a few nanometers
for 8h polishing runs at varying parameters on a 2m-class flat
that we recently produced to <12nm rms figure error, including
power and astigmatism.
c 2007 SPIE—The International Society for Optical Engineering