There is an increasing need for precision large aspheric optics with small focal ratios for astronomical and space
applications. However, testing such optics presents a challenge. Interferometric testing of aspheric surfaces often
requires the use of null lenses. Many of these null lenses are tested using a certification computer-generated hologram
(CGH) for better error calibration. We present a method that will measure large aspheres to a greater level of accuracy
than is presently possible. We use segmented and superposed CGH elements to certify and calibrate null lens errors
absolutely to a high degree of accuracy. In such holograms two different phase functions are encoded on the CGH by
means of aperture division. One subaperture generates a spherical wavefront that is used to determine the pattern errors
of the hologram while the second subaperture reconstructs an aspherical wavefront used to calibrate the wavefront errors
of the null lens. This careful calibration process involves the removal of both axisymmetric and non-axisymmetric
errors in the null test. Once this is accomplished, the null lens may be used to test the asphere to a high degree of
accuracy. Our initial results show that we can test 4-meter class aspheric mirrors to better than 1nm rms surface error.
In current experiments we have set a goal of measuring such mirrors to better than 1nm rms surface error.
Computer generated holograms (CGHs) have been successfully used for wavefront correction for measuring aspheric surfaces. Features on the CGHs have assisted the alignment of the optical test equipment. CGHs can also be used to provide alignment references for other complex optical systems. This paper discusses the types of CGHs that can be used for optical alignment and gives some examples.
We present a method for a cascading null test using twin computer-generated holograms to calibrate errors in null correctors. This will allow us to test large aspheres an order of magnitude better than current limits. We discuss various sources of CGH errors and how to calibrate them. We also mention some ways to measure and calibrate the errors in the test optics.
Large diameter, non-axisymmetric aspheric mirrors can be measured interferometrically using null correctors that
employ computer generated holograms (CGHs). The testing of off axis segments for the new class of giant telescopes
pose requirements that beyond the state of the art for CGHs alone. The long radius of curvature and the magnitude of the
aspheric departure require other lenses and mirrors to be used along with the CGH. The alignment of these systems is
very sensitive and the absolute accuracy of the alignment is critical to the system performance. We have developed
techniques that use diffracted light from patterns on the CGH to accurately define the alignment of multi-element null
correctors. We will present results from the null test of the 1.7-m New Solar Telescope primary mirror. The optical
shape of this mirrors is an off-axis paraboloid from an f/0.7 parent.
The Giant Magellan Telescope (GMT) uses seven 8.4-m diameter segments to create a giant primary mirror,
25 meters across with focal ratio f /0.7. The off-axis segments will be difficult to measure accurately, as they
have 14.5 mm departure from the nearest fitting sphere! The test configuration adopted uses a large 3.75-m
powered mirror to fold the light path and provide most of the aspheric correction, with a smaller mirror and
computer generated hologram (CGH) providing the additional correction. These optics will be aligned to a
vibration-insensitive interferometer using a combination of optical references created by the CGH and
metrology with a laser tracker. Some key challenges for this system are presented here including, the system
alignment, the large fold mirror, and the mechanical structure. Analysis of the optical test shows that it will
meet GMT specifications, including the difficult requirement that the separate segments have matching radius
of curvature. Additional corroborative testing will be performed to assure that the mirror segments are correctly
We have nearly completed the manufacture of a 1.7 m off-axis mirror as part of the technology development for the Giant Magellan Telescope. The mirror is an off-axis section of a 5.3 m f/0.73 parent paraboloid, making it roughly a 1:5 model of the outer 8.4 m GMT segment. The 1.7 m mirror will be the primary mirror of the New Solar Telescope at Big Bear Solar Observatory. It has a 2.7 mm peak-to-valley departure from the best-fit sphere, presenting a serious challenge in terms of both polishing and measurement. The mirror was polished with a stressed lap, which bends actively to match the local curvature at each point on the mirror surface, and works for asymmetric mirrors as well as symmetric aspheres. It was measured using a hybrid reflective-diffractive null corrector to compensate for the mirror's asphericity. Both techniques will be applied in scaled-up versions to the GMT segments.
We present a method to accurately measure the radius of curvature of a concave spherical mirror with a phase-measuring interferometer and a laser tracker. Use of a laser tracker eases the alignment of the testing system, eliminates the need to move the test piece during the measurement, and improves the accuracy of the distance measurement. Using this method, we measured the radius of curvature of a spherical mirror 0.5 m in diameter and about 2.5 m in radius of curvature. The accuracy of the measurement is better than ±20 µm.
Steward Observatory Mirror Lab is currently polishing an off-axis parabola which will be the primary mirror of the New Solar Telescope. To test this mirror, we built a test equipment to combine a spherical mirror and a Computer Generated Hologram (CGH) as null lens. The spherical mirror is tilted to compensate much of the astigmatism and some coma. And the CGH compensates rest of aberrations. The combination of a spherical mirror and a CGH makes the test system compact. The technology developed here will be used to test the Giant Magellan Telescope's primary mirror segment--a five times larger off-axis parabola.