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Optimum Configuration Of The Offner Null Corrector: Testing An F/1 Paraboloid
Abstract
Three designs of the Offner type that can be used to test an F/1 paraboloid mirror are discussed. A tolerancing analysis is performed in order to design a null corrector with a performance insensitive to small fabrication and alignment errors. It is found that there is no optimum configuration, but solutions with different properties do exist.
... Null optics are additional optics that must be added to the system, and they are often referred to as a null lens, null corrector, or null compensator. Refractive, reflective, and diffractive elements have been employed as null compensators (Offner 1963;Sasian 1989;McCann 1991;Offner and Malacara 1992;Chen 1993). Asphere wavefront at the location of the test part, leading to errors in the surface figure (Allen et al. 1990). ...
Aspheric surface testing would be greatly expedited by eliminating the
need for a null condition. To test in a non-null configuration, an
interferometer must be capable of measuring steep wavefronts. These
wavefronts contain aberrations introduced by the interferometer optics,
so non-null measurements must be calibrated. Sub-Nyquist interferometry
(SNI) enables measurement of large wavefronts. A simple Twyman-Green
interferometer was constructed to conduct SNI and to explore the
calibration problem. The characteristics of the interferometer were
investigated, and wavefronts with several hundred waves of departure
from a reference sphere were recorded and successfully reconstructed. A
defocused sphere was used to demonstrate the need for calibration and to
serve as a test subject for the calibration study. Reverse optimization
was used to calibrate wavefront measurements. Experimental wavefront
data were entered into the merit function of a lens design program, and
optimization was employed to retrieve the prescription of the
interferometer and the part shape. To simulate interference in the
design program, the reference wavefront was stored in a Zernike phase
surface on the image plane. The multiple configuration mode of the
program was utilized to optimize several test configurations
simultaneously, providing additional information to overcome a global
optimization problem and to remove uncertainty in the part alignment.
Errors introduced by pupil distortion were avoided by moving the stop to
the plane of the sensor in the design program. Measurements from a
defocused sphere were successfully calibrated using reverse
optimization. A surface departure of 100 lambda was calibrated to better
than lambda/4 PV. Because the interferometer could not be characterized
sufficiently for calibration of measurements from aspheres, simulations
were conducted. A lens design program was used to develop an accurate
model of an interferometer, including simulated fabrication, alignment,
and characterization errors. Reverse optimization was applied using
wavefronts generated by the model. Testing was simulated for several
conic asphere surfaces. Reverse optimization yielded calibration to
lambda/4 PV for conic aspheres with surface departures as large as
300lambda . The results suggest that it is feasible to calibrate
non-null measurements of aspheric surfaces using reverse optimization.
This paper presents a new kind of “Easily Measurable Aspheric Surfaces”(EMAS), which could be easily measured by a traditional optical interferometer. The measurement of EMAS is mainly based on using the multi-configuration feature in Zemax software. The first configuration represents the optical system with EMAS, and the second configuration represents the setup, using a parallel planes glass plate or a single lens as a null corrector to measure the aspheric surface used in the first configuration. The applications and advantages of this technique are illustrated by many examples, which could confirm the ease of manipulating and testing this kind of surfaces, compared with conical or general aspheric surfaces. It can also show its competence in minimizing the optical aberrations. © 2017, China Science Publishing & Media LTD. All right reserved.
Optical traditional interferometers are usually used to evaluate the form of spherical and plane surfaces; those systems could also assess the quality of aspheric surfaces which are quasi-spherical. But if a general aspheric surface is to be evaluated, this procedure requires the purchase of complicated and very expensive devices, such as aspheric interferometer, null-lenses, and computer generated hologram for each aspheric surface. This article presents a solid method allowing the measurement of a wide range of conical aspheric surfaces. This method is based on inserting plane-parallel plate (PPP) within the path of a laser beam coming out of a traditional Fizeau interferometer. This article also presents a mathematical justification that relates the conical surface parameters to the thickness of the inserted PPP. The article also presents the supporting computer simulations and some practical results of applying this method and its range of use.
For the large concave surface, the traditional testing method is null tests by using refractive compensator. However, not only it is difficult to align the optical elements of the system, but also it is expensive to fabricate them. In this paper the CGH fabricated on the compensator lens surface which is utilized in system to substitute the traditional compensator is discussed, and the practical optical parameters are given.For the sample surface with the aperture of 950 mm, testing result of P-V is 0.024λ.
A new type of catadioptric compensator is established. The tolerance values of lens spacing, lens thickness, and tilt in mounting are about three times higher than that of the Offner compensator. They overcome the localization of the Offner compensator, improve the test validity, and reduce the test risk of system detection by cross-validation with existing means. The performance of the catadioptric compensator is verified in experiments and theoretically.
We present a design and new approach to certify a null corrector with tight manufacturing tolerances. This involves a diamond turned asphere and a hologram that provide certification redundancy.
Introduction Some Methods to Test Aspheric Wavefronts Imaging of the Interference Pattern in Non-Null Tests Some Null Testing Configurations Testing of Convex Hyperboloidal Surfaces Testing of Cylindrical Surfaces Early Compensators Refractive Compensators Reflecting Compensators Other Compensators for Concave Conicoids Interferometers Using Real Holograms Interferometers Using Synthetic Holograms Aspheric Testing with Two-Wavelength Holography Wavefront Stitching
A parabolic liquid mirror obtained by the rotation of a mercury bath around a vertical axis has been built and its optical surface characteristics measured to demonstrate that it can be used in optical shop testing as a reference surface. A linear Hartmann test allowed us to check that the focal length is well related to the rotation velocity, following the theory, and that no spherical aberration is present, as assumed by previous authors. The spherical aberration has been found to be smaller than λ/50 at 633 nm. An interferometric test of the mirror compared with a null lens gave information about the quality of the optical surface for which the rms wave-front error, when the random errors are averaged, is ~λ/25. Because modifying the mirror diameter is cheap and fast and adjusting its focal length within a large range is straightforward, the parabolic liquid mirror can become a highly adaptable tool in optical metrology. In particular, it can be used in optical shop testing as a reference surface to test null correctors, to check any system developed to control the shape of large parabolic or quasiparabolic top-quality solid-state mirrors, or to make holographic references of such surfaces.
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