With our Custom CGH Nulls, you can test
aspheres using your existing interferometer. The Custom CGH Null transforms the spherical
or collimated test beam into the aspheric wavefront required to produce a null interferogram.
Our CGH Positioners and Alignment CGHs simplify the alignment of CGH to interferometer.
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Convention says that a CGH null should be used in single pass because of low diffraction efficiency. It should be located in a collimated beam to ease alignment tolerances. Test and reference beams should pass similarly through the CGH to cancel any substrate aberrations.
Our CGH Nulls typically violate these rules, so we must produce high efficiency CGHs on precision substrates and align them to micron tolerances. You benefit in simple test configurations that use your existing interferometer and accessories.
Our CGH nulls are encoded to meet your accuracy requirements. Substrate transmitted wavefront distortion, which is less than lambda/10, can be measured in zero order transmission and subtracted.
System errors of your interferometer and transmission sphere can be compensated in the usual manner—by measuring a good spherical surface and subtracting the result from subsequent interferograms.
Custom CGH nulls are available in about 6 to 8 weeks. The price will depend on your test requirements, but is generally less than for a conventional refractive null.
Designing a custom CGH null does not require raytracing of your interferometer or transmission sphere. The raytrace model begins and ends at a point source concentric with your transmission sphere.
We attempt to locate the CGH null at one of the twelve distances from focus accommodated by our standard Alignment CGHs. The CGH must be outside the caustic region where rays cross. We make the CGH aperture somewhat oversized so that your test optic is the only stop in the system. We favor a CGH aperture of 30 to 45 mm as a reasonable compromise between fabrication costs and alignment sensitivity.
Every CGH Null is shipped with full documentation. We provide an optical prescription of the CGH phase function and test configuration. We include a sensitivity and tolerance analysis which includes all CGH encoding and fabrication errors plus the alignment errors.
A CGH null reduces the intensity of the test beam, thus altering fringe visibility. For testing of bright mirrors, this will result in improved visibility, but in other cases visibility may be reduced. Diffraction efficiency is typically 10 percent for chrome CGHs and 35 to 40 percent for binary phase CGHs.
Generally, we recommend binary phase CGHs for testing of bare glass surfaces and chrome CGHs for testing of mirror surfaces. Unless your test optic is anti-reflection coated, fringe visibility is adequate.
Unwanted diffraction orders may create ghost fringes. These are minimized or eliminated by careful test configuration design.
