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When most people think of a microscope it is one that works in transmission with the light source on one side of the sample and the microscope objective and eyepiece on the other. An autostigmatic microscope (ASM) works in reflection, just like an autocollimator, so the light source is in the microscope body, and is almost always introduced via a beamsplitter close to the objective but between objective and eyepiece.

Back in the late 1990’s NIST had a number of firms that wanted to send their interferometer transmission spheres there for calibration but NIST was not in this sort of calibration business. While I was at NIST consulting for Chris Evans in the Precision Machining Facility we thought of the idea of a self-calibration test for transmission spheres that was a spherical analog of the plane surface test1 used to self-calibrate interferometric surface roughness testing microscopes.

The random ball test for calibrating interferometer transmission spheres1 was reported about 8 years ago but there did not appear to be an ideal ball. Now, nearly ideal balls are available.

Background: As the tolerances on optical figure get tighter and as the need to provide traceable test methods to fulfill I S 0 9000 standards becomes greater, the more there is the need for a ro- bust and reliable method to determine the residual error in interferometer test optics.

Centering with the Point Source Microscope (PSM) is a perfect application. When a spot of light from the objective focus is reflected back from a concave surface sitting on a rotary table, it will sweep out a circle on the video screen as the table is rotated unless the center of curvature of the surface lies precisely on the axis of rotation of the table.