Everything you always wanted to know about Aspheres. Season 2/ Part 4

The 5 reasons you won't get a perfect asphere. Reason 3 of 5: Imperfect Dial gauge on lens when blocking on mandrel

Mechanical dial gauges with resolving power of about 2 microns in addition to roughness and roundness of the lens barrel in a range of a few micrometers or more do not solve the problem of centrering the semi finished lens of an asphere to the mandrel. With deviation of less than a few arcmin of angle of the first mounted spherical or aspherical surface the center of curvature will not run on the center axis of the mandrel and therefore not on the aspherical axes to be grinded and polished in the next step.

The dial indicators do not have the greatest impact on the centering error, but measuring the bottom surface with just 2 microns on a 30mm HD mandrel affects the lens tilt error by 0.22 arc minutes. This represents almost 50% of the tolerance found in most precision asphere manufacturing applications.

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