August 1, 2011:  Different tests for different defects - a 10" mirror

I thought I'd share some test results for some nearly-spherical mirrors that I had around.  These needed to be characterized for later use and figuring, so I took the opportunity to run three different tests and take photos.

Tests include 1) Foucault test images, 2) interferograms from a spherical-wave interferometer, and 3)
artificial star test images.

The first mirror was a 10" f/6.3, roughly spherical, with an emphasis on rough!

Foucault tests show a fairly rough surface, and I may not have gotten all of the grime and slime off of the optical surface before testing!

Foucault test of 10"Foucault test 10"

The Foucault test clearly shows some patterns from prolonged pitch lap pressing.  It also shows a slightly non-symmetric figure and one that is not quite spherical (outer ~1.5" high).

Interferogram of 10"Interferogram of 10"

The interferograms clearly show the higher outer areas and some irregularity in the figure.  They don't really show the roughness.  A surface error plot based on the interferogram is at right above, and shows the outer part of the mirror to be higher than the rest, exactly what was observed with Foucault testing above.

Artificial star test of 10"

Finally, an artificial star test shows a slightly irregular shape and rings, and lacks a sharp focus.  This is a proprietary test that I use with a laser-illuminated pinhole source.  It will show more interesting results for the next mirror, a thin 17.5" that has been mistreated, so stay tuned for that one.

This 10" mirror is within radius tolerance, and will be polished to a sphere, qualified interferometrically, and used as a test plate for a convex spherical mirror which will be tested against it in contact interference.

Please check back for future installements of "In the Shop".

Mike Lockwood
Lockwood Custom Optics

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