November 25, 2011:  Not up to my standards - 16", Part 1

Well, this example shows quite nicely what a mediocre mirror is.

The client complained of..... well, he didn't complain, but he said the mirror was merely "OK".  He wanted a better idea of what was going on.

First I tested the elliptical flat, which turned out to be quite good.

Next came the primary.  Foucault testing showed a good amount of roughness on the optical surface, as shown in the images below.  Also seen is a slight zone near the center of the mirror, but likely behind the secondary's shadow.

The "roughness" is seen in the shadows - they look blotchy, and "not smooth" in general.  The mirror appears to have "texture" over most of its surface, and this is the roughness.  It may reduce the image contrast in critical observing, and also can affect test results when high precicion is desired.  Roughness is observed over most of the optical surface, and was likely caused by a heavy-handed optician, working in a hurry with a hard pitch lap.  Edge condition was OK, but not perfect, with a brighter diffraction ring on one side of the mirror than the other, though this is quite a minor defect even for a very good mirror.  The figure and roughness are of much greater concern.

Center zone~50% zone
Outer zones

Further testing showed the mirror to have a very good figure of revolution, but the figure was at best 1/4-wave on the glass, or 1/2-wave on the wavefront.  I know from experience that this is far too much error to be caused by the surface roughness affecting the Foucault readings.

The error plot below shows the error in the figure, as measured with Foucault testing.  It generally looks like overcorrection.  Also note the transverse error of 4.16 (slope error relative to the airy disk size), which indicates stars will definitely not be at their sharpest.

Error plot for rough 16" mirror

Obviously, refiguring and smoothing work is recommended, to which the client agreed.  It will be done when I can work it into my schedule.

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

Mike Lockwood
Lockwood Custom Optics

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