August 1, 2011:  Different tests for different defects - a 17.5" mirror

The results here are a bit different than for the previous 10" mirror.

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

This mirror is a rather thin 17.5" with good, flat back.  Its figure is close to a sphere (well at least closer to a sphere than a parabola).

Foucault test of 17l5"Foucault test 17.5"

Foucault tests above show a reasonably smooth figure and decent edge, although not perfect in either regard.  Some irregularity is visible in the non-symmetric nature of the shadows, but this could be due to the thin mirror itself.  More tests are needed to see what's going on.  We could rotate the primary and see if the shadows change, and that would help rule out test stand sag.

This is a very good time to pull out the interferometer and artifial star test to get a better ideal of what's going on, so let's do that, shall we?

Interferogram of 17.5"Interferogram of 17.5"

Yikes!!!  The interferograms tell the tale.  Note the triangular fringes in the image above at left, cleary showing trefoil is present.  It is likely that this mirror was pinched or clamped during previous attempts at polishing or figuring.  Note the non-parallel fringes in the image on the right also betray this defect, but in a different manner.  Analysis of the image on the right yields the contour plot below, which clearly shows three high regions.

Artificial star test of 17.5"

Artificial star test of 17.5"Artificial star test of 17.5"

Finally, an artificial star test shows an obviously triangular shape that is impossible to miss.  This is a proprietary test that I use with a laser-illuminated pinhole source, and which is very valuable for testing figure of revolution after polish and after figuring.

This test is also superb for testing the edge support of mirror cells while the primary mirror is nearly spherical (just after finishing polish).  If offer a mirror cell edge support testing service for mirror that I am making, and often work with JPAstrocraft on this.  They make the cells, and we test and adjust the edge support for best performance after careful testing in my shop.  We have seen triangular images like this in large telescopes and have fixed the problem.

John Pratte (JPAstrocraft) and I will be collaborating on and testing 40", 42", and 45" mirror cells in the future.

Clearly this 17.5" will need a lot of polish time to get it back to a good figure of revolution.  Then it can be figured.

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

Mike Lockwood
Lockwood Custom Optics

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