[ATM] The 15.5 Ultrathin Project, Astigmatism discussion

[vorblesnak@peak.org]

Dale Eason wrote:

> Concerning your astig seen in the ronchi images.
> Perhaps some of the astig is due to self weight of the
> mirror while on the test stand.  Have you tried
> rotating the mirror 45 to 90 deg and seeing if astig
> rotates with it?

An excellent suggestion.  I have rotated the mirror and it does not affect
the astigmatism.  The general orientation is always through the vertical
axis.  That is, the astigmatism does not rotate with the mirror.  However,
there are axis where no astigmatism is seen, though there are other issues
there.

> The reason I ask is that I'm astounded by how much
> astig is induced by the test stand on a 18 x 2 inch
> mirror that I measured with the interferometer.  Your
> supper thin mirror must have much more of that
> problem.  Perhaps so much that it shows up in images
> like it does.

Part of the lure of the thin slumped mirror is both in the shape and in
the weight.  The curved / shield shape of the glass adds strength and the
weight is greatly reduced.  The weight of the 15.5 for example is 3.5
pounds.

Thanks for the tip, loved your post on heat induced deformities.

David Davis
Toledo, OR 97391

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