Re: [ATM] Spherometer calibration, precision, etc.

[Dominic-Luc Webb <dlwebb@canit.se>]

On Thu, 22 Feb 2007, Mike Lockwood wrote:

> Well, I agree in concept, for most ATM mirrors, but not in the case of
> a Cassegrain secondary.  Extra precision here, before polish is begun,
> is advantageous.

Admittedly I do not know much about making Cass secondaries. Can't
this also be measured by testing the ROC of a glass tool with my
pinpoint light source and piece of paper? I recall once a long time
back going through the math and deciding this test was more precise
than a typical ATM spherometer. I can certainly get the ROC of my
more conventional concave mirror surfaces accurate to a couple mm,
and assumed a Cass secondary ROC was also flexible to within a couple
mm, presumably the position in the optical system could be adjusted
to accommodate the exact ROC obtained. As I understood it, the goal
is a particular type of curve (depending on type of Cass), not a very
exact ROC. The ROC actually obtained simply dictates the
amplification, which I thought is somewhat flexible. If your design
is for a hyperbola that is ROC 480 mm, will it actually matter if
the ROC comes out 479, 480 or 481 mm?

Dominic-Luc Webb

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