Re: [ATM] Malacara, Flats etc... topic was "They come through for me, again"

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

On Wed, 21 Feb 2007, Francis J. O'Reilly wrote:

> just a piece of float glass will do.

Andy from this list once sent me the generous gift of a screw with
something like 100 turns/inch and this works fine for a spherometer
for any sagitta I have ever needed. If you consider your measurement
requirements for the initial rough ground surface, and take into
account some uncertainty in the following stages of fine grinding,
and then accept trivial deviations from the design specs (e.g., F/5.00
vs F/5.05), nothing is accomplished with an optical flat as reference.
Actually, if I need tight precision, I directly measure the ROC with a
pinpoint light source and a piece of paper next to it and measure
where I get the light at perfect focus. This test has never failed me.
If I am making a convex, then I use a glass tool and make the above
measurement on the tool. One way or other, I have never felt a need
for a flat for this for any of my surfaces from F/1.0 to F/8.

Now as for the Mallarca book (again, many university libraries have
it), I specifically recall it shows a couple of simple interferometer
setups that use (and require) optical flats. So far, I have managed
to get by OK without an interferometer and am curious what your
project is that leads to your proposed need for a flat?

Dominic-Luc Webb


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