[ATM] Spherometer calibration, precision, etc.

[Mike Lockwood <melockwo@uiuc.edu>]

Dominic,

Dominic-Luc Webb wrote:
> 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.

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.

> 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.

It works well.

For me, using a flat to accurately zero a spherometer is mainly a time
saver.  The flat doesn't have to be perfect, but better than half a 
wave is nice.

With an indicator reading to 0.00005", you can grind a flat to within 
a couple waves of flat before polishing, provided you can zero the 
spherometer on a known good flat.  The spherometer is transferred
(gently) from the flat to the work until you get an idea what the
difference in the readings is.  Even with a spherometer reading to
0.0001" you can read fractional movements of the dial and get an idea
of what's going on in a few minutes.

You can also read the ROC of a convex or concave mirror to a fraction
of a percent of the ROC.  This is accomplished in about 15 seconds if
you calibrate the spherometer base on known mirrors.  I made and
printed out a table of sagitta-ROC conversions for both convex and
concave surfaces for the spherometer base, and then I simply look up
the reading and find the resulting ROC in the table.

The spherometers that I use are shown here:
http://bi-staff.beckman.uiuc.edu/~melockwo/mirror_making/sphero/sphero.html

Some readings with a 0.0001" indicator on some good and bad flats are
shown in this page, just below the top:
http://bi-staff.beckman.uiuc.edu/~melockwo/mirror_making/four_flats/four_flats.html

> 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.

Agreed.  It's more of a convenience.

	Mike Lockwood

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