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Re: (ATM) Fergy's Mirror 2
>
> In message <DKw7+qfFYkA@banyan_abq3.aquilagroup.com>
> kferguson@aquilagroup.com writes:
>
> > I think I will just build a spherometer and be done with it. I have a good
> > 3/4 travel indicator which aughta be the ticket. I am thinking of having
> > three pins with rounded points, set on a 1" radius with the indicator in
>the
> > center. Is there some problem with this design? (too big/small radius??).
>
> Too small. I made a ring spherometer a bit bigger than that and
> got very misleading readings on a ten inch mirror. And very
> different ones on the matching tool.
>
Too small. That size will would work ok on a small lens with
steep curves. I have had good luck with the two footed spherometer
that is almost as large as the mirror. They are easy to make
and the math is simple. In my spherometers I add a third member
at 90 deg. from the center of the beam so the thing will set on the mirror
leaving both free. The third leg is 1/2 the length of the long
beam. A micrometer head or dial indecater can be used to measure.
Any flat surface accurate to .001" can be used to zero the spherometer.
On my 24" f4.5 mirror I was able to get the 18 ft RC to within .5 inch
using a 18" two footed spherometer.
The ends of the feet have ball bearings silver soldered to them.
The distance between the ball bearings of the main beam should be
measured as accurately as possible. The centering of the indecater on the beam
can be off several thousands.
<html><pre>
Top view
-------------------------------
foot O (O) O foot
-----------------^-------------
l l\
l l \
l l \ Indecater
l l
l l
l Ol foot
</html></pre>
See Advanced Telescope Making Techniques Vol 2 p100 for the
construction and math.
Bob Pfaff