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Re: [ATM] Commercially available spherometer withMitutoyoDigimaticIndicator
Vladimir,
I don't think interferometry is a good idea for just taking quick
measurements between wets on grinding. I use a spherometer that is
considerably smaller than the mirror, and take multiple readings from
multiple positions, and draw a mental picture of the sagitta. I'm using
that information to make the determination if my next grinding session
should be MOT or TOT. That's all. The greater precision is desired just so
I can get closer to the desired focal length.
I would reserve interferometry for the end of fine grinding and polishing.
FYI, I've read their site several times, and am VERY interested in building
my own interferometer. It is on the agenda.
Regards,
Terin Lyr D'Amico
Camarillo, CA
-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Vladimir Galogaza
Sent: Wednesday, February 22, 2006 3:22 AM
To: ATM List
Subject: Re: [ATM] Commercially available spherometer with
MitutoyoDigimaticIndicator
Teryn,
>Any thoughts on a useful range of diameters?
I have some thoughts about spherometers in general.
Very high precision of spherometer digital gage micrometer is illusion
because what you are measuring is mutual position of plane defined with mere
3 points (in case of 3 legged design) and gage tip. What the rest of mirror
surface is like can not be deduced from this one measurement. You need
plenty of them and make mental picture ( I guess that even Mel will have
hard time). In this process the digimatic accuracy is gone, lost, and mostly
not required.
Diameter of spherometer defines how big part of the mirror surface can be
examined in one go. In extreme case when diameter of spherometer and mirror
are the same, only center of mirror is accessible for measurement.
If spherometer diameter is half of mirror's then only central half of the
mirror is accessible and accuracy is smaller. And so on. To measure great
part of the mirror, spherometer diameter should be small accordingly and
accordingly les precise measurements are obtained. Other methods to measure
curvature, though simple and not using high precision gadgets, are
sufficient for grinding phase. Those are Sharpie test, spit test,
curvature template method, to name some.
When it comes to polishing then required accuracy is so much greater than
possible with spherometer that only Foucault like tests or interferometry
are practical. Instead of building very nice spherometer with expensive
parts not to speak of buying one, it is far better to invest money and
effort into building interferometer.
Components are available (www.Ceravolo.com) or Bath variant for much less
than horrendous figures mentioned for spherometers or differential
micrometers (euro 400-500+).
Spherometer will be used (useful) in first phase of mirror making which is
not critical at all. Interferometer will serve you for life in much more
demanding situations where ATM really comes to its forte.
Just an opinion. More experienced should speak.
Regards
VLadimir.
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