Re: [ATM] Ain't got no shperometer

[David Adams <elrey314159265359@yahoo.com>]

A sphereometer tells you how deep your sagitta is, or, using a simple calcualtion,
what your ROC would be if you had a sphere.   It doesn't tell you how smooth 
your surface is, or what defects lie there, or if you actually have a paraboloid
or an ellipsoid or on and on.   By moving the sphereometer around you can 
find zones with different radii of curvature which would provide you some information
on lack of sphericity, but that is nothing compared to the information you 
get from the sharpie test.  If the sharpie marks disappear uniformly you know 
that your surfaces are mating to a tolerance that is as small or smaller than 
your grit size.   The only surfaces that can mate perfectly under random movement
like that are 1)  a sphere or 2) a plane.

You might use the sphereometer to look for holes or hills or a turned down edge,
but the sharpie test ought to be showing you the same thing, probably quicker
and easier.

Yes, the mirror edge is another good point.  The sharpie test gives you information
right up to the edge.   If you have a turned down edge and you are using 
a sphereometer one foot should dip as you get to the edge so you should 
detect some movement there--so I'm not saying the sphereometer is useless.

So in summary:  Sharpie test gives you a quick visual sense of where the 
surfaces are mating.  A spherometer can be used to hunt and seek for
irregularities.  Sharpie cannot quantify how far off you are where the spherometer can
to some degree.

You still need the spherometer (or a sagitta tester) to tell you if you have gone deep enough, 
unless you don't really care what your final ROC is as long as it is a 
quality sphere.    

Yes you can do sharpie with finer grits.  (I use pencil marks, BTW.)
At finer grits the spherometer use becomes more problematic.
And you have to be more careful about the spherometer feet and 
probe scratching your mirror.

Once you begin polishing you want to be using Ronchi.   Ronchi is 
much easier to set up and use than Focault, so save the Focault
for later on when you need more detailed information.

Here I am talking like a pro.  I'm not.  So take that for what it is worth.
I have only ever done one mirror (a 16.5 inch f 5.68) which I expect to
add the final finishing touches tomorrow night and then off to have it
alumninized.   I AM SO JAZZED!  (This is the result of the 2006-2007 class that was
offered by John Ziegler in Eagle Mountain, UT.)

I made some spherometers for some of the members of this class, and 
have some extra spherometer bases, and maybe some dial indicators
to go with them.  Contact me off list for more info if you are interested.

David

----- Original Message ----
From: ArtfulBodger <artfulbodger@earthlink.net>
To: David Adams <elrey314159265359@yahoo.com>
Sent: Tuesday, November 27, 2007 10:53:05 AM
Subject: Re: [ATM] Ain't got no shperometer



Michael, David, um ... a spherometer isn't good at testing 
sphericity?  How come?  Do you mean the fact that it can't check near 
the edge of the mirror?  I'm guessing that that's what you have in 
mind, but maybe not.  Could you (or someone) elaborate, please?

David, yep, Foucault would do the job; but I only have access to a 
Foucault tester on Saturdays.  Gotta build one of those, too: it's on 
The Other List, the scary list....

About the Sharpie test, I can still do it with the finer grits, can't 
I?  25u, 12u, and like that?  I guess the mirrors should be spherical 
before then, and they were, but I'd like to check 'em again (and 
again).  Make sure they're good & round before I start polishing. 
Can I do that?

Pete

-- 
Artful Bodger
http://www.artfulbodger.net





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