Re: [ATM] Grinding Circles ( Making the pitch tool)

["John Stephen" <atmlist.net@johnstephen.org>]

> "Grind in haste; repent at leisure".  But surely now you will nail the 
> ROC.
> After that, figuring your 8 inch f/4.000 to Strehl 99 will be a slam dunk.
> Your Foucault tester and mirror support are rock-slide steady, right?

Yeah, they're solid.  I built the ones described on the Stellafane site, 
though I modified the tester slightly due to lack of availability of those 
funky bell clamps they used and I also modified the test head to be narrower 
and include a brightness control for the LED.  I'm pleased with how it came 
out considering how little woodworking I do and how crappy some of my tools 
are.

> Is that the same spherometer that would not render consistent readings on
> your first mirror grind, the plate glass?  Or was that a different thread
> and project?  I wonder if the beginning letters of 'assume' have the same
> connotation in other languages as they do in English?

It is indeed the same one, though I think the inconsistent readings had more 
to do with the glass itself and that I was testing the mirror during very 
coarse grits.  I also was *somewhat* unfair in blaming the spherometer.  It 
isn't a professionally made instrument and last night I worked out the math 
and discovered that the guy who made it was only off by 0.033" when he 
drilled the holes for the posts (it's one of those three-post kinds). 
Having measured the actual focal length and actual sagitta of my original 
mirror, I calculated the correct diameter/radius for the spherometer and 
then used Excel to make a new chart to convert readings to ROC and FL.  The 
new chart predicted the focal length of my mirror-under-construction 
correctly so now I'm confident that I can use the spherometer to get the 
curve right this time.  I'd never considered how such a tiny amount of error 
in the construction could lead to such a large error in the mirror.

> I am not stupid, and caught on to the laughter real fast, though I had
> trouble joining in.  Due to the hecklers, though, most of the students
> probably missed the fine points of my invention.

But your A.S.S. will go down in the anals (sic) of history :-)

- John


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