Re: Re[2]: [ATM] back to longer FL

["Bob May" <bobmay@nethere.com>]

I concur with the approximately 3% error.  The thing here is that you want
to make the two mirrors read exactly the same.  The needle needs to sit in
exactly the same spot as an a half of a mark movement of the needle will be
about 1.5% error between the two.
The .01mm dial lindicator is reading about .25 thousandths of an inch which
is marginal for this use.
I'd more build another spherometer base that is almost the full width of the
mirror and that way you'll gain a lot more accuracy in your measurements and
be able to read the less than 1% error requirement with a decent movement of
the needle of the indicator.
I use a 4" spherometer to just get an approximate measurement of what the
ROC is for a surface and I have a .001" dial indicator for the measuring
tool.
I did one mirror to less than 1/4" difference in ROC and that was done with
the spherometer to get close and then used the Foucault tester fo finalize
the ROC.  About half of the error budget was done due to the vagarities of
figuring the final surface.
Bob May
bobmay@nethere.com
http://nav.to/bobmay
http://bobmay.astronomy.net

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