From: "Bob May" 
To: "atmlist" 
Reply-To: "Bob May" 


The accuracy of the Foucault test is from being able to measure the
differences in the ROC, not the absolute ROC.  Yes, there is an error that
happens when you don't have a good idea of the ROC but getting it to a
1/4" is more than sufficient for any but the most insane detail nuts. 
This accuracy is at the 1/2% of focal length to begin with for the typical
scope and is really quite accurate.  To make that start becoming a part of
the error budget would mean that the accuracy of the surface would have to
be better than 1/100th of a wave and that's not easy to do!  It is all in
where the accuracy is needed to be.  Remember that it is the difference in
ROC that determines the change of the slope of the glass, not the absoulte
value of that ROC.  It also allows for us to use simpler math for doing all
of this stuff - there is a set of math forumulas that does use the full ROC
but the numbers quickly become extremely large and small when using it.
Next is that the only optical aid needed is a telescope for those long FL
mirrors that don't subtend a large angle to the eye.  This is merely so
that you can measure the shade of the gray part that you are interested in
without problems.  For short FL scopes, you actually may want a negative
power telescope to do the viewing with as the contrast between looks best
at a particular spread of the zones.
As to the measuring equipment, the Dial Indicator (available from Harbor
Freight and Enco for under $20 for a .001" resolution and 1" of
travel instrument) will be just fine for doing both a Foucault tester and a
wedge tester for a refractor.  The Newport Glass webpages have a full
description of how to build and use a wedge tester. Bob May
http://nav.to/bobmay
bobmay{at}nethere.com
NEW! http://bobmay.astronomy.net

--- BBBS/NT v4.00 MP