Re: ATM "Flats" for Autocollimation Null Testing

[Ric Rokosz <bz737@freenet.toronto.on.ca>]

On Mon, 23 Nov 1998 DROWESMI@aol.com wrote:
> 
> Now, the real questions.  A)  Is it possible to maintain the spherical surface
> during polishing?  B) How can the surface accuracy be tested?  If the surface
> of the AM is truly spherical and has a very long radius of curvature, then it
> works fine in an autocollimation null test.  On the other hand, if it picks
> up, during polishing, a turned edge, a hole in the center, or any one of the
> numerous other possible defects, then it will not work as an AM.  How can one
> tell?  Is this the real reason that people make flats for autocollimation null
> testing?  Because a flat can be tested against two other flats, cyclically, to
> quantitatively determine flatness?


Having made a 12 inch flat -and flatness can vary as in plus or minus 
concave or convex flatness that can range from 1/2 to 1/1000000000000 
wave- the way I tested mine was by using a long focus 6 inch f/25 sphere 
as detailed in Texereau.You test for the sag and tan cut off and measure 
the difference.My flat has a central hole and the edge around it is 
turned and can be seen in the auto setup but you just ignore it.You can 
have  the surface full of say circular ripples and the "flatness" is 
still useful as long as the overall correction is in line.This has to 
do with the offense against the sin law as first outlined by Burch and 
the resultant formula he used,and as you saw comes out in raytracing aswell.


Ric