Re: [ATM] Fringe testing Dall Kirkham secondary

["Rod Brackenridge" <rjbr@lycos.com>]

Hi Bob,

Thanks for that.  I would like to check I have understood you correctly.  I 
just tested again.  I started with the bulls-eye this time and pressed on the side and the number of fringes increased.  Pressing in the centre seems to give about the same number of fringes.  It certainly doesn't seem to increase.  Therefore the convex is longer and I should polish concentrating on polishing its edges?  I plan to work with the convex on the bottom, using a polishing machine to supply a 1/3 centre over centre stroke.  Does that sound right?

Rod.
 
> The first thing when trying to match two surfaces is to find out which is
> the smaller radius.  If you get the bullseyes and press on one side and the
> count of fringes goes down, then the convex is smaller (more of a gap at the
> edges) while the pressing at the center and the count goes down is the
> opposite.
> What you need to do is to polish the secondary so that the edges get more
> polishing and that will shorten the radius or polish the center more and
> lengthen the radius.  You will be in extasy when the two surfaces match - I
> did the first time I got there - and you will then be finished with that
> optical surface.
> Bob May
> bobmay@nethere.com
> http://nav.to/bobmay
> http://bobmay.astronomy.net

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