Re: [ATM] Fringe testing Dall Kirkham secondary

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Bob May wrote:
> Actually, when you have equidistant rings, you don't have a spherical
> surface but rather a surface where the outer area is of a longer radius than
> the center.

Agreed.  Judging the fringe spacking is tough.  Best to take the time 
to match to a known good sphere, IMO.

2.5 waves is not a lot to polish off of a small mirror (but it's 2.5 
waves at the edge, so this will take longer than if it were that much 
too high in the center).

How large is the secondary?

If the curve changes very slowly, you may need to use a more 
aggressive polishing stroke.  Be careful not to turn the edge.  Press 
often, or the pitch may resist the change in curvature, depending on 
the hardness.  Hard pitch will likely not work so well here.

Pressing in a petal lap is another option.  Once it is close, press 
the petals away and spherize and tweak.

Good luck.

	Mike Lockwood

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