Re: [ATM] Posible variation on the Cassegrain secondary

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

vladimir sacek wrote:
> If the design bfl is ~300mm, that is quite a bit of
> secondary ROC deviation (~10% or somewhat more).
> As someone with zero hands-on experience, I wonder
> what makes controlling the ROC to within a couple of percent during the 
> figuring difficult?

Controlling curves on a small mirror can be tricky.

First, you're dealing with a small mirror, and that makes spherometry 
a little trickier.  It's easy to have the curve off slightly during 
fine grinding.  (I'm just speaking from experience here, not trying to 
question Jim's procedures.)

However, for me it's not the figuring, it's the polishing that screws 
things up.  The curve often changes significantly just while removing 
pits.  The ROC of the secondary can change significantly during 
polishing, and in my (and apparently Bob May's) experiences it likes 
to go longer (less convex).  Then it's tricky to go shorter (more 
convex) to get it back to where it approximately matches my test 
plate.  I'm talking a change of ~10-15 waves, an amount I can easily 
detect with a sperometer and a good indicator.

Problems compound when you calculate just how sensitive a cassegrain's 
focal length is to movement of the secondary mirror or changes in its ROC.

	Mike L.

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