[ATM] Parabolization

[Jerry]

Hello Bob,

Yes, and if you look around the entire edge of the mirror, some parts such
as to the left and to the right are not in contact with the lap almost the
entire stroke. Average contact time for the edge during stroking is much
less than half the contact time of the mirror's central area.  No wonder it
takes so long to get the edge polished with normal strokes.

Jerry 

> -----Original Message-----
> From: atm-bounces at atmlist.net [mailto:atm-bounces at atmlist.net] On Behalf
> Of Bob May
> Sent: Sunday, May 09, 2010 12:46 AM
> To: atm at atmlist.net
> Subject: Re: [ATM] Parabolization
> 
> Jerry, that is a fairly good explaination of what is going on.  I
> might add in the note that wherever you rub, that is where the
> material of the mirror is that you remove from the mirror.  Thus
> the MOT tends to make the ROC of the mirror larger and TOT tends
> to make the ROC smaller.  I've had grinders move the ROC of the
> mirror in polishing by more than 2" as they tried to parabolize a
> mirror.  Remember that the ROC of a parobilized mirror can be
> 0.2" different between the edge and center on a short FL mirror!
> Polishing the center area of a mirror will quickly shorten the
> ROC of the center and since the edge isn't worked as much as the
> center, the dreaded TDE can appear as the edge fails to keep up
> with the center's ROC change.
> Most of us don't recognize that this is part of what is happening
> because the Foucault test isn't kept setup and ready for the next
> test and as a result, don't notice the change of ROC as you keep
> on chasing the TDE problem.  When you do keep the test setup
> aligned and ready, you will notie that the ROC is slowly changing
> as you have to realign the ROC of the setup.
> Bob May
> 
> rmay at nethere.com
> http: slash /nav.to slash bobmay
> http: slash /bobmay dot astronomy.net
> 
> 
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