Re: ATM Mirror Evaluation (was: Getting close - figuring 21"f/6)

[Aplanatic@aol.com]

Hi Ken, Frank, Nils:

Ken, you wrote:

> Dave said:
> 
> > 4) It is simply not worth the time to try 
> > to get the edge perfect.  Start
> > with the knowledge that you're going to 
> > mask off 5 mm all the way around.

> Why not put a larger than normal bevel on 
> the mirror before figuring? That
> way the turned down edge will be 
> off the surface of the glass and of no
> consequence when you are done!

Unfortunately that doesn't work.  The edge turns down because of the glass-air discontiuity.  You need the glass there during figuring so that the TDE doesn't move even farther inward.

Frank, I'm not sure that I got your point, but let me make some comments anyway.  Some folks strive for perfection and I find that to be a most admirable pursuit.  If you get joy from making a parabolic mirror perfect to the very edge then I commend and support you in this interesting and difficult task.  On the other hand, I am a pragmatic fellow that likes to make things work well as fast as possible.  It has become clear to me that some minor masking or baffling of the edge of a parabolic mirror is by far the quickest way to my end goal.  (Goal is excellent optics with the minimum amount of time invested.)  I always have several interesting projects lined up and never enough time to get to them all.

Nils, thanks for the diffraction simulations.  Are you agreeing with my assertion that one should mask their mirror at the point where the outer diffraction ring is still dimly present?  From your simulation this looks to be the case.

Dave Rowe