From: David Lewis 
To: Marco Miglionico 
CC: Mark Holm , ea8yu{at}arrl.net, atm{at}shore.net
Reply-To: David Lewis 


"All this effort would be wasted", you say? With a 1/58 wave P-V
and a 1/260 wave RMS error due to the cell, how is your effort wasted?
These numbers are my results for f/5 200mm diam 25mm thick with 40mm
obsctruction.

David Lewis

Marco Miglionico wrote:
>
> Three points would probably be adequate for 25mm thickness (see plop plot).
> However this may satisfy Raleigh's criterion, it does not satisfy the
> requirements
> I would put on a planetary scope having a diffraction limited performance.
> If  I  had gone to the trouble of producing a mirror with a wavefront of
> accuracy 1/8th to 1/10th wave which is considered real diffraction limited
> performance, all that good work in parabolizing (theoretically) would be
> wasted and at the mercy of your three point cell.
>
> Lets say that the optical worker by, as you say luck, or skill does produce
> an excellent figure, maybe one night at the eyepiece for a fraction of a
> second the seeing steadies, then the 9 point cell will come into its own.
>
> Personally I think you are right Mark. I've never been able to tell the
> difference
> when I mounted my mirror (8.75in 25mm thick) on 3 or 9 point cells. But it
> is not exacting or complex  to make the 9 point cell so what is there to
> lose?
>
> Lastly 25mm glass is 1.95 times stiffer than 20mm glass. There is no way I
> would put
> a 20mm thick (8in dia) mirror on a 3 point cell.
>
> ----- Original Message -----
> From: Mark Holm 
> To: ; ;

> Sent: Thursday, May 08, 2003 3:39 AM
> Subject: Re: ATM First mirror prjoject, how thin?
>
> > Responding to Goran Hosinsky
> > Marco Miglionico wrote:
> >
> > > Don't go thinner than 25mm.
> > > Even mount this thickness of glass
> > > on a 9 point cell for planetary viewing.
> > >
> >
> >
> > Goran,
> >
> > At 25mm thick and 200 mm diameter, a three point mounting with the support
> > points located at 0.4 r  (40 mm radius), will be plenty good enough.  (I
> am not
> > being condescending.  It would be good enough for even the most demanding,
> but
> > still rational, ATM.  Even a professional would be satisfied with it,
> unless
> > perhaps he was going for diffraction limited work in the far ultraviolet!)
> This
> > is a result calculated by Plop.  See
> > http://www.telerama.com/~mdholm/atm/cells/index.html
> >
> > You can get that 1/8 wave, but it isn't always easy.  Some folks are
> either
> > skillful or lucky and seem to get there with not too much trouble.  Others
> (like
> > me) go through a long procession of ugly figures before arriving.
> >
> > You can go thinner than 25 mm.  I haven't done it, but many others have.
> 20 mm
> > should be doable in 200 mm dianeter, though that might be as thin as a
> beginner
> > should go.  If you can get 25 mm thick, it will be a little easier and you
> won't
> > have to be quite as careful.  Even at 25 mm, it is still quite possible to
> grind
> > in significant astigmatism if you don't take the standard precautions.
> >
> > More important than thickness is the uniformity and annealing of the
> glass.  A
> > well annealed, uniform 20 mm blank is vastly better than a poorly annealed
> 25 mm
> > one.  A lot of the glass in this world is not well annealed by optician's
> standards.
> >
> > You have some first rate observatories in the Canary Islands.  Do any of
> them
> > have a local optical shop?  If so, then they must get supplies.  Perhaps
> they
> > would be willing to add your order to one of theirs.  (Of course you pay
> them.)
> >   I am thinking that the shipping, and perhaps handling charges will be
> lower if
> > your order is part of a larger shipment.
> >
> > Mark Holm
> > mdholm{at}telerama.com
> >
> >
> >
> >

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