Re: [ATM] Mirror cell for an 8" mirror

["Olivier Biot" <olivier.biot@gmail.com>]

I'd strongly advise against the traditional (symmetrical) 9 points
cell as (1) its performance is usually worse than a 6 points cell and
(2) positioning the fulcrum of the (very flat) support triangles is
critical due to the natural imbalance between the weight carried on
the inner and outer points.

I ran your mirror thru PLOP with a classical 6 points cell, and here
are the results:
P/V error: 3.83nm
RMS error: 0.61nm
Radius of the 6 support points: 57.13% = 58.3mm

For a comparison, the traditional 9 points cell yields the following
error on your mirror:
P/V error: 8.07nm (2.1x worse than the 6 points cell)
RMS error: 1.62nm (2.7x worse than the 6 points cell)

If you allow the forces to vary between the 3 inner and 9 outer
points, then the 9 points cell still performs worse than the 6 points
cell. However in this case the 3 inner points only carry 21.8% of the
weight of the 6 outer points (!), which means any manufacturing error
gets severely magnified by the 1:5 force ratio.

Bottomline: adding points to a cell doesn't necessarily increase its
performance.

There's one 6 points cell I "fell in love with" ever since I saw it.
Two designs were made by Albert Highe, and are available from his
website (I hope it's OK that I post these links):
Cell 1: http://ultralightscopes.com/images/CellAsm.jpg
Cell 2, top: http://ultralightscopes.com/GrabNGoMarkIV/MirrorCell28AC2H.jpg
Cell 2, bottom: http://ultralightscopes.com/GrabNGoMarkIV/MirrorCell35AC2H.jpg

Cheers!

Olivier

On 11/2/07, Corona Australis <corona_australis@westnet.com.au> wrote:
> Thank you everyone for your feed back. I really apreciate it. I
> have heard of PLOP, but dont know alot about it. Will it run
> on an IBM clone running Win XP? can I down load it from somewhere?
> is it easy to use? sorry for all the questions
>
> Ok, I'm building a kriege style open back tailgate to build the
> mirror cell on. At around 40% out it looks to me there is alot of
> mirror hanging out there. I know this may sound overkill 8" how
> about a 6 or 9 point cell for a bit more stability. I was also
> thinking if I did a 9 point cell I could used the velcro idea
> Kevin suggested and not worry about how I was going to hold it
> there.
>
> Can I ask a kind soul to run a nine point cell for the same mirror
> through PLOP for me? pretty please  : )))
>
> here's there measurements again, F/6, 204mm neat in diameter and
> is of a thickness of 23mm
>
> A little more info...
>
> The scope is for my young sons, one is nearly 7 and the other is a
> little over one years old, although I dont see him using it for a
> few years yet. I dont mind the extra metal in the cell. As there
> is a lot of weight in the secondary cage to counter at the bottom
> end, although I think I'm pretty much on top of that. I'm also
> building it this way for a little bit of insight into the truss
> design, and cant believe how much fun it has been to boot.
>
> Thanks again
>
> Regards,
>
> Ozzy
>
> > I haven't run Plop for this mirror exactly, but many runs by myself and
> > others have shown the optimum for 3-point cells to be right around 40%,
> > so I agree with Olivier.
> >
> > --
> > Mark Holm
> > mdholm@telerama.com
> >
> > _______________________________________________
> > ATM mailing list http://www.atmlist.net/
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