Re: [ATM] 3 point cell for 12 inch ?

[Mark Holm <mdholm@telerama.com>]

I get from 0.40r to 0.41r depending on diagonal.  0.40r results from a 
diagonal diameter of 55 mm, 0.41r results from no diagonal obstruction 
at all.  Otherwise, my results are pretty close to Jeff's.  I agree with 
Jeff.  Build the cell reasonably carefully and make sure that the mirror 
is accurately centered on the cell.  If those two conditions are met, 
this three point cell should be fine.    A very nice thing about the 
three point cell, when Plop says it is good enough to use, is that it is 
easy to build one accurately.  Use pads around 12 mm in diameter, maybe 
a bit more.  That should actually improve the result a bit and make it 
less sensitive to small construction errors.

It is true, this result doesn't meet my 2.5 nm criterion, but Jeff's 
logic for relaxing that a little in this case seems sound.

Mark Holm

Jeff Anderson-Lee wrote:

> A 3 point support at 0,44r will have 21nm P-V and 3,9nm RMS error for 
> this mirror.  That's better than 1/60th wave on the wave front.  The 
> refocus change in focal length is 0.022mm, which about the same as the 
> focus tolerance of an f/4.8 mirror and likely less than the flexure in 
> the overall OTA.  Furthermore, you should not suffer a 2 to 4-fold 
> loss in performance due to construction errors because the cell is so 
> simple and the measurements are so large.  You have the added benefit 
> of no stiction, because there are no pivots.  It's well worth a try.
>
> Jeff
>
> Treur, Harro wrote:
>
>> Hello,
>>
>> I am rebuilding the mirrorbox for my 12,5" Pyrex f4.8 with thickness
>> 2"(51mm).
>> I got the advise that I should make 3 point cell.
>> But this type of cell I do not consider a Flotationcell, because 
>> there is no
>> rotational movement involved (like in a 6-point). Is
>>
>> And is a 3 point with 0,44r still a good choice?? (I am not 
>> experienced, but
>> 0,44r seems a bit small to me...)
>>
>> I would like this to be confirmed before I start cutting aluminum and 
>> steel.
>>
>> Harro Treur
>> Netherlands
>>

-- 
Mark Holm
mdholm@telerama.com


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