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Re: [ATM] The 15.5 Ultrathin Project, Astigmatism discussion

Testing Ultrathin.

I have tested with grate success mirrors up to 700 mm in Diameter using oil 
as reference flat. You can even use your favorite salad oil.

The mirror is upside down and the height of the testing setup is  'only' 
about focal length of the mirror.

The mirror is supported in its flotation system upside down.    The 
flotation system needs a little modification.
Doublesided tape functions perfectly in contact points( in this case the 
points should be small ' areas' . Non contacting safety clips should  be 
used ( as usual )  at the edge of the mirror.  The problem is merely getting 
the mirror of from the flotation system after testing  and not the falling 
of the mirror during  testing.

There is reflection from the mirror and from the liquid surface and 
depending on the setup an additional reflection from aluminized secondary 
mirror. Just like Newton telescope used at the zenith but now upside down..
More light is needed than in normal testing but 1mW laser(Diode ) or other 
bright source is enough
I use 20 mW green laser for interferometer testing and it is enough for four 
uncoated reflections.

Collimation is easy with laser collimator.

This testing setup is indeed  not difficult.

Consider the advantages:

This is perfect NULL test.

The distortion of the mirror in testing can be calculated and is just like 
predicted with PLOP
No more distortions like in sling testing.


If you want to try this  testing setup I recommend that you start taking 
small Newton telescope
( Or any telescope or objective ) and  just turn the whole telescope upside 
down ( modify the mirror zell first  !!!) and oil vessel under the 
telescope. Now the whole telescope should be adjustable with three screws to 
get it perfectly vertical and in a moment you have excellent undistorted 
null test for all your telescopes.


Martti Koskimo


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