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[ATM] Autocollimation Oil Test
I was asked to tell more about this testing setup and put it in the list
using a subject corresponding
more accurately the real subject
Well, here we go
The original post:
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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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Mirror support in flotation system upside down:
Upside down the levers and triangles ( as always ) must move sensitively and
without restrain. You can design the flotation system so that it
works in any position.
I meant it really that it can be difficult to get mirror out of Zell ( it
can also fall of so don't blame poor me)
Testing mirror over 400 mm in Diameter needs special care. Lifting big
mirrors high up needs special arrangements and safety( for you and the
mirror ) is priority one.
If you have high enough ceiling elliptical plane mirror is not necessary.
Small hole in center of the oil basin ( margin up of course) and you can
test in that point UNDER the oil plane ( I would have done this but did not
want to remove the whole ceiling.)
I had motorized collimation for the 700 (F4.4) . Three bars and two with
motorized screws.
I said that oil autocollimation setup is not difficult. Especially 'easy'
it is for those short Celestron and Meade Schmidt-Cassegrain
telescopes ( I hope the main mirror is not falling of)
Plumb-line and normal laser collimator (only mirror testing) speed up
collimation.
Just try it !!!
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