C014 testing an elliptical flat

testing an elliptical flat

The main mirror needs a large elliptical flat - ist's a Newton System, and you cannot buy that flat everywhere. This big flat looks very nice,
perfect overcoating. But does it works, what about the accuracy ? You can test that flat with a high quality sphere, made in England by
Orion optics. Diameter 250 mm and radius 2368 mm. You have to know the quality of that sphere, let's report about that in second step.

This shows the setup. This elliptical flat is fixed as it works later in an optical tube. As near as possible to the sphere you don't use the total diameter
of 250 mm.

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A foto of the tested flat .

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These are the dolorous results: The Ronchi test 13 lp/mm intrafocal shows an undercorrected shape, the fringes map also, that means, there is a radius
power on that flat. I'm sure, this flat creates no happiness.

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this is the reference interferogramm

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the wave front diagram

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and at last the poor result

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The sphere's quality

This mirror was made by Orion Optics, UK

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And this the results of the artificial sky test with that sphere at radius of curvature: the theoretical resolution and better.

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The Ronchi test 13 lp/mm intrafocal, single pass and the fringes map at 532 nm wave at RoC.

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For testing we just need the "diameter" of the elliptical flat from this sphere.

ellFlat_10.png

This shows the wave front of that smaller diameter.

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and this would be the result of the "smaller" sphere

ellFlat_12.jpg

At last I've tested the flat as the upper setup shows in a flat angle. If you test a flat in this way, there should be no radius on the surface that
causes astigmatism.

ellFlat_13.jpg

but this doesn't work in the optical tube. So the producer has a second chance to make it better. May be he will test it with a sphere like me.

ellFlat_14.jpg