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14.7" f/3 Dob/RFT

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#51 Oberon

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Posted 03 December 2019 - 02:46 PM

Doesn't that require a big expensive flat, or is that a different test?

It requires a bath, a liquid, and a gravity. You can use a cheap pan, oil, and the gravity comes free.



#52 Pierre Lemay

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Posted 03 December 2019 - 03:08 PM

Doesn't that require a big expensive flat, or is that a different test?

The Bath and DPAC are different tests. Bath is a simplified, common path interferometer test. You can learn more about this test, and building a tester, in this thread: https://www.cloudyni...-at-the-bottom/  . Using free software to interpret the data it will provide a quantitive, 3D mapping of the optical surface under test. 

 

The Double Pass AutoCollimation Test (DPAC) is one which calls upon the use of an smooth optical flat. See this thread for more details on how it works and is conducted: https://www.cloudyni...4046-dpac-test/ . Unlike the Bath, it’s more qualitative than quantitative but, if the optical flat is smooth, DPAC is capable of revealing 1/13th wave peak-to-valley geometrical defects on the mirror surface instantly, without having to resort to any software or measurements. It can also be used to test complete optical tube assemblies, wether Newtonian , refractor or compound instruments. 


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#53 stargazer193857

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Posted 03 December 2019 - 03:32 PM

Steve Dodds uses that test. I bet flats that size are expensive.... If you want to test for spherical undercorrection. If you only are looking for lumps and bumps, I wonder if smooth power would be acceptable.

#54 a__l

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Posted Yesterday, 12:47 AM

First of all you can determine astigmatism. From the stand (mirror cell) or from the mirror.

Price tag: small lens, flat mirror and beam splitter.

Laser, camera and X-Y-Z table. Plus open software (Dale)

Read what Dale et al writes.

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Edited by a__l, Yesterday, 01:11 AM.


#55 BGRE

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Posted Yesterday, 01:39 AM

Just don't try to make accurate measurements using that exact setup unless you're into rigorous diffraction calculations etc.

Either use a lens to image the test surface onto the screen or use a camera focused on the test surface.

 

Someone actually tried the depicted fringe projection setup using a camera to capture the fringes on the screen. 

The results weren't accurate due to diffraction and other effects.

When he was persuaded to use the camera to capture the interferogram directly, the accuracy and consistency improved.

The camera views the test surface through the interferometer and is focused on the test surface. 

 

For large fast test surfaces imaging distortion and other effects can be significant. These effects aren't limited to Bath interferometers.


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