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Testing a diagonal

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#1 wfj

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Posted 02 November 2018 - 02:07 PM

(Not to pollute rolo's 2045 thread further.)

 

It occurs that there are many ways to do this, some perhaps better than others.

 

Mirror diagonals are just like newtonian secondaries - so if you open up the mirror, you can place them against a flat to check for accuracy.

 

Prism diagonals also should have three flat surfaces as well in like kind.

 

After that, you're left with secondary lateral chromaticism if the glass isn't uniform in refractive index, or perhaps if the angle of a face is wrong.

 

If the glass is "swirled" or strained, it's non-uniform density will deflect parallel light, much like with air density in a Schlerien system - like when you do a Foucault test with your warm hand causing ripples.

 

Perhaps DPAC with the diagonal in front of the objective, with the mirror to the side "leg" of the diagonal would get that best. Perhaps use the objective dustcap with a central hole to support the diagonal with?



#2 rolo

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Posted 02 November 2018 - 09:47 PM

https://www.cloudyni...omparison-r2877



#3 starman876

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Posted 03 November 2018 - 09:03 AM

Yes you need a flat if it is a mirror diagonal.   I have been meaning to get a small flat just to do that very thing.    



#4 wfj

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Posted 03 November 2018 - 12:28 PM

Strip the reflective coating off a cheap Newtonian diagonal mirror.



#5 DAVIDG

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Posted 03 November 2018 - 04:00 PM

 If you use a  "cheap" diagonal mirror  as  the reference surface it needs to be flat to the accuracy you want to test the other surface too. For example a 1/4 wave surface can only show you errors no smaller than a 1/4 wave. Also you need to be able to see through the back of the diagonal to see the fringes. Most are either ground on the back or left "raw" with a rough surface from the mold.

   Your better off with something like this and wetting the fine ground back to see through it. https://www.surpluss...tem/PM1052.html

 

                      - Dave 


Edited by DAVIDG, 03 November 2018 - 08:59 PM.

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#6 dave brock

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Posted 09 November 2018 - 09:18 PM

Or just compare a star test with and without the diagonal.
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