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Complete Newb Vintage Optics Testing

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#1 Compressorguy

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Posted 25 January 2013 - 10:49 PM

at testing optics but always thought this was pretty cool. So I bought a tester and here is my first attempt at setting up and testing the primary from my vintage ToHo (Eikow) 114mm. It has a alot of very fine surface scratches from previous owners cleaning and needs a recoat any way, so I did not mind pulling it. Here it is set up in the stand.

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#2 Compressorguy

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Posted 25 January 2013 - 10:51 PM

The Foucalt Ronchi tester.

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#3 Compressorguy

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Posted 25 January 2013 - 10:54 PM

The set up with the ToHo and a 4" Uni in the background.

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#4 Compressorguy

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Posted 25 January 2013 - 10:59 PM

My very first vintage mirror Ronchi test. Success!! Like I said complete newb to testing and reading the results anyone care to evaluate?? I cant wait to knife egde test to view the mirror figure. This stuff is so cool!!!

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#5 Compressorguy

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Posted 25 January 2013 - 11:01 PM

One more

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#6 hottr6

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Posted 26 January 2013 - 09:19 AM

Well done! It seems to me that the Ronchi lines at the radius of curvature of the mirror (twice the focal length) are straight, indicating a spheroid. More experienced ATMers can chime in here.

Next step for you is Focault testing.

#7 Mirzam

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Posted 26 January 2013 - 09:38 AM

To me it looks like the lines are slightly bowed inward, indicating a small amount of correction (i.e. slightly flatter towards the outer part of the mirror as a paraboloid would be). Whether this is a good thing or not would depend on the f-ratio. It may be easier to see the curvature if you adjust the screen to show only 3 lines.

The edge of the mirror looks a little bit suspect--there may be a narrow turned edge. Again, 3 lines would help to show this and a larger image scale would also help.

JimC

#8 DAVIDG

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Posted 26 January 2013 - 11:13 AM

To me it looks like the lines are slightly bowed inward, indicating a small amount of correction (i.e. slightly flatter towards the outer part of the mirror as a paraboloid would be). Whether this is a good thing or not would depend on the f-ratio. It may be easier to see the curvature if you adjust the screen to show only 3 lines.

The edge of the mirror looks a little bit suspect--there may be a narrow turned edge. Again, 3 lines would help to show this and a larger image scale would also help.

JimC


Jim's analysis and advice is perfect. We also need to know on which side of focus the pictures were taken on.
For spherical mirrors the Knife/edge Ronchi is the perfect test since it is null test. Nothing to measure and the mirror should show a perfect null when knife edge tested and dead straight Ronchi lines with 3 showing across the mirror when the mirror is a perfect sphere. For parabolas and refractors a better method to test is double pass autocollimation using an optical flat. The flat is set up in front of the telescope and the tester at the focuser. Light passes thru the telescope twice so any errors are doubled making it a very sensitive test. It is also a null test, so when you test with a Ronchi screen your looking for dead straight lines. Any departure from straightness is direct indication of errors and there is nothing to measure, unlike a Foucault test. It is also a great way to test refractors.
A word of warning thou, you may discovered that many of your prized vintage optics aren't as good as you think and when you do find one that test very well, you'll soon seen the error in the images of the others scopes that you once thought were extremely good. You also find that many of the Legends in Optics weren't anywhere as good as has been stated.
Happy testing.

- Dave

#9 roscoe

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Posted 26 January 2013 - 06:17 PM

I'm assuming the green thing is the LED, the round dark thing is the ronchi screen, and the black rectangular item is a battery box, but what are the two blue-and-white items?

And I like the 'life' game as a stand!!

Russ

#10 Compressorguy

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Posted 27 January 2013 - 06:39 PM

Thanks all for your analysis and comments. This is such a new learning curve but very interesting. I know the set up is crude but just setting this up and actually seeing the ronchi lines appear on the mirror was very exciting. I know, I know, it doesnt take much. My wife did not share my facination either and all I got from the kids was a passing "that's cool". I will set this up again with 3 lines and verify inside or outside focus. If I had to guess, I would say I was outside of focus. The focal length is 1200mm (f/11). I also noticed something on the extreme edges (right and left side) of the mirror, but no sure of what I am looking at yet.

Hi Russ, those are on/off switches for the green and yellow lights.

Here are some more detailed photos of the tester setup. Maybe you can comment on how to better photograph the image. It is very difficult through a 1/4" hole. Can the viewing hole be larger as long as the ronchi grating covers it? Also, any good mods/changes to this setup you could recommend?

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#11 Compressorguy

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Posted 27 January 2013 - 06:44 PM

The rochi grating is fixed to a steel washer and the whole assembly is held in place by a strong magnet glued on the other side of the card.

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#12 Mirzam

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Posted 27 January 2013 - 06:58 PM

I have not really mastered the photographic challenge involved in these sorts of pictures. I do know that using a tripod is helpful. The hole size doesn't matter as long as the return beam is covered by the screen.

Pictures taken slightly outside of focus (3 bars) would be best. If you can't get a larger image scale then simply cropping the image before uploading to CN may be okay.

JimC

#13 DAVIDG

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Posted 27 January 2013 - 07:49 PM

Scott,
To tell which side of focus your on, push the grating/tester toward the mirror. If the number of lines visible on the mirror decrease, your on the outside of focus and moving toward the exact focal plane which with be when one line covers the complete mirror. If you push the grating/tester toward the mirror and the number of lines visible on the mirror increases then your on the inside of focus and moving farther away from the focal plane.
I agree that your most likely on the outside of focus. You don't have to have 1/4" hole in your tester, you can enlarge the hole to the size of the ronchi grating if you wish. What is more critical is that you place the return beam from the mirror close to the light source. This will minimize astigmatism in the testing procedure. With your small aperture and slow f-ratio mirror it won't make much difference, but if you try to test a larger aperture or a faster mirror like an F/4 you can introduce false astigmastim into the test.

- Dave






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