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Ronchi EP questions

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#1 oddog

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Posted 12 January 2013 - 01:14 AM

I have two questions regarding the use and interpretation of the EasyTester Ronchi eyepiece.

I have been testing an 8" Orion DOB, a 1978 Meade 856 8" f6 Newt, a 10" Antares DOB and a 12" collapsible Skywatcher DOB with the EasyTester on the brightest star I could see. All have parabolic mirrors. I am getting very consistent straight lines as you can see from the photos below (taken with my iPhone held to the eyepiece so some vignetting and the atmosphere was very turbulent).

Firstly, I have read that the lines for a parabolic mirror should not be parallel whereas they should be for a spherical mirror and have also read the exact opposite (the insructions for the EasyTester). I'm confused...

Secondly the patterns I am getting on either side of focus are significantly different from each other. Any idea what is going on?

Input needed!

Tom Duncan

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#2 oddog

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Posted 12 January 2013 - 01:16 AM

And the other side...

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#3 dave brock

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Posted 12 January 2013 - 01:34 AM

Hi Tom.
When tested on a star, the Ronchi lines should be straight and parallel for a parabola. They're curved when tested at the radius of curvature of the mirror (twice the focal length).
The different appearance of the lines on either side of focus is normal. The number of lines visible changes with the amount of defocus.

Dave

#4 oddog

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Posted 12 January 2013 - 03:25 AM

Ah, I get it, I wasn't aware of the two different viewing distances.

Regarding the differing appearances on either side of focus I was referring to the dark solid lines on one side vs the mushy diffused lines on the other. Is this what is normally seen? I ask as most images/diagrams of Ronchi patterns (including the ones that are on the printed instruction sheet that came with the EasyTester) show similar looking dark contrasty lines on either side of focus.

Tom

#5 MKV

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Posted 12 January 2013 - 06:38 AM

I am not sure why bands appear different inside and outside of focus. Their shapes will be reversed if the bands are not straight, but in a null (straight lines) it doesn't matter.

But you should be aware that testing at the focus (star testing) is not as sensitive as at the radius if curvature and that what may look like a "good" mirror may actually turn out to be quite poor.

Mladen

#6 GlennLeDrew

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Posted 12 January 2013 - 10:03 AM

Tom,
Your first image, with fewer bands visible, is a more reliable indicator of figure. The second image has too many bands across the aperture.

And from the first image I do see some error of figure, where in the outer part of the mirror the bands scrunch closer together toward the outer edge. If this is inside focus, the mirror is effectively a bit overcorrected, with the outer zone polished too far (a broad turned edge, if you will.) but if that first image was taken outside focus, then the outer zone is turned up, effectively resulting in some undercorrection.

#7 MKV

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Posted 12 January 2013 - 01:50 PM

Glenn, Tom used a phone to take these pictures, so it's hard to tell if these kinks are due to the atmosphere, the phone optics, the way camera was held, the mirror, you name it.

There are just too many unknowns to make a definitive call based on that one sample. Also notice that the lower left bright band doesn't exhibit the edge issue, and the lower central lines has a peculiar kink in it. We also don't know how long it took the camera to record this, etc, so questions remain a plenty.

Tom, you need to take these pictures near the Zenith when the seeing is steady, you need to indicate if the image is in or out of focus, you need to have all of them with the same number of bands (at least three full bands), you need to make more than one image for each setting for comparison, you need to describe the atmospheric conditions, etc. to be able to make any reasonable conclusions about the quality of optics, and even then it's with a grain of salt.

Mladen

#8 ccaissie

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Posted 12 January 2013 - 09:54 PM

Or, study what you're seeing over several nights, gain some skill at testing, and make notes. We all want photos here but that's sometimes a task. I find Polaris is bright enough to test on, and solves the tracking issue. The most interesting images are just in/out of focus at high magnification. Soon, you'll know which optics you have are the best, and why. This aspect of ATM is my favorite....visualizing optical quality.

#9 MKV

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Posted 13 January 2013 - 02:06 AM

This aspect of ATM is my favorite....visualizing optical quality.

That may be a "feel good" factor. However, what people claim they see differs for many reasons. Sometimes they see what they expect to see.

Mladen

#10 oddog

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Posted 16 January 2013 - 03:39 PM

To be more specific, I was looking for an explanation of the significant difference in the lines appearance on either side of focus, not looking for interpretation of the mirror figure. The seeing was too poor for any interpretation.

Tom

#11 Asbytec

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Posted 17 January 2013 - 05:40 AM

I've seen that effect, too. I'm curious as well. I am sure diffraction is playing a role, just not sure how.

#12 GlennLeDrew

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Posted 17 January 2013 - 10:24 AM

The grating I use is well made, on a glass substrate. It does exhibit this seemingly odd disparity of appearance on either side of focus, where outside focus the dark bands are more sharply defined. However, the difference is nowhere near as pronounced as shown above. To be honest, I've never bothered to learn just why this is; I simply accept it as a fact and work with it.

#13 ed_turco

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Posted 17 January 2013 - 11:45 AM

Nice discussion. I'd like to comment that a mere cellphone for photography in discussing optical errors is patently ridiculous. I'd use a medium quality digital camera that really shows something. And for astrophotos, it can't be beat.

As for phone calls, I have my land line. When Sandy hit, most of the cell towers in my state went down, but my landline kept the faith. :)

#14 oddog

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Posted 18 January 2013 - 04:31 AM

Ridiculous or not, the difference in general appearance of the pattern inside and outside of focus seems quite clear to me (and apparently others) and that is what I was asking about.

Tom

#15 ccaissie

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Posted 18 January 2013 - 08:41 AM

I'd say it's time to do another series of photos. Whatever weird artifacts are showing in the two photos, they don't make sense to me, and I've looked at a lotta ronchi images.

There's some kind of strange reversal going on. I can't think of any optical principle that allows photo 1 to turn into photo 2 by changing the focus. I've seen the multiple line effect in photo 2, but never as a mate to the strange fat dark :question:lines and thin bright lines in 1.

Try again, we're all eager to sort this out.






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