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Difficulties using a Ronchi eyepiece

Equipment Eyepieces
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Götz Golla avatar
Götz Golla avatar
Tonight for the first time I tried to use a Ronchi eyepiece to test my RC10 telescope. I have a nice description of the process by David Davies from 2020. I tried it on Deneb. I do get a Ronchi pattern with about 30-40 straight parallel lines. When I move the focus in, the number of lines decreases as expected. If I understand correctly one should decrease this number to about 5 and then see if the lines are still straight parallel lines.

Unfortunately I dont get that far. After about 10 lines the image gets so blurry that no lines are visible at all. Moving the focusser further to the other side of the focal plane finally 10 lines are emerging again and increasing.

I understand that right in focus there is the so-called null image which is supposed to be uniform. But I thought that just out-of-focus three lines and than 5 lines should be visible. Either I am wrong with this assumption or something else is wrong. Could it be that its because its almost full moon and its a rather bright night ?

How meaningful is it when in the Ronchi pattern with 10 lines the lines are perfectly straight and parallel ?
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andrea tasselli avatar
andrea tasselli avatar
I guess 10 is too few but I never actually used one of them in anger. I relay on Roddier test for a quantitative assessment and the symmetry about the focus point to test for quality. I don't expect moon is making any difference since you're using a first magnitude star.
Pedro Goles avatar
Pedro Goles avatar
In my experience doesn't matter the number of lines, in fact, the lines decrease as you get focused, to the point of "no line at all" and image displays some gray floculent pattern, that's your focal plane…
Tony Gondola avatar
Tony Gondola avatar

Don’t worry about the exact number of lines. The thing that’s important is that the lines are straight, inside and outside of focus. As you get closer to focus (the fewer lines you see) the more sensitive the test is. It is much that the star test in that regard.

Götz Golla avatar
Götz Golla avatar
I found the reason why I didnt  get less than 10 lines: There was a spider and its net inside the tube where the rays pass thru the primary mirror smile.
After removing the spider (it didnt want to go), the whole span of lines down to even two lines are visible. With 3 lines the pattern is barrel-shaped inside of the focus, but only very slightly. I have decreased the mirror distance a little bit. So far tonight star shapes are excellent. Will try another Ronchi test soon.

BTW: Right now it is really complicated to do this test: my setup is photographic-only and includes a reducer. Using the Ronchi eyepiece means I have to remove the main camera and reducer, add two of the big spacers for the RC10 (because of the longer focal length without reducer), add a 1.25" holder etc. Also , without the main camera I have to switch the setup of my ASIAIR to point with the guide scope. This is all complicated and time consuming. I wonder if there are better solutions ?
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Tony Gondola avatar
Tony Gondola avatar

I’m not sure you need a better solution as this is a test that you really only need to run once with any given OTA. As you know, The fewer lines the greater the sensitivity. It sounds like you got a good result.

Götz Golla avatar
Götz Golla avatar
Yeah, its probably OK now. Its more that I tend to be a perfectionist and I always want to know and see how thinks look like. It was good to understand and use a Ronchi once. Mainly I learned that the optics including the mirror distance of my RC is OK, spiders love RC-telescopes,  and how to quickly disassemble and assemble/collimate the image train.