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Ronchi grating eyepiece


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I think there's a possibility I may have slight astigmatism in my middle-aged right eye so to eliminate the possibility of the optics being at fault I'm considering buying one of these (top item in list) after reading this.

I'm finding it difficult to get pinpoint star images in my dob (even with a premium quality eyepiece - Pentax XW 30mm). It's hard to tell if the focussed stars are crosses (I've read this is an indicator of astigmatism) or not at a low magnification (x50) but when I slightly defocus in I see a short line and slightly defocus out I see another short line perpendicular to the first. I've tried rotating my head around to eyepiece to see if the line rotates with my eye position (which would indicate the fault is in my eye) but it's tricky and inconclusive.

I've been unable to do a proper star test due to the lack of a short FL eyepiece and bad seeing but the 'scope seems well collimated using a cheshire and a laser. I tried doing a star test last weekend, after the 'scope had been out for a couple of hours, using a 10mm with a x2 barlow but as a barlow is not recommended for star testing the results were inconclusive. I did catch a glimpse of circular diffraction rings inside and outside focus but there was a lot of turbulent air up there which boiled the rings away most of the time.

It seems a useful tool and will come in handy for testing any other equipment I may buy later.

Any thoughts?

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It may be easier to have an eye test and point out to the optician that you are an astronomer and use precision optics etc. Mine for years told me my right eye was fine (and it is compared to my left!). When I mentioned eyepieces and optics and how it didn't seem perfect to me he pointed out that I was very slightly short sighted and very slightly astigmatic - at the minimum that they could measure. It wasn't noticable during the day but is at night looking through an eyepiece!

That said I have a Ronchi eyepiece too and have confirmed (I think) that my optics are pretty reasonable. I need to do some more reading up to understand exactly what I'm seeing though!

James

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Would a visit to the opticians not be your best bet. They will establish whether or not you have astigmatism. If you are fearful of having that condition, then you should visit your doctor, and he will refer you to a specialist at your local hospital.

I believe we have an optician as a member here, and I apologize to him for

forgetting his name. He offered advice to anyone who might need it when he joined SGL.

Ron. :D

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The Ronchi grating will give an indication of the mirrors surface. Focusing on a fairly bright star will show the lines sillhouetted against the starlight. The nearer you get to the focal point of the mirror, the lines will curve ever increasingly from centre to edge, following the paraboloidal shape of you mirror. The curves should be uniform like a segmented orange. What you don't want to see, is the lines straighten up before they reach the edge of the mirror. An indication of spherical aberration. As you reach the exact point of focus, the light may black out altogether as one of the lines completely cover the diffraction spot. The point where all the light from the star is focused.

An uneven optical surface can be picked up by the Ronchi, but the test has to be made under Ideal conditions. Thermal equilibrium, steady seeing, all those conditions rarely found in the UK.

Ron.

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I agree a trip to the opticians would be cheaper but it's more astro gear innit? :lol: I like optical tools - don't we all? :lol:

I had an eye test less than a year ago for some new specs (I'm short sighted too James) but I don't find the short-sightedness a problem, apart from taking my glasses on and off all the time I'm out observing which is an inconvenience I've got used to now - I like to use the 'scope without my glasses.

I've a vague recollection that the optician said I had slight astigmatism in my right eye but I could be mistaken - I'll give them a ring - they must still have the test results on file somewhere.

Maybe my new glasses have corrected for it (if that's possible) so I'll try using them at the eyepiece next chance I get to see if there's any difference.

I'm going to get the Ronchi eyepiece though just to put my mind to rest on the optics - it'll fill up some space in an eyepiece case if nothing else :D

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... An uneven optical surface can be picked up by the Ronchi, but the test has to be made under Ideal conditions. Thermal equilibrium, steady seeing, all those conditions rarely found in the UK.

Ron.

Are you sure good seeing is necessary Ron? The reason I ask is because I was reading this page http://schmidling.com/ez-testr.htm for a similar product Stateside (only 133 lines instead of 250) and it says

"Although the standard "star test" is probably still the most precise test, it is very difficult for the inexperienced observer to comprehend and very subject to mis-interpretation. Furthermore, unless seeing conditions are excellent, even experienced "star testers" can learn little from the dancing images. The EASYTESTER, on the other hand, will tell its story under the most dismal of conditions, including fog, haze, turbulence, bright lights and full moon."

However, further down is this

"The ET can produce results under seeing conditions which would make "star testing" impossible. However, adverse seeing or tube currents will cause the image to be unstable but the average shape of the lines can still be determined."

which seems to contradict the above but I think it implies as long as the 'scope itself is thermally stable then the test can be done.

I have slight astigmatism (and short sighted) but it doesn't show at the eyepiece.

Can you webcam the effect on a bright star, that would clear up whether it was the scope or you eyes?

I haven't crossed over to the 'dark side' (imaging :D ) yet so unfortunately I don't have a guided mount or a webcam.

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Astigmatism in a mirror simply means that the same curve is not present on every diameter of the mirror.

You can test for this, but without taking the mirror out of the scope, you will only be able to test across one diameter.

If you want to do it. Find a reasonably bright star, and with the scope drive on and tracking, focus on a star without an eyepiece in place. All you will see is the light expand until it fills the field. Take a straight edged implement, I would suggest a razor blade, but it is perhaps a bit dangerous. The edge needs to be pretty straight, so a Stanley blade will do. Be careful though. Make sure the light is still in the eyepiece.

Slowly pass the blade horizontally across the end of the focuser. If the light blinks out altogether, focus in a little until the edge of the blade is standing out against the light. Move it across keeping it as level as you can. The blades shadow will move horizontally across the mirror.

If it stays on track and leaves the mirror on the same plane as it entered, then that portion of the curve is sound. What you need to do then is turn the mirror round a little and do the same thing again. and again and again. Now you see why it is better with the mirror out.

I will end by saying it is most unlikely your mirror will be astigmatic. This condition can only arise during the finishing process, when the mirror has not been rotated enough during the figuring process, and the curve is distorted in one area. No reputable mirror manufacturer is that careless.

Ron. :D

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A couple of things to try on the scope are to see whether either the primary or (more likely) the secondary are held in place too tightly by the clips, the latter problem crops up pretty often as a cause of astigmatism in Newts.

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A couple of things to try on the scope are to see whether either the primary or (more likely) the secondary are held in place too tightly by the clips, the latter problem crops up pretty often as a cause of astigmatism in Newts.

Is that because it becomes twisted?
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The secondary is supposed to be flat, if its pinched on too tightly at the edges it becomes slightly convex and the image suffers from astigmatism. It usually happens in manufacture/ assembly rather than by the end user.

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A couple of things to try on the scope are to see whether either the primary or (more likely) the secondary are held in place too tightly by the clips, the latter problem crops up pretty often as a cause of astigmatism in Newts.

I remember getting pretty paranoid about 'pinched optics' with my previous 8" fullerscopes newt 20 odd years ago. The secondary was mounted on a hollow tube with three clips holding it to the tube against some foam rubber in the tube.

Thanks for reminding me - I'll check this out too but the Skywatcher secondary housing is a different design with the secondary glued to a plate (no clips or possible pressure on the back of the secondary involved). I'll check the primary cell too - it'll be an excuse to blow some annoying specks of dust off the mirror.

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Took the 'scope apart last weekend and checked out the primary cell - no problems there but the rubber/metal clips holding the mirror to the cell are at an angle to the mirror edge when tightened (not too tight though, just enough to hold the mirror to the cell base). I think some extra rubber washers under the bottom of the clips (the end closest to the cell base) wouldn't go amiss to stop them angling up away from the mirror as the screws are tightened.

I was shocked to see just how much dust had accumulated on the mirror surface - looked pretty clean from the open end of the tube. I blew some of it off and carefully picked off the more stubborn bits with a piece of cotton wool but alot of it is just stuck to the surface so I might give it a rinse or I might just leave it alone (don't want to invite disaster).

Postie knocked on the door this morning and my Ronchi grating eyepiece from Orion Optics UK has arrived. The lines on the grating are so fine they can't be seen. It reflects at certain angles so 'something' must be there :D

Looking forward to using it.

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