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What causes astigmatism?


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...the "seagull" shaped stars towards the edge of my views.

All my eyepieces do it to some extent, probably my meade 5000 24mm plossl more than the others - though this may be more noticeable because I use it as my spotting eyepiece so spend more time looking at the edges of the field of vision

Is it a problem inherent in the eyepieces that I have? (therefore solvable by EP upgrade!)

Is it a scope collimation problem?

...something else perhaps?

:blob10::icon_scratch::)

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Astigmatism is a quite serious fault, be it with optics, or the human eye itself.

It a defect in the curvature of the lens/s, mirror, or a defect in the eye, which I couldn't possibly enlarge upon. That requires a professional optician. Astigmatism can be present in a mirror or lens from manufacture, but very rare, because Quality contol would pick it up. It can be induced by over tightening lateral screws contacting a mirror for example, or a lens cell gripping the lens elements too tightly. The same for eyepieces I suppose.

I really do not suspect you have multiple cases of astigmatism in you oculars.I suspect a problem resides in your telescope, and it needs to be investigated. What scope do you own, and it's specifications.?

Ron.

Edited by barkis
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...jeez, now i'm scared that my eyes are broken !!

did I get the terminology wrong - stars are elongated into short lines, wider in the middle, from about 60% out from the centre of the field.

I use a celestron nexstar 114slt (newt reflector with some form of internal jiggery pokery to increase the focal ratio to f10) using a meade 5000 24mm plossl.

Thanks

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After a recent visit to the optician I was told that I have astigmatism, I told him that astronomy was my hobby and he said that I should see "spikes" coming off the smaller stars in one direction.

I had never noticed this before as I was unaware of it, but guess what? I'm aware now and I can see the damned things.:blob10:

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Wide field eyepieces used in fast scopes (ie: F7 or faster) do show astigmatic stars towards the edges of the field of view unless they have been specially corrected for such scopes, in which case the eyepieces tend to be rather expensive (eg: Tele Vue Naglers).

I can't recall off hand if your scope is a fast one or not ?.

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Your telescope has an amplifying lens to increase the focal length, as you said. This extra element is probably located in your focuser.

I have had dealings with this in a friends telescope, he could not get a good focus, and when I checked it for him, the collimation was out, and this intermediate lens was the culprit. I removed it, and cleaned it, and also blacked the edges. I carefully replaced it making sure it was perfectly square in the focus tube. I rechecked the collimation, and tweaked the main mirror a little, took it outside to cool a while. Focused it upon a star, and it was perfect.

He collected the scope the following day, and later in the week he phoned to say it was performing as it should, and pleased as Mr Punch.

Unfortunately, these scopes depend on that lens, as without it, you would have a short focus scope that would deliver very mushy objects indeed. You wouldn't get anything to focus anyway.

I suggest you get the scope looked at if you know anyone such as a member of an Astro Society, who would correct the problem for you.

Ron.

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I suggest you get the scope looked at if you know anyone such as a member of an Astro Society, who would correct the problem for you.

Ron.

like that idea, my astro soc meeting is next week. I'll take it along and see if anyone can help. Thanks for that

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People seem to be getting a bit confused here...

Astigmatism is one of the most common eyepiece aberrations. You'll get it with almost any widefield eyepiece - even a Plossl - in an (e.g) F5 scope.

As you no doubt know, astigmatism is where the focus in one direction is different to the focus in another. In the human eye it happens because the eyeball is more rugby-ball shaped than spherical.

In eyepieces however, it's slightly different. Instead of being in a fixed direction (like eyeball astigmatism) it happens radially from the centre of the field. What many (most?) people on this forum refer to as "coma" is almost invariably eyepiece astigmatism, and not coma at all.

The solution is better-corrected eyepieces (e.g. TeleVue Plossls) or a slower scope.

Sometimes, using longer-length eyepieces with a Barlow can eliminate the problem.

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Had glaucoma and with Lazer treatment and drops ( all gone) must take drops for life

everyone starts to get something wrong with there eyes once you are over 50 years old thats when the wife said , get glasses or get longer arms whilst reading a paper:glasses1:, she was right:iamwithstupid: all to do with the astigmare in one eye for me some people can have lazer treatment if they are lucky

Doug

Essex

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not sure what your exact problem is but some good advice above and I think the best is get someone to have a look at it.

incidentally, you might be interested in my experiences recently with good collimation (in an f11 scope) and eyepieces but tired, ever so slightly watery eyes. when they were a bit watery I found I could induce a type of astigmatism (or similar) by looking 'above my lower eyelid' (can't think of a better way to put this!) and obviously through more fluid then usual. Therefore tired eyes probably don't seem to help although I am sure this is not your problem.

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A thought on the eyeball end of astigmatism. My experience has been that contact lenses are much better than glasses for sorting out eye corrections to allow good views through any scope. This is a very big subject so I'm only dipping in a little toe here!

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I have an astigmatism which is corrected by my glasses, I never new I had it till the optician told me, but if you have one, you usually can't focus on horizontals & verticals at the same time. The tests are more modern now, but on the eye chart type thing it used to be like a semi circular fan made of straight black lines, and you couldn't focus on the whole thing at once.

I use my glasses at the eyepiece but I don't particularly notice if I don't wear them, but if viewing with someone else it's better to keep them on or the focuser has to keep being adjusted for each person.

Also, a point to bear in mind if you are short sighted, (like me) , especially if webcam imaging, when focusing up and watching a lappy screen, take glasses off, you tend to move in close to the lappy screen and keeping glasses on can have a detrimental effect to your focusing abilities.

Edited by TopHouse
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