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0.25 astigmatism: glasses, dioptryx or wing it?


MiladyB

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I've been looking at eyepieces - specifically zoom ones such as the Svbony and Baader Hyperion mk IV - but am quite unsure what to do as far as my own eyesight goes with regard to the amount of eye relief.

I wear glasses for short-sightedness, and have slight astigmatism of 0.25 in both eyes. I bought binos with long eye relief, and have been wearing my specs to observe with them and my 6 inch dob (using the stock 25mm and 10mm Skywatcher eyepieces it came with), but I'm wondering if I really need to worry all that much about eye relief, and if I can 'get away' without wearing my specs, and be less limited in my potential eyepiece choices.

I've been reading up about how a dioptryx can help, but I must admit I've got myself a bit confused with exit pupils and so on and I'm not sure if I'd need one or not. Can anyone help shed some light on it for me please? 

Thanks 😊 

 

MiladyB x

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Did you see TeleVue's chart?

DioptrixAstigmatismVisibility.gif.913354394d37c893c1a484195a3a80ea.gif

So the wider the exit pupil, the more likely it is that you might see effects.

0.25 is I think the minimum increment for a standard eye test, so you don't have much. I have 0.5 and 0.75, and I notice it only when using EPs with the longest focal lengths, the rest of the time it's fine.

Do you have a range of EPs already that you can use to compare?

Edited by Zermelo
typo
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Fellow newb with a similarly slight astigmatism. I like taking my glasses off, too. My left eye, which I view through I believe is .25 off and I barely notice a difference glasses on or off. There is a difference, but it just means I need to position my head a little differently.

I noticed a difference going from crap plossls to a nice nagler but I'll still notice it. I appreciate the help but don't blame the nagler for still having issues, it's my eye that's mis-shapen not their lens ;)

I've always hated contacts as a matter of life or death but I'm considering changing that stance just to see if it makes a difference. I'll probably hate the contacts more but we'll see!

My guess is how much it matters and is noticeable varies from person to person. I'd relax and get data on your own preferences before purchasing anything.

You probably already googled this before you made the thread but I found this page and this blurb to be helpful as I was trying to discern what was me, what was the EP, scope, etc.

Quote

Finding the cause of regular astigmatism is pretty much a process of elimination. First, if you have any doubts, rotate your head/eyes and look through the eyepiece. If the orientation of the astigmatism changes as you move, it’s not your telescope –  you may just have very mild astigmatism in your eyes that hasn’t been noticed by a doctor. At higher magnifications, the astigmatism in your eyeball becomes less of a problem – as opposed to astigmatism from the telescope which becomes more of a problem at higher magnifications. If the problem is indeed your eyes, the solution might be as simple as wearing glasses when using lower magnifications, or just ignoring the issue.

 

Edited by HiveIndustries
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Thanks for the graph and the advice,@Zermelo. I spotted it  on the Televue website, but I got a bit lost with exit pupils.

I only have the stock eyepieces at present so not much to compare against.

Thanks for the info @HiveIndustries - it's good to know that it may not make too much of a difference. 

I keep seeing differing opinions on the eye relief on the Hyperion zoom, but I'm favouring that over the Svbony... Just don't want to make an expensive mistake! 

MiladyB x

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So you probably don't even notice much of a difference in real life with your level of astigmatism and your noticeable correction likely comes from your distance correction.

Quote

Mild Astigmatism: < 1.00 diopter

Nearly 33 percent of Americans have astigmatism to some degree. Mild astigmatism is considered normal. Most people have it and do not need corrective lenses or surgery to see clearly. You may not even notice any symptoms with a mild case of astigmatism.

I wouldn't quote me as gospel on this but my own personal way of handling was completely ignoring my eye, specifically when purchasing the EP because of having such a slight diagnosis. When I finally understood what I was seeing in my EP was related to any kind of astigmatism, eye or equipment, I learned for me it wasn't the end-all. I'm still learning myself but I think the only real effect it has is I pull the target (manual dob, no tracking) back into the center sooner as the aggregate of distortions including the astigmatism reaches a point where I do that.

Before my distance got worse I was even allowed to drive without glasses and my astigmatism in my right eye I believe is .50 or .75 (can't remember + or -).

For me, in a sea of confusing new information it was nice to be able to not have to worry about my own eyesight and understand the effect was minimal and completely livable for me. I definitely have not even thought about it once I went from my 20mm Orion 1.25" Plossl that came with a 4" reflector to a 17mm TeleVue Nagler Type 4. Who knows 6 months from now when I have a couple hundred hours through the EP, maybe I'll have a different opinion but it's also an EP that has value on the secondary market like your Hyperion, so I can address that if it happens, which I consider unlikely atm.

Edited by HiveIndustries
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19 minutes ago, MiladyB said:

I spotted it  on the Televue website, but I got a bit lost with exit pupils

To work out the exit pupil for a given eyepiece in a given scope, divide the focal length of the eyepiece by the focal ratio of the scope.

For example, using a 10mm eyepiece in an F5 scope gives an exit pupil of 10/2 = 5mm. If you're unsure of the focal ratio of your scope, divide its focal length by its aperture. So a 130mm reflector with a focal length of 700mm would have a focal ratio 700/130 = 5.4ish.

If you believe the Televue chart, eye astigmatism with a diopter of 0.25 would only be noticeable for exit pupils of 7mm and larger. Well, it's very unlikely that you would be using an eyepiece that would give you that - remember, the fully dilated pupil of a human adult is around that size, and in most observing situations it will be smaller. Also, there is a difference between "noticeable" and "so bad that I can't put up with it".

I agree with the other advice, assume that you won't be needing to wear glasses.

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I have 0.5 astigmatism and don't bother with specs. I see a little distortion with exit pupils > 4mm and it's slightly distracting at 5mm.

I'll use my specs with my 30mm and 35mm EPs if I want better views across the whole field of view of a large starfield.  But at that power I'm usually looking at (or trying to find) large nebulae with a filter, so the distortion is not noticeable and I'll dump the specs.

With 0.25 I'd think you'd be absolutely fine without specs for pretty much anything.

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Astigmatism can be seen at smaller exit pupils that it is bothersome in focus.

I'll explain.

Defocus the eyepiece on both sides of focus.  Does the star image go oval in one direction and then go oval in a direction 90° away from that angle on the other side of focus?

That is astigmatism.  If I am fussy about having the out of focus star images be completely round, I can detect astigmatism down to about 4.7mm of focal length in my f/5.75 scope.

But, if I evaluate astigmatism seen in the eyepiece IN FOCUS, I simply cannot see it below about a 9mm eyepiece.

My prescription for correction of astigmatism says I shouldn't need correction below 11.5mm focal length, but I can see it down to 9mm, likely indicating that astigmatism with a dilated nighttime pupil

is a bit worse than the daylight prescription I was given.

 

So, there you go: what your prescription is, what you can see in focus, and what you can see out of focus--3 different levels.  Technically, you want to correct all of them until all out of focus

star images are perfectly round, but since we use our scopes primarily in focus, the second level is what to correct for.

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Another newbie here.  I'm totally lost by exit pupils, afov, and any other ep quality!  Likewise I have a slight astigmatism and wear glasses for driving (fairly sure I need stronger specs though, and may be getting long sighted as I age, I've an eye test booked).  Given my confusion I've decided I need to try different EPs myself.  Ideally I'd get to try lots at some star parties but young family, lack of time etc means this is a self learning journey in my garden.  So be it's the classifieds, buy, try, sell on what I don't enjoy.  Postage costs aren't much so it shouldn't cost loads....I hope! I've bought a 6.5mm Morpheus, a 17mm Hyperion and both tuning rings and a skywatcher Barlow to add to two stock skywatcher EPs.  I'll learn what they agree good for and if I like them as I learn the hobby, if I don't like them, I'll sell them on.  Hopefully I'll understand through experience!

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22 hours ago, Don Pensack said:

My prescription for correction of astigmatism says I shouldn't need correction below 11.5mm focal length, but I can see it down to 9mm, likely indicating that astigmatism with a dilated nighttime pupil

is a bit worse than the daylight prescription I was given.

 

I've found the same.  The prescription for my observing eye is 0.75.  I tested both 0.75 and 1.0 models.  At night the 1.0 was better.

It's not just astigmatism that gets affected at night.  I have myopia (short sight) and use glasses for distance.  I also have some night driving glasses with an extra -0.25 correction.   However, they're still not perfect for astronomy.  So my friendly optician in London (Neil Gordon) lent me some flipper lenses to hold in front of my glasses to see what was best for stargazing.  It turns out that I need a further -0.375 correction in my observing eye and a further -0.25 in the other eye.  This effect is called "night myopia" and it's worth doing a Google search to find out more - there's an old Sky & Telescope article on it.

Edited by Second Time Around
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I have bad astigmatsm which is above 1, I cannot remember exactly what it is but according to the chart above from televue I need glasses or diopter down to about 1.5 degree exit pupil.

However my own experience is that I am happy viewing once the exit pupil hits 2.5mm or less in my F4.9 dob. 

When using my F9.8 frac I don't bother at all usually.

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I have .75 astigmatism and never wore glasses at the eyepiece until 3 years ago, when I was bemoaning the terrible coma in one of my scopes to a fellow observer with an identical scope.

We swapped scopes and the issue followed me, it’s my astigmatism.

Swapped to Longer eyerelief and wear glasses at the scopes now, and boy do I see more in familiar objects.

But I take glasses off at 7mm eyepieces down and all is good, that’s focal length of eyepiece not exit pupil.

All I can say is look at double stars, glasses on and then off, if the views better with glasses, then you have your answer.

 

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Thanks for all the responses! Lots of great advice and info. 

After much deliberation, and a spreadsheet to work out exit pupils and magnification for a few eyepiece options in my current scope as well as the 200p I've got my eye on as an addition to my collection, I've ordered an 8mm BST Starguider as my next eyepiece - as an upgrade to the stock 10mm - which should hopefully be OK for me in my Heritage 150p without glasses. 

MiladyB x 

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