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Long or short distance glasses?

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#1 Padremo

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Posted 30 July 2021 - 06:08 AM

Hi all

After a recent eye test I now need glasses for both long vision and short vision. I've not needed short vision (reading) glasses before, so previously I never wore glasses when looking into the eyepiece as the image was slightly better without.

When looking at Jupiter last night, I noticed that Jupiter appeared less focused when looked at directly through both glasses, but my averted vision improved considerably in that I can see more detail in the clouds on Jupiter with the new glasses on - but only by averted vision.

I'm wondering if this is because over the years I've trained my brain to view without glasses when maybe I should have worn them, and now my vision's changed slightly I need to retrain my brain, if that makes sense.

Should I use the distance or reading glasses when at the eye piece? On the one hand I'm looking at something that's focused to infinity, but on the other hand I'm looking at light generated from the eyepiece?

#2 kathyastro

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Posted 30 July 2021 - 06:14 AM

If your glasses are purely for distance accommodation, rather than for astigmatism, then it doesn't matter.  A small adjustment of the focuser will do the same job as the glasses, so you don't need them.

 

On the other hand, if your glasses are correcting for astigmatism, the focuser won't help: you will need to wear glasses.  But again, the distance compensation of the glasses is not important, since the focuser can make up for that.


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#3 astrokeith

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Posted 30 July 2021 - 06:20 AM

I think perhaps two possible issues

 

As Kathy has said, astigmatism. Did your test find astigmatism? (usually denoted as 'cylinder' on the prescription).

 

Did you buy 'varifocal" lenses, ie the lenses change from near (at the bottom) to far (at the top) without a sharp change? These can be bad for astronomy as the positioning of your eye & glasses at the eyepiece so as to ensure you are not looking through the transition section is tricky.


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#4 Padremo

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Posted 30 July 2021 - 06:36 AM

The glasses aren't varifocal, but they did mention very slight astigmatism. Sorry, forgot to mention that. I asked if this would affect my hobby and they said no as it's very slight, but that could be why averted vision was better.

I guess I have to do a few sessions with and without the glasses to see how bad the astigmatism actually is.

Thanks for pointing out the focuser does the job of the glasses, seems obvious now you said it!

Just to add I've had astigmatism in my right eye for years.

Edited by Padremo, 30 July 2021 - 06:47 AM.

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#5 TOMDEY

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Posted 30 July 2021 - 06:38 AM

Be sure to try both eyes. The other one might actually see things better than your regular observing eye.

 

If your eyes are astigmatic "cylinder", then your glasses should improve, not diminish your experience at the eyepiece. That may take some fiddling around to find the best way to address the eyepiece; some eyepieces do not have enough "eye relief" to accommodate eyeglasses and still see the entire field. The TeleVue Dioptrx is a lens that attaches to the eyepiece to correct your astigmatism so you can use it without wearing the glasses.

 

It's also possible that your eyeglasses need new lenses if they make things worse rather than better!    Tom

 

Here's what the Dioptrx looks like attached to an eyepiece. You order one that matches your prescriotion's astigmatism number.

Attached Thumbnails

  • 78 Dioptrx on TeleVue Nagler T5 26mm Tom Dey.jpg


#6 astrokeith

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Posted 30 July 2021 - 06:45 AM

The glasses aren't varifocal, but they did mention very slight astigmatism. Sorry, forgot to mention that. I asked if this would affect my hobby and they said no as it's very slight, but that could be why averted vision was better.

I guess I have to do a few sessions with and without the glasses to see how bad the astigmatism actually is.

Thanks for pointing out the focuser does the job of the glasses, seems obvious now you said it!

If the astigmatism is slight (do you have the prescription?) then it would be best to observe without glasses.

 

Mind you an optician's definition of slight might not be the same as an astronomers!


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#7 Richard O'Neill

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Posted 30 July 2021 - 06:45 AM

"I guess I have to do a few sessions with and without the glasses to see how bad the astigmatism actually is."

 

Yes, you will. The degree to which astigmatism affects the view is related to the exit pupil size of the eyepiece in use. Higher magnifications (smaller exit pupils) will reduce the effects of your eye's astigmatism, which will be most pronounced when using low power eyepieces that deliver a larger exit pupil.


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#8 spereira

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Posted 30 July 2021 - 07:29 AM

Moving to General Observing.

 

smp



#9 csrlice12

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Posted 30 July 2021 - 10:01 AM

Did the doc look for early cataracts?  It's a bit odd that Jupiter would be sharper using averted vision...Jupiter is kind of the text book direct viewing target.



#10 tkay71

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Posted 30 July 2021 - 10:25 AM

My eyes are farsighted, slightly astigmatic, and perceive sharpness much differently. Like you I recently obtained distance and desk eyeglasses. My doctor said the eye is a living organ and would rapidly adapt to the prescriptions. They have: I now rely on the desk prescription for sharpness perception to several feet and much prefer actually wearing the distance prescription for everything beyond.

 

For monocular observation I use zoom EPs exclusively: by day with a 45 degree prism (Amici), by night with a 90 degree mirror. Exit pupils range from 3.42 to 0.47. Eye relief varies from ~22mm - ~12mm. My binoculars are 8x56; 7mm exit pupil, 18mm eye relief. In none of the these configurations do I notice any effect of exit pupil on astigmatism, so that's a happy result.

 

My observation is probably 60% terrestrial/40% astronomical. Due to the very real change in vision induced by the new prescriptions, last month I purchased an Orion Pro Lanthanum 8-24 because it solves for me nearly all problems associated with wearing eyeglasses at the eyepiece. Sharpness is constant through the full zoom range ('parfocal'), even with focal extension. Eye relief gives no vignetting from 24FL-16FL without a Barlow and from 24FL-14FL with a Barlow. This makes it a 'X'-24 EP with good a) eye relief, b) color rendition, c) field flatness, d) absolute sharpness, and e) granularity of magnification from 23x-175x. So with one EP and one Barlow I am set for everything (lunar, planetary, double stars, wildlife) but wide-field astronomical observation, for which I usually prefer binoculars anyway.

 

I have an ED80 scope with 11:1 focuser. In my right eye sharpness perception can be modulated but not optimized by merely moving the focuser: I must wear the eyeglasses to improve sharpness perception at the limit. This difference is decisive and reproducible in all hardware configurations. In my left eye sharpness perception can be more or less optimized exclusively with the focuser, but that eye suffers from strain without eyeglasses. So focusing once with eyeglasses very effectively balances sharpness and thus reduces left eye strain both intrinsically and via load distribution to the right eye without perceivable image degradation.

 

Individual variances/preferences will determine your optimal setup. Mine is to minimize the hardware and keep both eyeglasses handy by night (driven mount/wifi app) and deploy them according to need. By day I just wear the distance frames and have done. I notice little if any difference in the image presented at the eyepiece between the distance/desk prescriptions. With binoculars I of course remove the eyeglasses and use the diopter.

 

Jupiter is a favorite target. For my equipment/location, actual seeing conditions are the major variable in detail resolution. I think I have noticed neither a sharpness nor a brightness modulation with averted vision. Cleardarksky says seeing conditions should be ok tomorrow morning, I'll try to notice this effect.



#11 Padremo

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Posted 31 July 2021 - 03:40 AM

My eyes are farsighted, slightly astigmatic, and perceive sharpness much differently. Like you I recently obtained distance and desk eyeglasses. My doctor said the eye is a living organ and would rapidly adapt to the prescriptions. They have: I now rely on the desk prescription for sharpness perception to several feet and much prefer actually wearing the distance prescription for everything beyond.

For monocular observation I use zoom EPs exclusively: by day with a 45 degree prism (Amici), by night with a 90 degree mirror...



Many thanks for all your detailed replies, it's helping me understand the complexities of the human eye (it's amazing how things are taken for granted until they go wrong)

Obviously, now that I need some time with the scope to see what works best for me there's cloud cover forecast for the next week or two!
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#12 tkay71

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Posted 02 August 2021 - 08:15 AM

I looked at Jupiter, Saturn, and E-Lyra this morning. The typical summer murk wasn't too bad. With the distance frames averted viewing blurred the image, direct viewing resolved the image. I didn't try the desk frames.

 

Prior to the new prescriptions I mostly favored unaided observation at the eyepiece. But now that I have come to rely on them, I have adapted my habits/hardware and am pretty pleased with the result. It's not perfect, but it certainly could be a lot worse, and really my left (sharper, but strained) eye is a lot happier now, so I suppose in the end this is an improvement.

 

Good luck!




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