Eye sight differences when viewing through telescope

[Iancovici]

I'm nearsighted and my girl friend's farsighted. When I focused clearly on my telescope and I showed to her, she said she couldn't see clearly and she had to re-adjust for her to see clearer.

But I was wondering, is this the case really? I never really thought about it until now and thought once focused we all see same though the scope..

[mdstuart]

It makes for fun when doing public viewing sessions with a telescope!

You could both put your glasses on and then the focus will be the same for both of you.

Mark

Sent from my BlackBerry 9320 using Tapatalk

[VigdisVZ]

I'm no expert but since the eye has a lens with unique properties, if it's different, I'm assuming that the focus should have to be changed aswell. Just think of the eyes as the final lens in the optic train

[Rockrae78]

My OH has to refocus after I've focused it perfectly for me!

I don't wear my glasses when looking through the EP (I don't like to), but I do wear glasses for distance. My OH doesn't need glasses.

[Iancovici]

So there's a difference in clarify based on each person's eye sight!? I thought it might.. but then it's really inconvenient to look through with glasses on and when I had couple nights ago at my girl friends home, I showed the entire family the view of moon and saturn, and they all said they see good and fantastic... but I suppose not all may have seen as I have focused for my eyes w/glasses off!! oh dear..

[mdstuart]

Well you now know for next time.

To be honest saturn looks good even when its fuzzy!

Mark

Sent from my BlackBerry 9320 using Tapatalk

[Stargazer_00]

It's easy to assume that when focusing you are bringing the light to focus on the exit glass of the eyepiece but in reality the eyepiece isn't designed that way. It's part of a system where the final lens is actually the one in your eye! You need to get all the rest of the system in place so that the light exiting the eyepiece is perfectly tuned to resolve past the lens in your eye and into your brain.

The only reason I think people wear glasses for astronomy is to correct astigmatism.

[Paul M]

Another consideration is that when stargazing our eyes are fully dark adapted (hopefully!) and therefore the pupils are wide open. That not only allows more light into the eye but also allows more off-axis and out of focus light "rays" to reach the retina. This means that optical imperfections in both the telescope and eyeball are more noticeable. Things such as astigmatism poor accommodation (flexibility of the eyes own lense) become more critical.

An example of this is my need for a stronger left lense in my glasses for night vision than I need in day light. I can see at distance perfectly in daylight with +2.00 dioptres for my left eye but at night it would still be struggling so I have an extra +0.25 dioptre in my prescription to allow me to see clearly in darkness, particularly when driving. I don't notice it during the day (luckily). My right eye is the same prescription in night or day although it is my weaker eye!

So from my own experience I can see that the focusing of a telescope is actually quite critical and individual and changes from user to user. Even for people who sport 20-20 vision. They have maybe never had their focusing tested so accurately as a telescope requires.

People with considerable astigmatism might want to keep their glasses on at the eyepiece as otherwise they might struggle to get a comfortable focus. I'm in varifocals which aren't best for that but I don't have too much astigmatism so manage fine without.

In bright sunlight I don't need glasses at all for most activities. That's the pinhole camera effect in action.

[Naemeth]

People with varying levels of short sight and long sight focus at slightly different points, that's why you need to change the focus if your eyes are even a dioptre apart.

What's weird for me is that I have to change focus between eyes. My left eye is long sighted, at about +0.75 dioptres, and my right eye is short sighted at about -1.50 dioptres. Can't remember exact figures...

[Iancovici]

Thanks Stargazer_00 and Paul M for your inputs.

With my Celestron 70 travel scope, when viewing Saturn at 120x, for instance, and needs re-focusing for another person, most of the time the object moves out of sight when re-focusing as it shakes away or moves the telescope a little..! Does this happen to the telescope you fellas have as well?

[Stargazer_00]

Not really, mine are quite stable. I use my 350p at over 300x and it doesn't move much when I focus. In fairness my technique for finding perfect focus is to find a bright star, then in the eyepiece move to a star as close to mag 7 or 8 as I can find and focus that to a perfect point. I find brighter stars don't show perfect focus as well as dim stars which are much more pin point, where brighter stars aren't quite as pin point (due to small diffraction spikes mostly). I'll then lock the focuser in place using the screw underneath and leave it that way while that eyepiece is in the focuser knowing that when I swing the scope around from target to target the eyepiece is at perfect focus.

Occasionally when viewing a DSO, if i think the view isn't in focus, i'll unlock the screw, move to a nearby dim star and pull it back to focus, lock it up and swing back to my target. The moon is about the only target I actually focus on the target itself, even planets I focus on dim stars before viewing them as I've found that seeing is variable and there is no telling if the focus point you've found on a planet is perfect or not and therefore you might miss out on the perfect moments by being slightly out of focus. dim star points are less effected by poor seeing I've found and prove to be a better subject on which to get perfect focus.

EDIT: also it's worth noting all my scopes have 10:1 focusers and the micro focus wheel is perfect for reducing the wobble introduced when focusing at the eyepiece as they require the lightest of touches. I'd expect your scxopes focuser is rather more coarse and therefore will introduce more wobble when focusing.

[Iancovici]

Ah.. I see.. yes, my telescope does wobble pretty much when focusing, but that don't put me off really, I just think what hard time man like Galileo and other fellows from old days.. mine is a luxury compare to that!

[Rockrae78]

Ah.. I see.. yes, my telescope does wobble pretty much when focusing, but that don't put me off really, I just think what hard time man like Galileo and other fellows from old days.. mine is a luxury compare to that!

You could try putting a weight (like a sandbag, or a bag of flour/rice) on the cross bar of your mount. This will help minimise the wobbles.

[Iancovici]

You could try putting a weight (like a sandbag, or a bag of flour/rice) on the cross bar of your mount. This will help minimise the wobbles.

Ah.. that might work... shall try..

[Rockrae78]

i use a brick on the accessory tray for my 130SLT as this really wobbles when trying to focus (and when people walk near it, or round it) and it definitely does help! It doesn't get rid of the wobbles completely, but it does dampen them down!

I've never had the scope move a little when focusing. Have you ensured that all knobs and clamps are secure?

[Iancovici]

I've never had the scope move a little when focusing. Have you ensured that all knobs and clamps are secure?

Most of the time I don't tighten it all the way, i keep it slightly loose so as the object slowly moves away i can re-adjust and follow; but because of this when another person re-focuses, sometimes it moves out of sight you see.

[paulastro]

I have astigmatism in both eyes and at low poweres my glasses need to be kept on to prevent distortion. At higher powers I can get away without having them on. I've never experimented to find out exactly when my eyes can cope without glasses, certainly by x150, but not much lower in my experience.

I have one or two astronomer friends who claim they don't need to refocus after I have been using a telescope. Frankly I think this is nonsense. I've never known anone to have exactly the same focus point as me.

The only exception may be when using a telescope with a high focal ration, say f12 to f15. I once observed a shadow transit on Jupiter with two refractors side by side, one a 4" f15 and the other a 90mm f6. In the f15 the shadow was ever present, whilst in the f6 it could be missing for up to about 30 seconds at a time - when it reappeared it was very clear. This is because like using a 'slow' telephoto lens compared with a 'fast' one, The 'slower' telephoto lens and the higher focal ration telescope will have a greater depth of focus. With the shadow transit, the greater depth of focus of the f15 was able to accommodate the changing seeing whilst the f6 could not do so.

So, it may be that if we don't want to change focus for different observers we all need to get f15 instruments!

[LukeSkywatcher]

The other night there was a 13yr old,39yr old,48yr old,53yr old and a 66yr old looking at the moon through my scope. Each one of us had to tweak the focus to make it suitable for our eyes. We were all in awe of the 13yr old's eyesight. Us oldies (me,aunt,uncle,mum) were turning the focuser for what seemed like an eternity from where the 13yr old left it.

[Dave]

I have astigmatism in both eyes

Astigmatism cannot be corrected by changing the focus, so your glasses will need to be worn all the time when viewing.

[Joe_L]

Aside from astigmatic and focus differences between my l & r eyes I also notice a colour difference.The effect is small and best noticed in a dim room such as when having a lie-in and peering at the white sheet in front of me. I need to open and shut alternate eyes (like an astronomer looking for a new planet). One definitely has a pinker cast and the other greener. I've been trying to work out how to check it's not an angular effect.....

God knows what the OH thinks!

[Geordie mc]

I'm very shortsighted and have tried using my glasses, contact lenses and nothing. I much prefer to use no correction and just focus using the scopes optics. It always seems clearer. My wife then has to adjust quite a bit to see the same object clearly. I also find my glasses are needed for charts etc which is annoying. Perhaps I should wear just one contact lens and use one eye for observing and the other for reading charts?

[Iancovici]

I'm very shortsighted and have tried using my glasses, contact lenses and nothing. I much prefer to use no correction and just focus using the scopes optics. It always seems clearer. My wife then has to adjust quite a bit to see the same object clearly. I also find my glasses are needed for charts etc which is annoying.

I concur, I can then bring my eyes much closer to the bino/scope and much more comfortable [though I have to put it back once I look back at the sky! or i just look up with the bino that's hanging on me neck].

Perhaps I should wear just one contact lens and use one eye for observing and the other for reading charts?

That'd be utterly bizzare and confounding!

[cotterless45]

My optician relates that because the eye is focussing parallel light from a distant object , glasses should be worn to get the best image. This apparently particularly in the case of glasses which attempt to correct astigmatism.

I've tried, but prefer the closer no glasses view. It's interesting to compare left and right eye and glasses.

He's quite welcome to come and have a shot,

Nick.

[Geordie mc]

Yep, I use right eye and no glasses whenever I can. Do use my glasses or contacts with binoculars though.

[whooshbang]

The other night there was a 13yr old,39yr old,48yr old,53yr old and a 66yr old looking at the moon through my scope. Each one of us had to tweak the focus to make it suitable for our eyes. We were all in awe of the 13yr old's eyesight. Us oldies (me,aunt,uncle,mum) were turning the focuser for what seemed like an eternity from where the 13yr old left it.

wow, does this mean when you take a photo of a DSO it is out of focus for the rest of the family?