Eye anatomy and observing through telescope

[AlcorAlly]

Normally, our eye muscles relax and the lens flattens when viewing distant objects, but the lens "bulges up" for close objects. How does this work when looking at distant astronomical bodies through a telescope? Does our eye lens treat this the same away as when when you look a photo of far away mountain (i.e. your eyes adjust for the close distance of the photo, not the far distance of the mountains in the image.) Is it exactly the same with a telescope’s image plane? Or is it different in some ways? 

What about binocular vs one eye observing? Does it make a difference to the eye lens shape and how it accommodates? 

[andrew s]

I am not an expert but as at focus the eyepiece gives parallel rays the eye would be relaxed and focused at infinity. 

It can tolerate small amount of defocus caused by, for example, seeing but not more.

This link may help https://www.telescope-optics.net/eye.htm with a general overview of the eye/telescope. 

Regards Andrew 

[vlaiv]

Yes, eye is focused to infinity if relaxed when observing.

Sometimes there is a bit of field curvature and our eye accommodates to it like it does in normal circumstances. It does so with seeing as well if seeing changes are not too fast.

[MikeAa]

Which is why it's rubbish that I am short sighted, astigmatic and slightly colour blind. Also have tinnitus in one ear, but I don't use that for astronomy!!

[vlaiv]

2 minutes ago, MikeAa said:

Which is why it's rubbish that I am short sighted, astigmatic and slightly colour blind.

These might not be as big of an obstacle as it may seem at first.

Any near/far sightedness is compensated by focus position (when you focus you adjust for your dioptre).

Sometimes, astigmatism can be alleviated by choice of exit pupil. Small exit pupils will only use small portion of your eye lens that might not be as distorted as the whole thing. Many people with astigmatism get sharp views when looking at the planets at high magnification because of that - as most of the time, planets are viewed with high magnifications and exit pupils of 1mm or less.

On the hand - observing very dim objects like DSOs - does not trigger color sensitive cells so objects are viewed in black and white - no need to worry about any color blindness .

[AlcorAlly]

7 hours ago, vlaiv said:

Sometimes there is a bit of field curvature and our eye accommodates to it like it does in normal circumstances.

Is this why it feels more relaxing to observe with contacted lenses which correct for my myopia (no astigmatism)?  Without the lenses, even though myopia is compensated by the focus position the eye lens still needs to do more accommodating than it would with contacts on, causing a little more eye strain. 

[vlaiv]

9 minutes ago, AlcorAlly said:

Is this why it feels more relaxing to observe with contacted lenses which correct for my myopia (no astigmatism)?  Without the lenses, even though myopia is compensated by the focus position the eye lens still needs to do more accommodating than it would with contacts on, causing a little more eye strain. 

Could be.

It requires scope / eyepiece combination that has some field curvature to check it out.

Usually fast refractor with wider field eyepieces will give you such combination. You can check this if you focus in center and while keeping eye relaxed - look at stars at the edge. If they are larger / a bit out of focus and you can "fix" this by either straining your eye or adjusting focus - then you have case of field curvature (where center of the field and edge can't be in the focus at the same time) - then it is just matter of testing with / without contact lenses - if it's harder to do the "switch" - from center to edge without touching focus - it is the reason.

[Gfamily]

Under Ideal conditions, an astronomical telescope would have the final image at infinity - and that gives the 'correct' magnification based on the ratio of the focal lengths.

If you took your spectacles off and had the final image at your eye's 'limit of accommodation', then the magnification will be different. 

[AlcorAlly]

How does the eye/brain know when to perceive an object at infinity? From what I've read, the determining signal is the convergence of the two eyes. But when looking through a telescope the projected image on a focal plane is at physical proximity with the eye. Why does the eye lens still relax? 

The link @andrew s shared explains some of it but I still don't fully understand what triggers the lens in the eye to "perceive" a telescope's projection as an image at infinity, despite the physical proximity of the image plane. 

[michael.h.f.wilkinson]

There are two mechanisms at play: stereo and focus. The stereo mechanism uses disparity of positions of features between left and right images to estimate distance. The muscles surrounding the eyes will rotate the eyes accordingly to put the object of interest in the foveae of both eyes. The difference in orientation gives the estimated distance. At large distances this does not really work. Focus simply changes the curvature of the eye lens to get the sharpest possible image on the fovea. When looking through a telescope, it is best to relax the eyes, so the lenses revert to their infinity setting (or its nearest equivalent for) and changing the focus of the telescope to get the sharpest possible image. Note that the image plane might be close, but the eyepieces transforms this into a virtual image at infinity at proper focus.

[AlcorAlly]

Thank you @michael.h.f.wilkinson

46 minutes ago, michael.h.f.wilkinson said:

The stereo mechanism uses disparity of positions of features between left and right images to estimate distance. The muscles surrounding the eyes will rotate the eyes accordingly to put the object of interest in the foveae of both eyes. The difference in orientation gives the estimated distance.

Does the stereo mechanism still work if you're looking with one eye? I image so because the muscles surrounding the open eye will still rotate accordingly.

46 minutes ago, michael.h.f.wilkinson said:

Focus simply changes the curvature of the eye lens to get the sharpest possible image on the fovea.

I understand that the telescope basically creates an optical illusion which allows the eye muscles to relax and perceive a projected image at infinity even though it is at a reading distance to the eye. What is the difference between the three scenarios below, in terms of how 'relaxed' or 'accommodated' the crystalline lens? 

A. Looking through a telescope eyepiece 

B. Looking through a digital eyepiece, such as the Nikon electronic eyepiece provided with eVscope 2 Digital Telescope 

C. Looking at 2D image depicting an optical 3D illusion of a stars or a distant landscape, such as the one below, by Tracy Stum

Would the crystalline lens be in the relaxed state in all of the above scenarios?  

Edited January 24 by AlcorAlly

[Mr Spock]

On 13/01/2024 at 21:46, vlaiv said:

Sometimes, astigmatism can be alleviated by choice of exit pupil. Small exit pupils will only use small portion of your eye lens that might not be as distorted as the whole thing. Many people with astigmatism get sharp views when looking at the planets at high magnification because of that - as most of the time, planets are viewed with high magnifications and exit pupils of 1mm or less.

This is true. Next week I'm at the opticians for some glasses to watch TV. My left (observing) eye has pronounced astigmatism making the TV blurred. Yet I have no problem planetary observing.

[AlcorAlly]

What about devices like Apple Vision Pro? It requires optical inserts to correct for myopia yet the projection is in close proximity to the eyes. I'm not sure why. Is the eye lens relaxed when using Vision Pro or other virtual / augmented reality device to look at, say, a night sky projection?

Edited February 13 by AlcorAlly

[Paz]

On 24/01/2024 at 09:09, AlcorAlly said:

How does the eye/brain know when to perceive an object at infinity? From what I've read, the determining signal is the convergence of the two eyes. But when looking through a telescope the projected image on a focal plane is at physical proximity with the eye. Why does the eye lens still relax? 

The link @andrew s shared explains some of it but I still don't fully understand what triggers the lens in the eye to "perceive" a telescope's projection as an image at infinity, despite the physical proximity of the image plane. 

How it feels to my eye/eyes trying to look at something focused too far away feels different to trying to focus on something too close. When I'm focusing I'm half thinking does it look in focus and I'm half thinking does my eye feel relaxed. On very dim objects I will be thinking more about if my eye feels relaxed to establish focus.

Sometimes I get better results if I'm focused very slightly inside of infinity. I can tell my eye is not completely relaxed  but only very slightly so.

[Geoff Lister]

I have short sight and astigmatism, but I tend to observe without my glasses. I can get a wider FOV, with my eye closer to the EP, and the anti-reflective coatings on my telescope optics are far superior to anything on my glasses. My binoviewer helps to get rid of the effects of "floaters".

Geoff

[AlcorAlly]

More questions on this topic. 

If you have myopia, say -4 diopters prescription, when is your eye lens relaxed? I assume it's not when focused on infinity because you can't focus on infinity. 

In the case of myopia the eye is focused in front of the retina rather than directly on it, causing distant objects to appear blurred. So for someone with myopia of -4 diopters, their eye is naturally focused at a point that is 1/(-4) meters in front of the retina. Therefore, the relaxed state of their eye, where distant objects are in focus, occurs at a point 1/(-4) meters away from the eye i.e. 25 cm in front of the eye. Is this correct?

Edited March 6 by AlcorAlly

[AlcorAlly]

Here's a helpful and detailed book I came across while researching this topic, in case it's of use to anyone else. 

 HumanEye.pdf

[Carbon Brush]

Thank you for linking to this fascinating book.