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I'm on a space walk and comet Kohoutek is passing by and I've brought my specially designed space binoculars with me to look at it.

I left my Nikon Monarch 8x42 binoculars inside because with an eye relief of only 18.4 mm they would be nearly useless held against the faceplate of my helmet. I would be able to see an extremely tiny field of view; their 4.2 mm exit pupil at say 15 cm from my face would present an apparent angular field of view of not 51.3°, but only 1.6 degrees. I'd see tiny dots with stars in them, but it would be really hard to locate anything.

Question: How far would you have to hold "space binoculars" from your eyes in a space suit? I guessed at 15 centimeters but I have no idea, and probably different helmets for different suits present different distances.

These are going to be pretty crazy looking binoculars!

Related:


Screenshots from How to Adjust Your Binoculars (Presented by Nikon Canada) and Understanding Binoculars: Eye Relief showing how eyecups can be adjusted to place the exit pupil of the eyepiece at the entrance pupil of the eye. For those wearing eyeglasses (a glass barrier fixed in front of the eye) one retracts the eyecups.

A helmet with a transparent face plate would place the binoculars much farther from the eyes than eyeglasses do, so the exit pupil positions of the eyepieces of a pair of space binoculars would have to extend much farther.

screenshot from How to Adjust Your Binoculars (Presented by Nikon Canada) screenshot from Understanding Binoculars: Eye Relief

See also Nikon Monarch Binocular Eyecup Repair How-to DIY


What it looks like when you are too far away from the eyepiece. From Wikipedia's exit pupil. If you were looking for something in a field of stars this "tunnel vision" would make it a lot more difficult.

loss of apparent field of view when viewing beyond the exit pupil

Cropped and annotated from here

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    $\begingroup$ I'm pretty sure gun sights normally have large eye relief, so $\endgroup$ – ikrase Mar 21 at 1:50
  • $\begingroup$ @ikrase hunting for a dim comet in a field full of stars at night may be a different task though. Find me one with a 50° apparent angular field of view and a 15 cm relief and I'll find you an eyepiece that is so huge that it's impossible to put another one next to it in order to make a pair of binoculars! Remember I've "stepped outside" to explore and enjoy the heavens, not to be a marksman (at least on this particular spacewalk). $\endgroup$ – uhoh Mar 21 at 1:57
  • $\begingroup$ @ikrase that's why I said "These are going to be pretty crazy looking binoculars!" I think this field is ripe for some interesting alternative technology. The first step is to establish just how crazy/awkward/huge normal eyepieces would be, and to do that we need to figure out how far away the faceplate puts anything outside it from the user's eyes. $\endgroup$ – uhoh Mar 21 at 2:06
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    $\begingroup$ Another possibility is less bulbous helmets. $\endgroup$ – ikrase Mar 21 at 4:49
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    $\begingroup$ I'm trying to understand which distance this question is asking for: the typical eye to helmet surface distance, helmet surface to telescope distance, or eye to telescope distance? $\endgroup$ – DrSheldon Mar 22 at 23:42
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To avoid this particular engineering problem I imagine space binoculars would just come with a screen so that you can hold it at your preferred viewing distance :P I'm imagining something akin to a camera with a telephoto lens

Edit: an example (not telephoto but an example of what a screen view through a high optical zoom camera lens would look like)

Something around the size of this camera is probably reasonable to mount on the helmet which would keep it fairly stable.

8x42 binoculars offer 8x optical zoom from my understanding and the camera in the video goes up to 83x so it should be simple to replicate the zoom level.


If they went with analog binoculars they would still likely be the kind which you press against the faceplate instead of held out in front of you though. As mentioned in the comment, stability is important when looking at magnified things with handheld devices.

Rifle scopes are designed to be viewed at a similar distance from the face and maintain a fairly high angular field of view.

man looking through rifle scope about 12 inches from face

It would probably be a monocular because there's no point in 2 lenses if you can't get one image for each eye.

What I'm getting at is that they have the technology to make optical devices for a variety of viewing distances and so it's unlikely that the viewing distance will be a device limitation.

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  • $\begingroup$ Great! Thanks for the edit. $\endgroup$ – uhoh Mar 21 at 8:26
  • $\begingroup$ "You can see what the view through a scope looks like above" well that view in the video is taken through another camera of unknown specification, not a human eye, we don't know what the apparent angular field of view is inside a space helmet but it certainly is not 50°. About the probably monocular, most high end microscopes have binocular views through a single objective and there are plenty of binocular eyepieces for single telescopes celestron.com/products/stereo-binocular-viewer $\endgroup$ – uhoh Mar 21 at 15:17
  • $\begingroup$ also payszpz.gq/… and aliexpress.com/i/32785667708.html $\endgroup$ – uhoh Mar 21 at 15:17
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    $\begingroup$ Anecdotally I have used a scope and it is close enough to the video that I feel confident using it as evidence, however I concede that this is insufficiently rigorous for SE, I'll edit the answer. Regarding monocular view: To the best of my knowledge it's impossible for humans to independently focus each eye on a different image when those images are around 12cm away from the face :P Hence why I'm assuming it would be monocular when wearing a space helmet. $\endgroup$ – Taha Attari Mar 22 at 14:13
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It's certainly possible to build a telescope (binocs are just pairs of 'scopes properly boresighted) with a distant pupil plane, or to build a focussing system with a digital display, as answers and comments already noted. The drawback of the digital display system is loss of stereoscopic vision, which doesn't matter if you're looking at stars but would matter if you're looking at some stuff nearby when you're on the Moon's surface, for example.

Given the overall cost of a modern spacesuit, I think it would be reasonable to design one with "flip-down" optics, the eye-lens of which are inside the faceplate and the field-lens outside. The whole assembly would be mounted on a common axis (bearings on each side of the helmet) to maintain alignment.

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As close as is practical

The aperture of a lens makes an angle (the angular aperture) with the pupil of your eye. The closer the lens is to your eye, the bigger the angle becomes:

angular aperture

When you look through a lens, you see an image behind the lens. However, you won't see any portion of the image that is outside the material of the lens. So you only see the part of the image that is within the angular aperture.

This means that the best way to see the image is to put the lens as close to your eye as possible, maximizing the angular aperture. Otherwise, you will only see part of the image; you can see the rest of the image by moving the lens or your eye side-to-side.

If you wear eyeglasses, try this. Look at your computer screen. Now take off your glasses, hold them a foot away from your face, and look through them at the screen. You probably won't see the whole screen. (It might be blurry, too, but that is a different issue.) Move your head side-to-side. Move the glasses side-to-side. The rest of the screen is there, but you only see a portion of it when the glasses are away from your eye.

An astronaut trying to use binoculars while inside a spacesuit will have the same problem; they will only see a narrow bit of what they are trying to look at.


Even if you were somehow able to make this work, only the astronaut can see what is happening. Why not replace the eyepiece with a video camera, so everyone can see what is going on? Then you could give the astronaut a video screen, too, and the optical issues would be solved.

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  • $\begingroup$ Okay, @uhoh, enough answering your questions. It's time I get to ask some as well. $\endgroup$ – DrSheldon Mar 23 at 15:46
  • $\begingroup$ You have only re-explained the problem that I've already explained! You're simply restating the problem. There is no information here about the distance that an astronaut's helmet will impose. $\endgroup$ – uhoh Mar 23 at 18:30

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