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!


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
    Commented Mar 21, 2021 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
    Commented Mar 21, 2021 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
    Commented Mar 21, 2021 at 2:06
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    $\begingroup$ Another possibility is less bulbous helmets. $\endgroup$
    – ikrase
    Commented Mar 21, 2021 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
    Commented Mar 22, 2021 at 23:42

4 Answers 4


Not much of an answer as others kind of cover it.

I just wanted to add this though:



NASA astronaut John Glenn uses binoculars to look at the Earth through the window of the Mercury-Atlas 6 spacecraft during the Friendship 7 mission February 20, 1962 in Earth orbit.

So, he tried, and maybe it worked because the visor he had on then looks a lot closer to the face than modern EVA space suits.

But as previous posts suggested, if sighting is an issue the most obvious way is to go with some form of televised sights.

It is an issue the military have been looking at for years.


If you still wanted the display helmet mounted, as mentioned before, easily done, even moreso given that current space suit helmets are fixed and not mobile. The issue is just then how the wearer adjusts the length and angle to best fit within their vision, which again is something looked at for years (NVG for air crew for example)


Again, for where the eyeball cannot quite get to where you need it, for a variety of reasons, you have ideas such as this.


so good, copied by these guys


and COTS ideas:


And of course, bringing the two together, as mentioned by others.


People are looking into it:


Consequently, a space suit helmet display system capable of meeting the demands of future space suit helmet display requirements in a decoupled-helmet, helmet-mounted design with sufficient eye relief is desirable.

From which appeared these:

FIG. 2 is a simplified top down illustration of an astronaut's head inside a helmet according to an exemplary embodiment;


FIG. 2 is a simplified top down illustration of an astronaut's head 202 inside a helmet 200 according to an exemplary embodiment. FIG. 2 is not to scale, but provides an example of the relative placement of features; additionally, although helmet 200 may comprise multiple layers and various shapes, the embodiment depicts helmet 200 as a circular barrier around the astronaut's head 202 that protects a pressurized bubble of oxygen-rich atmosphere 212 for the astronaut from the atmosphere 214, or lack thereof. The astronaut's eye 204 is shown having a direct viewing path 206 to the focusing lens assembly 210 located within housing 208. The distance between the pupil of the eye 204 and the focusing lens assembly 210 is a first predetermined distance, referred to as the first eye relief 216. Housing 208 may include other features of the display system. It is readily appreciated that housing 208 may be of any shape or volume, material, transparency or orientation that is suitable to meet the environmental and design requirements of the space suit helmet display system. Additionally, the housing 208 or individual components of the display system may be placed at any location on the helmet, and may be designed to operate with the right or left eye individually or placed centrally so that either eye may comfortably view the image generated by a single display.

NASA closed this one:

The Heads-In Display must work inside a spacesuit without being cumbersome and must be optimized for the proximity of the helmet bubble to the crewmember’s eyes.


Key performance parameters (targets) include:

Graphical Data Presentation: SXGA @ 40 deg FOV (possibly biocular);

Decoupled from User's Head - Large Eyebox: 100 mm x 100mm x 50mm (D);

Sunlight Readability: 500 fL inside visor, 1800 fL outside visor (>10 to 1 contrast).

NASA developed a sextant that an astronaut could use, both without helmet and with helmet visor on:


NASA again:

Two NASA HMD prototypes: (a) Wright-Patterson AFB HMD layout

(b) Technology Innovative Group HMD layout


NASA has conducted several researches for the implementation of HMDs in its EMU. This effort resulted in the production of four HMD prototypes. The Wright-Patterson AFB HMD (1987), the Hamilton Standard HMD (1988), the APA Optics HMD (1991) and the Technology Innovative Group HMD (1991).

All the NASA HMDs are mounted on the bubble helmet.

This prevents the use of the ExtraVehicular Visor Assembly.

Moreover these systems have high power consumptions 5 to 20 W and use high voltages to control the image source (mini CRT).

The Hamilton Standard HMD model is the only one that uses a LCD backlighted by a halogen lamp as image source.

Obviously not adopted.


Theres 2 further links that might have been useful in there but one is dead and the other is behind a paywall.

But ripe for picking up and improving upon. Many ideas or combination of ideas could work here.

The helmet is typically about 13inches and you can extract maybe a number from that but most information I have come across carries no inside dimensional details of the helmet.


does not have dimensions.

My search terms might be lousy though.

Anyway, decided to see for myself:



assume 13 in EMU helmet

assume head

6.33 in wide

8.11 in deep inc. nose

(the average human head measures 6-7 inches in width and 8-9 inches in length)

results in:

eye to visor = 3.55 in (assuming flat plane)


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.

  • $\begingroup$ "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
    Commented Mar 21, 2021 at 15:17
  • $\begingroup$ also payszpz.gq/… and aliexpress.com/i/32785667708.html $\endgroup$
    – uhoh
    Commented Mar 21, 2021 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. $\endgroup$ Commented Mar 22, 2021 at 14:13

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.


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
    Commented Mar 23, 2021 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
    Commented Mar 23, 2021 at 18:30

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