Oversized telescopes dangerous?

[Superdavo]

Hi everyone!

I've been thinking, and have heard that with a 25+ inch scope, your averted vision can be affected, and it's got me wondering if in a truly monstrous scope (1 or 2 meter aperture?) could ever actually damage your eyes on any object other than the Sun? I know nobody will have an instrument of that scale, but would it be possible?

David

[swamp thing]

In a word no. Extended objects like the moon are no brighter through any size scope than with the naked eye.

[LukeSkywatcher]

As ST says, the only object we can see that will damage your eyes is the Sun. Although,i must admit that the first time i observed the Moon with my 8" scope, it did seem about a million times brighter then in my 5" scope and was unpleasant for a few seconds.

[JamesF]

Weren't there some pictures linked a while back of a chap taking delivery of his one metre-ish mirror?

James

[Moonshane]

Hi everyone!

I've been thinking, and have heard that with a 25+ inch scope, your averted vision can be affected, and it's got me wondering if in a truly monstrous scope (1 or 2 meter aperture?) could ever actually damage your eyes on any object other than the Sun? I know nobody will have an instrument of that scale, but would it be possible?

David

as above, it's not possible to damage your eyes with anything other than the sun as you are looking at either point sources, faint extended objects or bright planets/moon which have the same amount of reflected light as with naked eye. you are just stretching it over a wider area with more magnification.

when you say your averted vision, do you mean you night/dark adaptation? if you do then looking at brighter objects through any scope will do this (I mean solar system objects) but I doubt it's possible with anything else. if the object is so bright your dark adaptation is affected then you'll start to see colour in the object as your cones light up. with some large scopes and dark skies you can see colour in some objects of course but even then I doubt dark adaptation would be affected.

[nytecam]

Hi everyone! I've been thinking, and have heard that with a 25+ inch scope, your averted vision can be affected, and it's got me wondering if in a truly monstrous scope (1 or 2 meter aperture?) could ever actually damage your eyes on any object other than the Sun? I know nobody will have an instrument of that scale, but would it be possible? David

David - fear not you're safe but some folk do own BIG scopes - like Jimi Lowrey in the States with his 48" Dob - but I'm not worried as I can snap in a few seconds via my 12" SCT from suburbia what he and guests just about see high in the Rockies

Mind you silly amateurs using big scopes can fry the camera sensor - heard of one accredited school using a remote 3m Faulkes telescope aimed it at Sirius eg the brightest star in the sky apart from the sun

[acey]

I've been thinking, and have heard that with a 25+ inch scope, your averted vision can be affected

Where did you hear that? I've never heard anything like that, and can't see why it would be true. The biggest danger with a scope that size, I should think, is falling off the step ladder you'd need to reach the eyepiece (assuming it's a Newtonian).

[umadog]

No optical device can make an object brighter than it already is, if it did it would violate the conservation of energy. All a telescope can do is magnify whilst conserving brightness. Brightness is a function of the exit pupil size. So the moon at a 5 mm exit pupil through a 25" is the same brightness as the moon through an 8" with a 5 mm exit pupil. The difference between the two would be magnification.

If you're really dark adapted (super dark site) and you look at the night sky through any telescope with a large (i.e. >6mm) exit pupil then you will mildly alter your dark adaptation in one eye. I've experienced this many times. You blink your eyes back and forth afterwards and the difference is mild but just discernible.

[alan potts]

Telscopes of that size can damage the bank balance and your back but i believe thats about all.

Alan

[rowan46]

theoretically yes. you could fall off the stepladder in the dark, landing in such a way as to injure your eye

[laser_jock99]

The most damaging part of the solar spectrum with regards to eyes & telescopes is in the IR region (i.e. heat radiation). With the moon & DSO's you just don't get that. Put another way - you can't fry an ant with a magnifying glass under a full moon!

[cotterless45]

Trouble there is to get them to come out under a full Moon !

[AndyWB]

Yeah, I've read something like this before - and I still don't get it.

I mean, consider the moon, sat there reflecting photons all over the place. I look at the moon, and the photons that happen to be flying in the direction of my iris pass through and land on my retina.

Next, I take my telescope and look at the moon, with low enough magnification to fit the whole thing in. This is so we're still dealing with the same source. Now photons sleeting through the universe that enter my OTA get funnelled into my eye. (Again, possibly stopped by the iris). The telescope has a much bigger surface to acquire those photons over, so it must gather more of them, right? And won't they end up on my retina? If not, why can I see much dimmer objects with the scope than my eye?

Or to put it another, an ant on the patio in sunlight is fine. An ant on the patio under a child's magnifying glass is a scorched ant.

So ... what did I miss? I've read enough things agreeing with the comments above, so clearly I've missed something.

[Moonshane]

I am starting to edge towards the cliff of ignorance as my knowledge of optical theory is not large but I think the following is about right. the moon is viewed with reflected light only and therefore the surface can only reflect X photons off the surface. the moon sits in the line of sight of the sun and the photons that hit it and bounce off it are seen as the portion of the moon that is lit up. these then head towards your eye and if you are looking at the moon with naked eye you collect the maximum possible photons that head your way (this is likely to be different for every person). the moon is at its brightest at this point although of course some of the photons might miss your eye but you cannot do anything about this as you have an iris (which might differ in size from your fellow observer).

the moon covers a small amount of your view (retina) and therefore the resolution of the (6mm) aperture is low and therefore you cannot see fine detail. when you use your telescope you increase the aperture which generates more resolution and therefore you see more fine detail. using an example of a scope, if you look at the moon at 50x in a f5 6" newt, the exit pupil would be 3mm and therefore dimmer than your fully dilated pupil so you see more detail at a higher power but the image is dimmer than naked eye. if you double magnifcation, you halve the exit pupil so dimmer again.

in short if you increase aperture, the same object at the same magnification will have more resolution and therefore detail (and probably in the case of faint objects more apparent size) than with a smaller aperture. it's not that more photons are collected, it's that more becomes visible for the reasons above.

if the above is not right, please correct me as that's my own understanding of this somewhat counter-intuitive aspect of astronomy.

[acey]

I can point a lens or telescope at the sun and burn a hole in a piece of paper. Could I do the same with the moon if I had a big enough telescope? No - if I could then I would have a way of manufacturing heat: I'd use the very cold moon to make paper hot (a violation of the first law of thermodynamics). A telescope can't make energy. The reason why I can burn holes using the sun is that the sun is hot.

Think of the total amount of light coming from a source of a certain (angular) size. Now double your distance from that source. The intensity of light is reduced by a quarter (because of the inverse square law) and the apparent size is reduced by a quarter (because of perspective). So the "brightness per angular area" is the same: surface brightness is independent of distance from the source. The surface brightness of the sun seen 93 million miles away is the same as the surface brightness seen a hundred times closer (if we ignore stuff like atmospheric absorption). Magnify the sun by a hundred in a telescope and you effectively bring it a hundred times closer. This is not good for your eye, and nor is staring directly at the sun with the unaided eye. In either case your blink reflex should save you pretty quickly.

For a telescope of aperture A, and eye-pupil diameter p, the light grasp G is defined as (A/p)². When an object is magnified in a telescope by a factor m, the total brightness of its image is multiplied by G, while its area is multiplied by m². Hence its surface brightness is multiplied by G/m². But the lowest useable magnification is (A/p), hence the surface brightness is multiplied by a number less than or equal to one. A telescope makes extended objects larger but no brighter; the advantage of aperture is that it minimises the amount of dimming.

You can make the light grasp as big as you like - make a bigger and bigger mirror. But you've got to fit the image inside your eye, which means the exit pupil can be no larger than the eye pupil diameter, which means you're never going to be able to manufacture excess surface brightness and burn a hole in your retina with moonlight.

But why can we see dimmer objects with a telescope than with the naked eye? Two reasons. In the case of stars the telescope doesn't make stars any bigger (because they're point sources, if we ignore atmospheric seeing, diffraction etc) so it intensifies their light while also magnifying and dimming the background sky. In the case of extended sources (e.g. galaxies) the telescope magnifies and dims both the galaxy and the sky by an equal amount, so the object's contrast remains the same. At low light levels our contrast threshold depends on size: we can read large print by moonlight but not small print. The telescope makes the dim object big enough to see. This will only work if the object already has enough contrast against the sky - which is why a large aperture scope is useless if the sky is too bright. No amount of aperture will show you galaxies in daylight.

[IanL]

Magnify the sun by a hundred in a telescope and you effectively bring it a hundred times closer.

Another way to think of this is to imagine you were on the surface of a planet so close to the Sun that it filled most of the visible sky. Would you worry about burning your retina by looking up at it? No you wouldn't because you'd be too busy worrying about your hair catching fire, rapidly followed by everything else that matters to you. Yet the Sun in that sky has the same surface brightness as the Sun we see from Earth for the reasons stated above.

This is a really good site that goes through all the theory in detail explaining why a telescope can't increase the surface brightness of an extended object :

http://www.rocketmim...Brightness.html

[umadog]

The really easy way of thinking of this is as follows. You can think of the telescope as an instrument for bringing an object closer. If you were standing on the moon, you could look down at the ground and see the surface. It would be similar to looking at a concrete parking lot during the day here on Earth. You don't burn your eyes looking a road surface and you wouldn't burn your eyes looking at the moon's surface (although there would be more UV on the moon). Thus, there's no way that any telescope of any size could burn your eye when looking at the moon. The reason the sun is dangerous through a telescope is because the surface is hot.

As Steve says, IR is a major problem with the sun. IR damages the retina by heating. There's also UV damage, though. Even if you blocked out the IR, you could still damage your retina by UV phototoxicity. There's a really interesting article here: http://mintaka.sdsu.edu/GF/vision/Galileo.html [NOTE: it's an interesting article but I suggest doing some of these experiments yourself isn't a good idea... ]

[ronin]

I don't get the question.

The OP says affect your averted vision, if the scope were big enough and so a bright enough image then you would not be using averted vision you would simply look at it directly without averted vision.

If you mean bright enough to knock your night adapted vision out for a while that's easy. A reasonable scope 4-8" on the moon will do that and leave a nice residual image for a couple of minutes that makes walking round highly amusing.

FLO will sell you a filter by the way.

[Vapouriser]

One if the people at Greenwitch( sorry cant remember who) a couple of years ago told me a young person working for them took a large telescope out and pointed it at a full moon all night. Well hours anyway. It certainly caused him problems as all he could see for 2 days was the a full moon! He couldnt drive because if it. But it didnt actually damage anything. Did nt even get a decent photo!

[Obsession 30]

Looking through my 30" at the moon certainly mess up your night vision for a while

[Moonshane]

One if the people at Greenwitch( sorry cant remember who) a couple of years ago told me a young person working for them took a large telescope out and pointed it at a full moon all night. Well hours anyway. It certainly caused him problems as all he could see for 2 days was the a full moon! He couldnt drive because if it. But it didnt actually damage anything. Did nt even get a decent photo!

but did they also do a test with naked eye and looking at the full moon all night? I'd have thought that anything that is not good to look at anything, lit in any way, 'all night'.

[eifionglyn]

This is a really good site that goes through all the theory in detail explaining why a telescope can't increase the surface brightness of an extended object :

http://www.rocketmim...Brightness.html

That is good. Not sure I understand it all. The maths is basic enough for even me to follow, but I can't seem to link what it says to my real-world experiences. In my suburban garden I cannot see M31 with the naked eye. I can through my scope. M31 is big enough to see with the naked eye so how does my scope let me see it if it doesn't make it any brighter? I think I get the bit about point sources of light that a scope increases the contrast by dimming the background, hence taking you from say 5th to 12th magnitude for example, but with an extended source like M31 won't it get dimmed just as much as the background sky glow?

Dammit I thought I understood how my scope worked, having been looking through it for the last four years. Now I'm not sure I understand what it's actually doing at all!

[michael.h.f.wilkinson]

Looking through my 30" at the moon certainly mess up your night vision for a while

So does looking through an 8". If I want to see DSOs I do not want to look at the moon at all.