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why only one eye ? Cyclops rules

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I was going to post this in equipment discussion but I think this is more appropriate.

New as I am to astronomy I'd like to know why telescopes always only have one eyepiece (after all we have of course 2 eyes).

The other reason for asking this , is that whilst trawling through ebay I see several items advertised (admittedly by the likes of Seben) which allow you to use both eyes with your scope ; which surely must be more comfortable and natural??.

So... are these adaptors useful ??, why does everyone only use one of their eyes ?

Incidentally, I'm halfway through a book called "Astronomy for Dummies" by Stephen Maran (which is really informative and should be read by anyone thinking about this as a pastime) but he hasn"t referred to this yet !

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Good question :)

Technically, we should see less thru a binoviewer (the prism block reduces light) but, being a million+ year old species, we have evolved to use both eyes and if our brain is allowed to process info from both eyes, we see more.

(Strangely, after we have noticed something extra thru a binoviewer, we can then see it thru a single eyepiece...)

However, binoviewers are not suitable for all telescopes and cheap ones are to be avoided! Those offered by Stellarvue and Williams Optics are the first of the good ones:



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Well, as we know, there ARE indeed binoviewers. My (supposed) objection is added weight and also additional optical path which might make coping with the in-focussing hard with some telescope designs. Catadiopteric systems are more suitable perhaps, but there are solutions for other setups. Then there's the financial impact of "doubling up" on some of your eyepieces. But I suspect I'll have a go one day! :)

Clearly (I'm open to debate!) you aren't going to get much in the way of conventional stereoscopy with objects at "infinity". But I recall there are sound reasons why TWO eyes can be better than one. IIRC, the EYE, like most electronic(!) detectors, suffers from noise, "floaters" etc. When two eyes combine, there are various "cancellations" which improve the binocular view over the Cyclopsian(?)! in several ways... :lol:

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A binoviewer splits the incoming light into two paths, each of which is fainter than the original one. On bright objects like moon or planets this isn't important, but for viewing faint DSOs you want to make the most of whatever light there is, so splitting it between two eyes is not a good idea.

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I think it might matter less than you might think for DSOs. The information from the right and left eyes converge in the visual cortex. Sure, you may loose out on the super-faint, near-threshold, stuff but chances are that even with DSOs you will pick out details you won't have seen monocularly. Your visual system is designed to work with both eyes.

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Sure, you may loose out on the super-faint, near-threshold, stuff

That happens to be what I'm interested in - it would also apply to faint details within DSOs. You're right that theoretically at least, a binoviewer should be no worse than a monocular view. The generally accepted correction to contrast threshold for monocular/binocular vision is root-2, i.e. the threshold for monocular vision is 1.414 times higher than it is for binocular vision. And a binoviewer reduces the surface brightness at each eye by root-2 compared to the monocular view, so the two effects ought to cancel out: a slightly dimmer view but a better perception of contrast thanks to using two eyes. But in practice you're putting extra glass into the system, and you're losing a bit more light.

So for DSOs at least, the potential advantage of the binoviewer (comfort of use) tends to be outweighed by the disadvantages: light loss, and having to buy two of every eyepiece.

There's also the problem, already mentioned, that in many scopes a binoviewer won't reach focus. The work-around is a negative lens element (i.e. Barlow) which gives the binoviewer a higher magnification than many users would like.

If you want the low-power, wide-field experience of ordinary binoculars then the only way to do it is to put two telescopes together.


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