Are there any astronomical bodies that would be dangerous to my vision to view through a telescope? Obviously the sun is dangerous, but are there other bodies at night I should avoid?
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$\begingroup$ I think telescopes already have some kind of filter to prevent eye damage, which are good enough to view e.g. the Moon without a problem. $\endgroup$– DanuJun 5, 2014 at 13:53
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$\begingroup$ @Danu would that include smaller commercially-available amateur telescopes? $\endgroup$– garrettendiJun 5, 2014 at 13:54
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$\begingroup$ Yes. I am quite sure that I read this somewhere on this very site, but have been unable to locate the question right now. Maybe someone else will know where to find it: It was someone asking about what he thought was a small defect in his telescope, but it turned out to have been caused by a filter which is apparently present in all (most?) telescopes. $\endgroup$– DanuJun 5, 2014 at 14:00
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4$\begingroup$ I have a moon filter for my 10" telescope, but it is an accessory. It is not a standard component of a telescope. $\endgroup$– DavePhDJun 5, 2014 at 14:19
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$\begingroup$ If you happen to watch in the wrong direction at the wrong time: youtube.com/watch?v=dtWeH4-Ugy4 $\endgroup$– Count IblisJun 5, 2014 at 15:08
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2 Answers
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Compared to naked eye view, a telescope image never increases surface brightness. This fact is related to the concept 'etendue'.
However, although the image formed on your retina is never brighter than the corresponding naked eye image, the image through a telescope is magnified. This means that looking through a telescope at the sun can expose your whole retina to the brightness that in naked eye view would be limited to only one or a few retina cells. So the difference is that with naked eye view you tend to destroy a few cells, while with a telescope you can instantly destroy your whole retina.
On observing the moon: the surface brightness of the moon is comparable to that of deserts at earth exposed to bright sunlight. So watching the moon through a telescope that yields maximum brightness (a telescope and eyepiece that combine to create an exit pupil as large as your eye pupil) is like walking through a desert without sunglasses. It is therefore more comfortable to have some grey filter in the light path. Having said this, and although I am no medical specialist, I do want to offer the following consideration. I would be surprised if watching the moon through a telescope without filters can inflict permanent damage to healthy eyes: the harmful UV components won't pass thru the eyepiece glass.
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I add this personal experience in addition to the other (correct) answers. Using my 4.5 inch diameter telescope (11.4 cm) with a 20 mm eyepiece (and a 900 cm primary mirror focal length, so magnification 45x) to view the full or nearly full moon without a filter is painful for my eyes (not nearly as bad as the sun, but still too much). Even when I reduce the aperture diameter to about 3 inches the full or nearly full moon is still very difficult to view comfortably with my eyes. A crescent moon is just fine though with the smaller aperture.
With a telescope of my size though there is nothing other than the sun and the non-crescent moon that is too bright for me to look through with my telescope. The next brightest objects in the night sky after the sun and the moon are the ISS and the planets and I've viewed Jupiter, Saturn, and Mars in perfect comfort even with my telescope acting on maximum magnification (which is 225x).
answered Jun 5, 2014 at 15:00
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$\begingroup$ An Iridium flare might be fairly dazzling, but probably wouldn't cause permanent eye injury. I suppose the same would hold true for a meteor fireball. Dunno about the Moon cranked up very high. The only astronomical object that would definitely be harmful would be the Sun. Note that cheap telescopes have a filter (welder's glass filter) that screws into an eyepiece -- avoid these, as they get very hot and can shatter without warning. Use a proper full-aperture solar filter if you want to observe the Sun, and make sure it's very securely fastened! $\endgroup$ Jun 5, 2014 at 18:59
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$\begingroup$ @PhilPerry Wouldn't this comment be better placed as a comment to the question itself? Or as an actual answer? $\endgroup$ Jun 5, 2014 at 23:59
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$\begingroup$ It's probably worth noting that when you look into your telescope your eyes are probably pretty well dark-adapted. This is enough to cause discomfort in the same way that walking from a dark room into sunlight is painful. $\endgroup$ Jun 6, 2014 at 20:39
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$\begingroup$ That definitely adds to the discomfort. $\endgroup$ Jun 6, 2014 at 22:02