![]() Comparing Light Collection Between Pupil and Telescope?.Eyepiece diameter for small focal length planetary viewing on a large dobsonian?.What range of exit pupils work for observing the full moon?.The block quote relates small exit pupil diameters to "so little light to the eye that they are functionally useless." That might apply to extended objects differently than it would to unresolved objects like stars, asteroids or even distant planets in modest telescopes.Īstronomical seeing and diffraction may also come into play these effects can spread even unresolved objects in to apparently extended objects. ![]() Question: How does eyepiece exit pupil diameter affect image clarity and viewing experience? Can large scientific telescopes observe the Moon without being damaged?. ![]() Can a telescope ever increase the apparent luminance of an extended object?.My reference point here are my binoculars which have large diameter eyepiece optics which allows them to have a long eye relief passing them back and forth between an eyeglass-corrected vision person and someone without glasses is just a quick rotation of the eyecups to different distances.īut I never looked at the diameter of the exit pupil, as it seems not to be an independently adjustable parameter of the optics the exit pupil diameter is "baked in" to the choice of objective aperture and desired magnification.Īs a side note it's worth mentioning that our eye's entrance pupil diameter is often cited around 6 mm or 7 mm (in the block quote) so anything larger than that is wasted. I've always paid attention to eye relief of the exit pupil, which is how far it falls beyond the eyepiece. Either formula results in the same answer. Thus, a 10 mm ocular in our f/10 (100 mm clear aperture and 1000 mm focal length) telescope has a 1.0 mm exit pupil (10/10=1). OCULAR FOCAL LENGTH / TELESCOPE FOCAL RATIO = EXIT PUPIL Thus:įor example, our 100 mm clear aperture telescope with a 10 mm ocular is operating at 100x magnification and therefore has a 1.0 mm exit pupil (100/100=1)Īnother way to calculate exit pupil is to divide the eyepiece focal length in millimeters by the telescope's focal ratio (f/stop). Actually, I like exit pupils of at least 1.0 mm for decent viewing.Įxit pupil can be calculated by dividing the telescope's clear aperture (in millimeters) by the magnification produced by the ocular in use. Exit pupils of less than about 0.5 mm are so small and pass so little light to the eye that they are functionally useless. On the other hand, as the exit pupil decreases below 7 mm, lack of light becomes the basic limiting factor to what you can see at night. ![]() The pupil of fully dark-adapted human eye can dilate to about 7 mm diameter, so an exit pupil in excess of 7 mm is passing more light than the eye can accept. The exit pupil is the diameter of the "light pencil" that emerges from the eyepiece. A comment there cites Sky & Telescope's Stargazer's Corner: Adventures Under the Night Sky's Simple Formulas for the Telescope Owner for the math. ![]() This answer to Not Able to View Objects with Barlow Lens includes a calculation of exit pupil diameter as a way to address the limits of useful magnification when observing planets with a small telescope. ![]()
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