![]() ![]() So while a 6-inch telescope certainly can be used to view the Moon and planets at 300X on a good night, the view will always be dramatically better at 300X in a 12-inch telescope on that same night, and probably somewhat better even with apertures as big as 24 to 30 inches.Īnd for viewing galaxies, there is in practice no limit to the useful aperture even on the worst nights. Second, diffraction effects start to kick in - a point of light such as a star starts to appear as a visibly fuzzy disk even when the air is perfectly steady. Beyond 200X, many people will find even the Moon to be annoyingly dim. First, of all, the view becomes dimmer as the available light is spread out over a bigger area. Beyond an exit pupil around 0.75 mm (200X on our 6-inch scope), people may actually experience a decrease in detail, especially when viewing the dimmer, less contrasty outer planets Jupiter and Saturn. The gains continue beyond that for most people, but at a much slower rate. What if you're viewing a bright subject like the Moon or a planet? Most people see a fairly rapid increase in detail down to an exit pupil of 1 mm (150X on our 6-inch scope). ![]() And at an exit pupil smaller than 1 mm (150X), the galaxy may disappear entirely. If you're viewing a faint subject such as a galaxy, the view becomes increasingly dim, as the light gets spread over a bigger and bigger area. What happens when you use exit pupils smaller than 2 mm? In other words, going back to that 6-inch telescope, what happens when you use magnifications higher than 75X? So for viewing dim subjects, the best telescope is one that delivers an exit pupil in that range. In dim light, most people's pupils open to anywhere from 5 to 7 mm. At 300X, that means an aperture between 600 and 750 mm, or 24 to 30 inches. That hints that for viewing bright objects, the best telescope aperture is one that delivers a beam matching that width, with an exit pupil from 2 to 2.5 mm wide. In bright light, most people's pupils are 2 to 2.5 mm wide. Your own eye matches the stargazer of my thought experiment: it has a fixed magnification of 1X, but it can vary its aperture by opening or closing the iris that surrounds the pupil, just like the f/stops on a camera lens. Exit pupil equals aperture divided by magnification, so a 6-inch (150-mm) telescope at 300X has a 150/300 = 0.5-mm exit pupil. ![]() This is best understood in terms of exit pupil, the width of the light beam issuing from the eyepiece. But for most people, the most pleasing views will probably be through telescopes with 24 to 30 inches of aperture. Then the question becomes: "On a good night, at 300X, what's the best possible aperture for viewing Saturn's rings?" Again, people's answers will vary. So it's reasonable to ask questions like "On a good night, with a 6-inch telescope, what's the best possible magnification for viewing Saturn's rings?" Different people will have different preferences, but 300X is a plausible answer.īut now let's do a thought experiment and say that this same stargazer has access to any telescope he/she wants, but is stuck with 300X. In practice, stargazers are stuck with the aperture of their telescopes and get to vary the magnification by using different eyepieces. ![]() Second, just because a 6-inch telescope can deliver 300X doesn't mean that it's the best possible telescope for viewing at 300X. Magnification is important too, but it's definitely less important than light-gathering. A telescope's main job is to gather lots of light. First of all, it overestimates the importance of magnification. A 6-inch telescope can deliver 300X, so why would anybody use a telescope with more than 6 inches of aperture? People say that the maximum useful magnification for a telescope is 50X per inch of aperture, and that Earth's atmosphere rarely allows sharp images through any telescope at magnifications higher than 300X. A number of people in this forum and others have recently asked why big telescopes are useful, given the limitations imposed by Earth's atmosphere. ![]()
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