Opinions about the real limit will vary between 10" and 16", with most of these scopes being f/4 to f/5. If it's a tube design, instead of truss, then it must be even smaller. Many old experienced users say that a 300 mm (12") f/5 truss dobsonian is the biggest that can be used without major effort. So, realistically, there are limits to what a big scope could do. If you own a huge 24" dob then you could only exploit all its light-gathering power in those rare places with zero light pollution. You'd have to drive an hour or two to the dark skies away from the city to see all the really faint fuzzies. But remember, if you live in the city, there's something called light pollution, due to all the lights around you, that makes the sky glow with light, so the faint objects are hard to see. So it would allow you to see fainter objects. A 300 mm scope will capture 4x more light than your 150 mm. Beyond that, it's pretty rare that you could crank up the mags without getting a large blurry blob instead of a crisp image. Related to resolution, an alternative way to look at it - magnifications up to 100.150x are basically almost always usable. People living in the american north-east often find that instruments bigger than 4.6" are seeing-limited a lot. How often you'll get 1 arcsec seeing, or better, in the place you live? Not very often, unless you live in California, or Hawaii, or some other place with excellent seeing, far from the jet stream. The bigger the scope, the less often you're able to use its full resolving power, because atmosphere is turbulent and keeps churning all the time.Į.g., a 100 mm (4") instrument will provide 1 arcsec resolving power. Realistically, instruments bigger than yours (which has about 0.7 arcsec resolving power) will use their resolving power much less often, only during those fleeting moments of perfect seeing. There's something called "seeing", which is the stillness of the air that allows you crisp sharp views, and it varies a lot with the place and time. You'd think a 300 mm (12") aperture scope would provide 2x more resolving power, but in reality it's not that simple. In terms of resolution, things go downhill pretty fast after 150 mm (6"). There are two things that a bigger aperture will provide: more resolution, and more brightness. Iįind even a 12-inch Dob becomes bulky, cumbersome, and heavy. Nicest telescope in the world is useless if it never gets used. Above 10-inches, the more aperture the better, provided you canĬomfortably transport, set up, and operate it.Spot." It's about the smallest aperture to show significant detail inĭeep sky objects, yet is compact and light enough to be easily A 10-inch Newtonian on a Dobsonian mount is something of a "sweet.Experienced observersĬan tease amazing observations out of tiny scopes, but most of us Show you much, especially if you're a beginner. Unless you have some specialized purpose, don't consider anything.There are lots of mathematical answers to this question, but I'd like to make a few qualitative observations instead, based on 54 years using telescopes of all kinds and sizes, from 40mm refractors to 74-inch reflectors.
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