Dobsonian Telescopes, and Why Experienced Observers Recommend Them

200mm is a different category of instrument, not an incremental upgrade. Against the 70mm refractor at the other end of this list the light-gathering ratio is 200 divided by 70, squared: 8.2 times more light reaching your eye.

Two times 200mm gives 400x of useful magnification, which is more than the atmosphere delivers on all but a handful of nights a year. In practice this telescope is limited by the sky rather than by its optics, which is exactly the position you want to be in.

Dawes' limit at 200mm is 116 divided by 200, which is 0.58 arcseconds. The Cassini division, at roughly 0.7 arcseconds, is comfortably inside that. So is the Great Red Spot, and so are hundreds of Messier objects a 70mm scope simply cannot reach.

The honest cost is bulk. The tube is long and the base is wide, and this is a telescope you carry outside in two trips. Everyone who owns one tells the same story: they used it far more once they stopped storing it somewhere awkward.

This is the aperture-per-dollar champion of the beginner shelf and it is not close. You get the same 130mm parabolic mirror found in telescopes at twice the price, and you get it without paying for a tripod, a slow-motion assembly or a hand controller.

Two times 130mm is 260x of useful magnification. Dawes' limit at 130mm works out to 116 divided by 130, which is 0.89 arcseconds. The Cassini division in Saturn's rings spans roughly 0.7 arcseconds at its widest, so this telescope sits right at the edge of showing it on a still night and will show the rings themselves easily.

The catch is the word tabletop. It needs a surface at roughly chest height when you are seated. A garden table works, a plastic patio chair does not, and buying one without thinking about where it will stand is the single most common regret with this design.

The tube collapses, which is what makes it live in a cupboard rather than a garage. That matters more than it sounds: the telescope you can carry outside in one trip is the telescope you use.

150mm sits in a genuinely useful place. Two times the aperture gives 300x of useful magnification and Dawes' limit works out to 0.77 arcseconds, so the Cassini division is within reach on a steady night.

Against the 8-inch it gives up 78 percent of the light, which is Sky-Watcher's own published comparison rather than ours. Against a 100mm refractor it gains 232 percent, also their figure. Those two numbers together are the whole argument for buying by aperture.

The practical case for the 6-inch over the 8-inch is transport. It fits across a back seat, one person can move it in a single trip, and a telescope that gets used beats one that is technically better and stays in the garage.

This is a floor-standing Dobsonian, so unlike the tabletop models it needs no furniture. You sit on a low stool beside it, which is the most comfortable way to observe there is.

This is the Heritage 130's argument with 33 percent more mirror. Light gathering scales as the square of the aperture ratio, so 150 over 130, squared, is 1.33 times the light of its smaller sibling.

Two times 150mm is 300x of useful magnification and Dawes' limit is 0.77 arcseconds, which puts the Cassini division inside its reach on a good night.

At f/5 the 750mm focal length keeps the tube short enough to collapse, which is the entire reason this design exists. A conventional 150mm Newtonian tube is over a meter long and lives in a garage.

The same tabletop caveat applies as with the 130: it needs a stable surface at seated chest height. Check you have one before you buy, not after.

203mm gathers 841 times the light of the naked eye, which is Celestron's own published figure. Against the 130mm scopes further up this list it is 2.4 times the light, from the ratio 203 over 130, squared.

Two times 203mm gives 406x of useful magnification; Celestron publishes 480x. Dawes' limit is 0.57 arcseconds, so the Cassini division, the Great Red Spot and hundreds of Messier objects are all inside its reach.

The published limiting stellar magnitude of 14.2 is the number that explains what deep sky means. It is roughly 4,000 times fainter than the faintest star a good naked eye sees from a dark site.

The reason to choose this over a plain 8-inch Dobsonian is the phone finder. An 8-inch tube with a 0.57-arcsecond resolution limit is wasted on somebody who cannot locate anything to point it at.