6 Inch vs 8 Inch Telescope

This is the aperture step where most people stall, and the reason is that the optics and the logistics point in opposite directions. Every optical figure on this page favours the 8 inch. Every practical figure favours the 6 inch. Neither set of numbers is wrong, and the decision is genuinely about your house rather than about your sky.

The committed short answer: buy the 8 inch if the telescope will live on a ground floor with a door to the garden, because 1.78 times the light is a real gain you notice every session. Buy the 6 inch if it has to be carried up or down stairs, or stored in a flat or a car boot, because a telescope that takes two trips gets used on planned nights only, and planned nights are much rarer than casual ones.

What does the extra two inches of aperture actually buy?

Light gathering scales with the area of the aperture, not its diameter, so it scales with the square. Going from 6 inches to 8 inches multiplies the collecting area by 1.78. That is the headline figure and it is the honest one.

Resolution follows a different rule. The Dawes limit, the closest double star an aperture can separate, is 4.56 divided by the aperture in inches, in arcseconds. So a 6 inch resolves 0.76 arcseconds and an 8 inch resolves 0.57. And maximum useful magnification is roughly 50x per inch of aperture, or 2x per millimetre, which puts the ceiling at 300x and 400x respectively.

Two rows in that table deserve reading twice. The limiting magnitude difference is about six tenths of a magnitude, which sounds small and is actually the difference between a galaxy being a suspected smudge and being an obvious one. And the exit pupil row explains something people notice but rarely name: at the same magnification the 8 inch delivers a wider cone of light to your eye, 2 mm against 1.5 mm, so the image is simply brighter at identical power. Exit pupil is aperture divided by magnification, and it is the number that decides how bright a view feels.

Worth knowing: a 6 inch f/8 Dobsonian and an 8 inch f/6 Dobsonian both have a 1,200 mm focal length. The same eyepiece gives the same magnification and the same true field in both. The difference is entirely brightness and resolution, not framing, which makes them unusually directly comparable. The magnification calculator and the field of view calculator will confirm that with your own eyepieces.

What can you see with each in practice?

Numbers only matter if they change what appears in the eyepiece, so here is the honest target by target account.

The Moon and planets look almost identical on an average night. Both apertures exceed what typical seeing allows. Atmospheric turbulence usually limits detail to around 1 arcsecond, and both of these resolve better than that, so the atmosphere is the bottleneck rather than the telescope. On the handful of genuinely steady nights a year the 8 inch pulls ahead clearly, showing festoons in Jupiter's belts and the Encke minima in Saturn's rings that the 6 inch only hints at. If planets are the main goal, the difference is real but smaller than the price gap suggests.

Globular clusters are where the gap opens. A 6 inch resolves the outer stars of M13 into points against a grainy core. An 8 inch resolves it much closer to the centre, and the difference between "granular" and "a ball of individual stars" is the single most convincing demonstration of aperture in the sky.

Galaxies are where the gap is largest. Most galaxies are small and faint, and faint objects respond directly to collecting area. A 6 inch under a dark sky shows the bright Messier galaxies as elongated glows with brighter cores. An 8 inch starts showing dust lanes in the brightest of them and pulls in a substantially longer list of fainter ones.

Nebulae depend far more on your sky than on your aperture. This is the honest complication. Emission nebulae have low surface brightness, so skyglow competes with them directly, and under a bright suburban sky both apertures show much the same disappointing amount. Driving to a darker site changes the view more than two inches of aperture does. That trade is worked through in how to deal with light pollution.

What neither aperture will do, at any price, is show colour in a nebula. Human night vision runs on rod cells, which are nearly colour blind at these light levels, so the Orion Nebula is a grey-green mist in a 6 inch, an 8 inch and a 20 inch alike. Set that expectation before you spend, with what you can actually see with a telescope and Messier object visibility by aperture.

What does the 8 inch cost you in weight and bulk?

This is the half of the decision that no specification sheet frames properly, and it decides whether the telescope gets used.

An 8 inch Dobsonian such as the Sky-Watcher Classic 200P weighs close to 48 lb assembled. Nobody carries that in one piece, so it becomes two trips: the base, then the tube. A 6 inch Dobsonian like the Orion SkyQuest XT6 is around 35 lb, which many adults manage assembled, in one movement, through a door.

That difference sounds trivial written down. In practice it is the difference between going outside for twenty minutes on a Tuesday because the sky happened to clear, and deciding that tonight is not worth the setup. Over a year, the smaller telescope that goes out forty times shows you far more than the larger one that goes out eight.

Storage compounds it. Both tubes are roughly four feet long, so neither fits a cupboard shelf, but the 8 inch base has a noticeably larger footprint and the tube is materially fatter to get an arm around. If the telescope has to live in a bedroom corner or share a car boot with luggage, measure before you commit.

Which should you buy?

Buy the 8 inch if the telescope lives near a door

Ground floor, garden or patio, and somewhere the base can sit permanently. Under those conditions the 8 inch is straightforwardly the better telescope: 1.78 times the light, better resolution on the steady nights, and globular clusters that resolve rather than shimmer. The Classic 200P is the plain version and the Apertura AD8 is the version that includes most of the accessories you would otherwise buy separately. If finding things is the part you expect to struggle with, the StarSense Explorer 8 inch Dobsonian adds a phone dock that plate solves the sky, for roughly $150 over the plain version.

Buy the 6 inch if you carry it up stairs, or if it travels

Flats, upstairs bedrooms, anyone who drives to a dark site, and anyone who is honest with themselves about how much setup effort they will tolerate on a work night. The SkyQuest XT6 is the long standing reference here, and a 6 inch f/8 is a genuinely excellent planetary telescope because the long focal ratio is forgiving of cheap eyepieces and produces a small central obstruction. This is not a consolation prize.

Buy neither if you cannot store a four foot tube

The honest third option. If a 48 inch tube genuinely does not fit your life, a 130 mm tabletop Dobsonian like the Sky-Watcher Heritage 130P collapses down, goes outside in one hand, and shows real structure in the Orion Nebula and the Cassini division on a good night. It gathers less than half what the 8 inch does, and it will be used ten times as often, which is a trade many people should make deliberately rather than reluctantly. A folded 8 inch Schmidt-Cassegrain such as the NexStar 8SE is the other escape route: the same aperture in a tube you can carry one handed, at roughly twice the price, and the Schmidt-Cassegrain versus Newtonian comparison covers what that costs you optically.

What to spend the difference on if you choose the 6 inch

The gap between a 6 inch and an 8 inch Dobsonian is roughly $250, and there are three things it buys that will improve your views more than aperture would under a typical suburban sky.

• A good high power eyepiece. The bundled eyepieces on most Dobsonians are the weakest optics in the light path. A proper eyepiece set or a single well corrected planetary eyepiece changes every night, where aperture only changes faint targets.
• A better finder. A Telrad reflex finder projects a bullseye on the sky at true scale, which is the difference between star hopping working and not working. See how to find objects in the night sky.
• Fuel. Driving forty minutes to a darker site changes the sky more than two inches of aperture does, particularly on nebulae, and it costs nothing after the first tank.

If you already own a 6 inch and are contemplating the upgrade, one honest test: keep a log for a month of which objects disappointed you. If they were faint galaxies, buy the aperture. If they were nebulae or the planets, buy a filter, a better eyepiece or a darker site instead, and see the Dobsonian roundup and how to choose a telescope before committing.

We review them on their own too, in full detail: the Celestron NexStar 6SE and the Celestron NexStar 8SE.