On a Dobsonian, almost the whole price is the mirror. That makes the decision unusually clean: pick a diameter and everything else follows. What follows is worked out below — first what each rung buys you optically, then what it costs you in weight, boot space, cupboard space, and the hour before the image settles down.
We earn a commission if you buy through our links, at no cost to you. We have never put any of these telescopes under a sky. The optical figures here are computed from each listing’s own published aperture and focal length, which is why you can check every one of them.
The four rungs, side by side
Aperture sets four things at once, and no eyepiece, filter, coating or mount moves any of them. How faint a star you can reach. How close a double you can split. How far you can push magnification before you are enlarging blur rather than detail. And how much light arrives at your eye compared with your naked eye alone. Every cell below is calculated from the diameter in the first column.
Size
Aperture
Faintest star
Splits doubles
Useful max
Vs. your eye
6″
152.4 mm
mag 13.6
0.76″
305×
474×
8″
203.2 mm
mag 14.2
0.57″
406×
843×
10″
254.0 mm
mag 14.7
0.46″
508×
1,317×
12″
304.8 mm
mag 15.1
0.38″
610×
1,896×
Limiting magnitude from 2.7 + 5·log₁₀(D) and valid only for a genuinely dark site — from a suburban garden expect one and a half to two and a half magnitudes worse on every row, which shifts all four rungs down together and changes none of the comparisons. Resolution from Dawes’ 116/D. Useful magnification at the conservative 50× per inch rather than the 60× per inch manufacturers print, because on an ordinary night the atmosphere is the limit and not the mirror.
Those apertures are the exact conversions — 6 inches is 152.4 mm. Manufacturers round, and Sky-Watcher sells its 6-inch as a 150 mm and its 12-inch as a 305 mm. The gap is worth 0.03 of a magnitude, which is far below anything an eye can register. Nobody is being cheated; the round number is just easier to print.
What each step actually buys you
This is the part that gets exaggerated in both directions. People either treat a two-inch jump as transformative or dismiss it as marginal, and neither is right. The honest version is arithmetic:
Step up
Extra light
Fainter by
Resolution gain
6″ → 8″
+78%
0.62 mag
0.76″ → 0.57″
8″ → 10″
+56%
0.48 mag
0.57″ → 0.46″
10″ → 12″
+44%
0.40 mag
0.46″ → 0.38″
Read the middle column again, because it is the one that decides this. Going from eight inches to ten collects 56% more light, which sounds enormous, and it is worth 0.48 of a magnitude. Ten to twelve collects 44% more and is worth 0.40. Each of those steps is about half a magnitude.
The same four rungs drawn to true scale. The figure inside each circle is how much more light it gathers than a dark-adapted eye. Six inches to twelve is a doubling of diameter and 4.0× the collecting area — which is why every two-inch step reads as a large percentage while the circles themselves look only a little wider.
Half a magnitude is real. It is roughly the difference between a galaxy you can see and a galaxy you can see the shape of; between a globular cluster that looks like a fuzzy ball and one that grains up around the edge. It is not nothing, and observers who chase faint objects from dark sites care about it a great deal.
But it is not what people picture when they read “fifty-six percent more light”. The eye responds to brightness roughly logarithmically, which is exactly why magnitudes exist as a scale. A half-magnitude gain does not make the view half again as bright to look at; it makes the threshold move by half a step. The percentage figure and the magnitude figure describe the same physical fact, and only one of them matches what you will experience.
The other half of the arithmetic is that resolution improves in a straight line with diameter while weight and bulk climb far faster. That asymmetry is the whole reason this page has a second half.
Six inches: the one that stays available
A 150 mm Dobsonian reaches magnitude 13.6 from a dark site, splits doubles to 0.77″, and tops out around 300× before the atmosphere takes over. In capability terms that is enough for Cassini division in steady air, globular clusters resolving at the edge.
The reason to take this rung seriously is not the optics, which are unremarkable. It is that a tabletop six-inch is a piece of furniture-sized object rather than a project. It comes out of a cupboard with one hand, it goes on a garden table, and the whole operation from decision to first look is a couple of minutes. That number — minutes from deciding to looking — predicts how often a telescope gets used better than any optical specification does.
The compromise is genuine and worth stating: a tabletop Dobsonian needs a table, and a solid one. On a flimsy garden table every focus adjustment becomes a wobble, and you have reintroduced the exact problem a Dobsonian base exists to remove. Budget for a steady surface the same way you would budget for an eyepiece.
Sky-Watcher
Sky-Watcher Heritage 150 Tabletop Dobsonian Telescope - Perfect for Beginners, Easy Setup, Portable, and Fun (
$355.004.3(102)
A 150mm mirror on a base that lives in a cupboard. Aperture you actually carry outside beats aperture that stays in the garage — this is the rung where that trade is most obviously in your favour.
Aperture
150mm
Focal ratio
f/5
Max useful
300×
Resolves
0.77″
Reaches magnitude 13.6 from a dark site — 459× the light your eye gathers alone. Shows Cassini division in steady air, globular clusters resolving at the edge.
Eight inches is where most experienced observers point beginners, and the reason is not sentiment. At 203 mm you reach magnitude 14.2, resolve to 0.57″, and gather 841× the light your unaided eye does. That is enough for spiral structure in the brightest galaxies from a dark site, and it is the first rung at which the aperture-derived capability list stops being about the Solar System.
It is also the last rung that most people can move without thinking about it. A solid tube eight-inch is two objects — tube and base — and both are objects a single adult can pick up. Above this the answer changes, and it changes fast.
The step down from eight to six gives up 78% of the light and 0.62 of a magnitude — the largest single jump on this page, and larger than either step above it. If you are choosing between six and eight purely on optics, eight wins by more than ten wins over eight. If you are choosing on logistics, that calculation can flip, and it legitimately does for some people.
Sky-Watcher
Sky-Watcher Classic 200 Dobsonian 8-inch Telescope – Solid-Tube – Simple, Traditional Design – Easy to Use, Pe
$725.003.9(241)
Eight inches on a base that is a box and a bearing, which is why nearly all of the money went into the mirror. The listing does not publish a weight, so treat the tube and the base as two separate loads until you have checked the manufacturer's own figure.
Aperture
203mm
Focal ratio
f/5.9
Max useful
406×
Resolves
0.57″
Reaches magnitude 14.2 from a dark site — 841× the light your eye gathers alone. Shows spiral structure in the brightest galaxies from a dark site.
Ten inches sits in an awkward and genuinely attractive place. It is 56% more light than an eight, reaching magnitude 14.7 and resolving to 0.46″, and it is meaningfully easier to live with than a twelve. For a lot of people with a garden, a shed and a car, ten inches is the correct answer.
We do not currently list a 10-inch Dobsonian we would recommend. The set of telescopes we have specification data for does not contain one, and we are not going to invent a pick to fill the gap in a table. The rung is covered here because the size question is real; the absence of a product is the honest state of our inventory, not a judgement on the size.
If a ten is what you want, the thing to check before you buy is the same thing that decides the other three rungs: the published tube length and the published assembled weight, from the manufacturer rather than from a marketplace listing. The section below explains why we make that distinction.
Twelve inches: the one you plan around
A 305 mm mirror reaches magnitude 15.1 and resolves to 0.38″. Set against the eight-inch it is 126% more light and 0.88 of a magnitude — the two half-magnitude steps stacked, which is the point at which the difference stops being subtle.
What it does not do is add a new category of thing to look at. The aperture-derived capability list we compute does not gain a line above eight inches, and that is deliberate rather than an omission: what a twelve gives you over an eight is contrast, and how often the detail is present rather than hinted at, not a different set of objects. Galaxies that were smudges acquire structure. Planetary detail that appeared in flashes of steady air stays put for longer. It is a better version of the same night, not a different night.
The collapsible truss designs exist precisely because of everything in the next three sections. A twelve-inch solid tube is close to five feet of rigid object; the same mirror in a collapsible tube is a much shorter thing to get through a doorway and into a car. You pay for that in price and in a light shroud you will want on any night with a streetlight in view.
Sky Watcher
Sky Watcher Sky-Watcher Flextube 300 Dobsonian 12-inch Collapsible Large Aperture Telescope – Portable, Easy t
$1895.003.8(48)
Twelve inches with a collapsible tube, which is the design answer to the problem that a twelve-inch solid tube is roughly five feet long. It will show structure in galaxies an eight only hints at from the same site. The honest question is whether you will move it twice.
Aperture
305mm
Focal ratio
f/4.9
Max useful
610×
Resolves
0.38″
Reaches magnitude 15.1 from a dark site — 1,898× the light your eye gathers alone. Shows spiral structure in the brightest galaxies from a dark site.
Here is the practical test that decides more of these purchases than any optical figure: can one person get the telescope from where it is stored to where it will be used, in one trip, without planning? For a tabletop six the answer is yes. For an eight it is usually two trips, tube then base, and most people stop noticing within a week. For a twelve the answer for most people is no, and that no is the entire reason twelve-inch telescopes sit unused.
We would like to give you exact weights for each rung. We can only give you what the listings publish, and what the listings publish does not survive being put in a table next to itself:
SkyWatcher Flextube 300 SynScan Dobsonian 12-inch Co
305 mm
1500 mm
13.7 pounds
Celestron StarSense Explorer 12" Smartphone Guided D
305 mm
1362 mm
83.6 pounds
Two listings for the same 12-inch collapsible Dobsonian publish weights that differ by more than a factor of two, and one of those figures is lower than the tabletop six-inch immediately above it, which cannot be true of a telescope with four times the mirror area. A third 12-inch publishes a figure several times larger again. The eight-inch does not publish one at all.
The reason is mundane: a marketplace weight field can hold a shipping weight, a single-carton weight for a multi-carton product, an optical-tube-only weight, or an assembled weight, and nothing forces the seller to say which. We are not going to build carry advice on top of that. What we will say is the structural fact, which does not depend on any of those numbers: a Dobsonian is always two objects, the tube and the base, and the assembled weight you care about is the sum. Look the figure up on the manufacturer’s own specification page before you buy, and look for a tube weight and a base weight separately. If only one number is published, assume it is the tube.
Whether it fits in your car
This one you can settle exactly, tonight, with a tape measure and no purchase. A Newtonian tube is at least as long as its focal length, before you add the focuser, the closed end and the mirror cell. Focal lengths are published, so the floor is knowable:
30″ minimum tube length — Sky-Watcher 150 mm, f/5
47″ minimum tube length — Sky-Watcher 203 mm, f/5.9
59″ minimum tube length — Sky Watcher 305 mm, f/4.9
59″ minimum tube length — Sky-Watcher 305 mm, f/4.9
54″ minimum tube length — Celestron 305 mm, f/4.5
Now go and measure the longest straight line inside your car with the rear seats folded down, and the height of the boot opening. That is the whole test. A four-foot rigid tube goes in a hatchback with the seats down and goes diagonally across the back seat of a sedan, awkwardly, with a blanket. A five-foot rigid tube does neither in a small car, and no amount of enthusiasm changes it. This is why the collapsible truss designs charge what they charge: the mirror is the same, the transportability is not.
If the telescope will never leave the garden, this section does not apply to you and you should ignore it entirely — and that is a real answer, not a consolation. A telescope that lives at home and gets carried ten metres can be much bigger than one that has to reach a dark site.
Where it lives between sessions
Storage is the constraint people discover after buying rather than before. A Dobsonian base is a box roughly as wide as the tube is across, plus room for the altitude bearings, and the practical requirement is not floor area but that the thing can stay assembled. A telescope that has to be dismantled into a cupboard and rebuilt in the dark adds ten minutes to the front of every session, and ten minutes is enough friction to lose a marginal night.
None of the listings we have data for publish base dimensions, so we are not going to print a footprint figure. The rule of thumb that matters is simpler: find the place it will stand, fully assembled, indoors or in a dry shed, before you order. If you cannot picture that place, choose a smaller rung. This is the single most common way a twelve-inch becomes a six-inch that cost three times as much.
One related point, since it costs nothing: a garage is usually the wrong place. Garages run warmer than outside air in the evening and colder in the morning, and the temperature difference is exactly what the next section is about.
Cool-down: the hour nobody mentions when selling one
A mirror that is warmer than the air around it does two things, and both of them ruin the view. The glass itself expands unevenly, so the figure of the surface is temporarily not the figure it was polished to. And the warm surface sheds a thin, turbulent layer of rising air that sits directly in the light path — every ray you are trying to focus passes through it twice. The result at high magnification is mush: a planet that boils, a star that will not come to a point no matter how carefully you focus.
The fix is time, and the amount of time scales with the mass of glass, not with the diameter. A mirror is thicker as well as wider as it grows, so a twelve-inch is not twice the thermal mass of a six — it is several times it. The figures amateur astronomers quote are roughly half an hour for a small mirror and an hour or more for a large one, longer again on a night when the temperature is falling quickly, because the mirror is then chasing a moving target. We have not measured this ourselves and we are not going to publish a number as though we had.
What that means for the size decision is concrete. If you set up at nine and want to look at Jupiter, a six-inch is close to ready and a twelve-inch is not. You either put the telescope outside an hour before you intend to use it — which requires somewhere safe to leave it and a decision made an hour in advance, on a night whose weather you cannot yet trust — or you accept a degraded view for the first part of the session. Cooling fans help and many larger Dobsonians ship with one; they shorten the wait, they do not remove it.
Note what this does to the comparison. The twelve-inch is 0.88 of a magnitude deeper than the eight-inch when both are at ambient temperature. For the first hour of a spontaneous session it is not, and it may well be worse.
The arithmetic that actually decides this
Every figure on this page is about what a telescope can do on a night it is used. The variable none of them contain is how many of those nights there will be, and that variable has a far larger range than aperture does.
An eight-inch used fifty times is fifty nights of magnitude 14.2. A twelve-inch used five times is five nights of magnitude 15.1. The second telescope is better by 0.88 of a magnitude and worse by a factor of ten in every other respect that matters, and there is no version of that trade where the twelve wins. This is the conclusion nearly every experienced observer arrives at eventually, usually by owning the big one first.
So the question to answer is not “how much aperture can I afford”. It is: where will it be stored, fully assembled; how many trips is it from there to the sky; does it fit the car if it needs to; and am I the sort of person who will start an hour before I want to look. Answer those four honestly and the rung picks itself, and it will usually be a smaller one than the specification tables suggest.
For most people, most of the time, that answer is eight inches. Not because it is a compromise between the other three, but because it is the largest rung that stays easy, and easy is what determines whether the mirror ever sees the sky.
Where this leaves you
Eight inches is the rung that stays easy
Reaches magnitude 14.2 from a dark site and resolves to 0.57″ — computed from the listing's own aperture, not copied from a box.
841× the light your unaided eye gathers, and the first rung where the capability list stops being about the Solar System.
Two objects, tube and base, both of which one adult can lift — the last rung where moving it needs no planning.
Cools faster than a ten or a twelve, so a spontaneous hour at the eyepiece is actually an hour at the eyepiece.
Not the right answer for everyone. If it will never leave a garden and you have somewhere it can stand assembled, the twelve above is the better telescope — and if it has to come out of a cupboard onto a table, the six is.
Sky-Watcher
Sky-Watcher Classic 200 Dobsonian 8-inch Telescope – Solid-Tube – Simple, Traditional Design – Easy to Use, Pe
We have not observed with any of these telescopes. There is no test bench behind this page and no reviewer with an invented biography. Giving that up costs the usual affiliate posture and buys something better: every claim here is one you can check against a published number or a formula printed on the page.
The aperture and focal length of each telescope come from that listing’s own specification table — never parsed out of a product title, because titles are marketing and one listing in this set states an aperture its own detail table contradicts. The optical figures are computed from those specifications by a single shared module, so no two pages on this site can disagree about what a 203 mm mirror does. Prices were captured on 2026-08-20 and will drift.
Where the data will not support a claim, we have said so rather than filled the gap: there is no 10-inch pick because we do not have one, no weight comparison because the published weights contradict each other, and no storage-footprint figure because none of the listings publish base dimensions. A confidently wrong number is worse than a missing one.
For what this method genuinely cannot judge — how a particular focuser feels after a year, whether a base binds in cold weather, which supplier is honouring warranties this season — the people to ask are on Cloudy Nights. They pay us nothing and they will tell you things we cannot.
