Refractor vs reflector: the honest price of a millimetre of aperture

The short answer

For a first telescope bought to look at things, a reflector gives you more aperture for the money — usually a great deal more — and aperture is the specification that sets every hard limit on what the instrument can do. A refractor gives you a sealed tube, nothing to align, no central obstruction in the light path, and an instrument that will still be working in twenty years with no maintenance beyond keeping the dust off. You are trading capability for convenience, and the exchange rate is steep.

The rest of this page puts numbers on that sentence, because “more aperture for the money” is the kind of claim that gets repeated without anyone checking the size of it.

What the two designs actually are

The reason the prices diverge is manufacturing, not marketing. A lens must be transparent all the way through, free of bubbles and strain, and figured to an optical tolerance on both of its faces — and because different colours of light bend by different amounts, a usable astronomical objective needs at least two elements of different glass types cemented or spaced together, so that is four surfaces to polish. A mirror is figured on one surface only, the glass behind it never has light pass through it so its internal quality barely matters, and a mirror bends every colour identically so there is no colour error to correct in the first place. Scaling a lens up gets expensive very fast. Scaling a mirror up gets expensive slowly.

What the money actually buys

The obvious way to settle this is dollars per inch of aperture. Below is that figure for every one of the 18 telescopes we hold trustworthy specifications for — all of them, not a chosen few, because picking the rows would let us prove whatever we wanted. Prices are a snapshot taken on 2026-08-27 and they move, so treat the ordering as the finding and the exact dollars as perishable.

A note on the “Design” column: it is filled in only where the listing’s own specification table states the design. Amazon publishes no structured field for it, and we will not read an optical design off a marketing title — so a dash there means unsourced, not unknown to science.

Now the honest part, because this table does not say what we expected it to say. The spread is a factor of 10.9× between the cheapest and dearest inch — but the cheapest inches belong to the smallest telescopes, and the dearest to the largest. Dollars per inch rises with aperture no matter which design you are looking at. It mostly measures how small a telescope is, and on that metric our 90 mm refractor at $63 an inch looks better value than the 203 mm reflector at $91.

That comparison is a trap, and it is worth naming because it is the one most buying guides fall into. You do not observe with dollars per inch. You observe with total aperture, and the two numbers pull in opposite directions: the way to win on cost per inch is to buy a smaller telescope. The question that matters is what a fixed budget buys — and there the answer flips, because the largest aperture we can source as a refractor at any price is 90 mm, while mirrors in this same data run to 305 mm. Past a certain size the lens is not expensive; it is simply not offered.

What aperture buys, drawn to scale

Aperture is not a linear specification and this is where intuition fails people. A telescope collects light over the area of its opening, so the gain goes as the square of the diameter. The 90 mm refractor and the 203 mm reflector below differ by a factor of 2.3 in diameter — and a factor of 5.1 in light gathered.

Three hard limits follow from that opening and nothing else — not from the brand, not from the eyepieces in the box, not from the number printed on the front of the carton. The faintest star the instrument can show, the closest double star it can split, and the highest magnification that still puts new detail in front of your eye are all functions of aperture alone.

Limiting magnitude assumes a dark, transparent site. From a suburban garden expect one and a half to two and a half magnitudes worse on both instruments — light pollution takes the same bite out of each.

The Dawes limit row is the one worth sitting with. It is the angular separation at which two stars stop being two stars and become one smudge, and the 203 mm mirror resolves detail 2.3× finer than the 90 mm lens. No eyepiece, no Barlow and no amount of magnification recovers that; it is set by the physical size of the opening. This is what the figure below shows.

The magnification claim that gives a listing away

Small refractors are where the worst marketing in this hobby lives, and there is a single check that sorts the honest listings from the rest. The conservative field rule is that an instrument supports about 2× magnification per millimetre of aperture — 50× per inch. Manufacturers generally quote 2.36× per millimetre, 60× per inch, and we show both rather than hide the disagreement. For a 90 mm objective that is 180× by our rule and 213× by theirs.

The 90 mm refractor on this page is itself sold under a model name containing the number 600×. We are not going to quietly leave that out because we link to it. Here is its own claim placed against what a 90 mm objective can actually resolve.

This does not mean the telescope is a fraud — a 90 mm achromat is a perfectly real instrument that will show you the Moon, the phases of Venus, Jupiter with its four bright moons and Saturn’s rings as an unmistakable ring. It means the number on the box describes an eyepiece swap, not a capability, and a manufacturer willing to lead with it has told you something about how the rest of the listing was written. Work the arithmetic yourself with our magnification calculator before believing any figure printed on a carton.

So what does the refractor buy back?

Everything above is the case for the mirror, and it is a strong one. But a refractor is not simply a worse telescope that costs more, and the reasons people keep buying them are real:

• Nothing to align. A Newtonian’s two mirrors drift out of alignment with transport and temperature, and a misaligned reflector shows mush at high power no matter how good its optics are. It is a fifteen-minute skill, not a crisis — see how to collimate a telescope — but it is a recurring chore a refractor simply does not have.
• No central obstruction. The secondary mirror sits in the light path and diffracts some of it out of the image, which slightly softens contrast on planets and puts the familiar four-point spikes on bright stars. A refractor’s beam is unobstructed, which is why small refractors have a reputation for crisp lunar and planetary views that outrun their aperture.
• A sealed tube. Closed at the front by the objective, so no dust on the optics and far less of the internal air turbulence that plagues an open tube while it is still cooling to the outside temperature.
• It survives being owned. A refractor tolerates being carried, knocked and stored badly. This matters more than any optical argument if the telescope is going to live in a cupboard and come out four times a year.

There is a fair summary hiding in that list: the reflector wins on what the instrument can do, and the refractor wins on how often you will actually use it. A 203 mm Dobsonian that stays in the garage because it is awkward to move shows you less sky than a 90 mm refractor that lives by the back door.

The two instruments, side by side

Sky-Watcher

Sky-Watcher Classic 200 Dobsonian 8-inch Telescope – Solid-Tube – Simple, Traditional Design – Easy to Use, Pe

$725.003.9(238)

The most aperture on this page by a wide margin — 841x the light of the naked eye and a Dawes limit of 0.57", against 165x and 1.29" for the lens. It costs more per inch, not less. You collimate it, and you carry it in two pieces.

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.

View on Amazon

Gskyer

Gskyer Telescope 600x90mm AZ Astronomical Refractor Telescope for Adults Astronomy, German Technology Scope

$222.994.2(927)

A real 90mm refractor with a nonsense number on the box: nothing to align, a sealed tube, and a genuine ceiling near 180x rather than the 600x advertised. At f/6.7 expect some false colour on the Moon and bright planets — that is what a two-element lens at this price does.

Aperture

90mm

Focal ratio

f/6.7

Max useful

180×

Resolves

1.29″

Reaches magnitude 12.5 from a dark site — 165× the light your eye gathers alone. Shows Saturn's rings as a distinct ring, not a bulge.

View on Amazon

If the deciding factor is how much you will see, the 203 mm mirror is not a close call — it is 5.1× the light and 2.3× the resolution for roughly 3.3× the price.

If the deciding factor is that it has to be simple, sealed and ready in ninety seconds, the small refractor is the one that will actually get used.

The third design nobody mentions in the question

“Refractor or reflector” leaves out the catadioptric — a design that uses both a mirror and a corrector lens to fold a long focal length into a short tube. The Celestron on our cost table is one: 203 mm of aperture and 2032 mm of focal length in a tube you can carry under one arm. It is the same aperture as the 203 mm Dobsonian and therefore has identical hard limits — the same 14.2 limiting magnitude, the same 0.57″ Dawes limit — at $188 per inch against the Dobsonian’s $91.

You are paying that premium for packaging and, on this particular model, for a computerised mount that finds objects for you. Whether that is worth roughly double per inch is a genuine judgement call and depends entirely on whether the alternative is leaving a heavier telescope at home. What it is not is more telescope.

How to decide, in four questions

1. Where will it be stored, and how far will it move? Answer this first and honestly. It eliminates more telescopes than any optical consideration, and a 203mm solid-tube Dobsonian is a two-handed object.
2. Planets and the Moon, or faint fuzzy things? The Moon and planets are bright and small, and a good small refractor handles them gracefully. Galaxies and nebulae are large and extremely faint, and there is no substitute for aperture — which in practice means a mirror.
3. Are you willing to learn collimation? If the honest answer is no, buy the refractor and enjoy it rather than buying a reflector you will never align and will slowly conclude is faulty.
4. What is the real budget, per inch? Divide the price by the aperture in inches before comparing anything. It is the single most clarifying number in telescope shopping and almost nobody computes it.

If those four questions point you at a mirror and the remaining question is only how big a one, the sizes and what each rung costs in weight and cool-down time are worked out in which Dobsonian size to buy.

What we did not test

We have not observed through any of these telescopes and we are not going to pretend otherwise. Everything on this page is arithmetic performed on the specifications each listing publishes, using formulae we show so you can repeat them. That means we can tell you with confidence what a 90 mm objective cannot do, because those are physical limits — and it means we cannot tell you how well any individual unit was figured, how smooth its focuser is, or whether the mount wobbles. Those are questions for owner reports, and the forums are genuinely better at them than any site like this one.