Updated 20 August 2026
Aperture sets a hard physical limit on what any telescope can show you, and it can be calculated in one line. Most telescope advice never does the calculation. We start there.
| Aperture | Faintest star | Splits | Useful max |
|---|---|---|---|
| 70 mm | mag 11.9 | 1.66″ | 140× |
| 114 mm | mag 13.0 | 1.02″ | 228× |
| 203 mm | mag 14.2 | 0.57″ | 406× |
| 305 mm | mag 15.1 | 0.38″ | 610× |
Every recommendation checked against the optics.
How the equipment works, and what spec tables leave out.
Work out magnification and eyepieces for your own telescope.
Head to head — including when the question has a false premise.
Telescope listings compete on magnification because magnification is easy to print and impossible to check in a photograph. It is also the least informative number on the box. Three others decide what you will actually experience, and all three are published, so you can settle most buying decisions before reading a single review.
The diameter of the main lens or mirror. It is the only figure that sets how much light the instrument collects, and light is what limits how faint an object can be before it disappears. Double the aperture and you gather four times the light, because area scales with the square of diameter. A 203 mm mirror gathers roughly 841 times what your dark-adapted eye does on its own.
Aperture also fixes resolution — how close two stars can be before they merge into one smudge — and it fixes the highest magnification that will ever be useful. Both follow from the physics of diffraction, not from build quality, so no amount of money spent on eyepieces moves them. This is why our buying guide sorts by aperture first and treats everything else as a tie-breaker.
Focal length divided by aperture. A low number like f/4 gives a wide, bright field that suits large faint objects and photography; a high number like f/10 gives a narrow, high-magnification field that suits planets and double stars. Neither is better. An f/5 reflector and an f/10 catadioptric of the same aperture see equally faint stars — they simply present them at different scales, and they behave very differently when you start swapping eyepieces.
Focal ratio also governs exit pupil, which is the width of the beam of light leaving the eyepiece. Above about seven millimetres that beam is wider than a dark-adapted pupil can accept, so the surplus lands on your iris and is wasted. That single relationship is why a 40 mm eyepiece is a poor purchase for a fast telescope and a sensible one for a slow telescope, and it is the sort of thing a specification table will never tell you directly.
A telescope you cannot hold steady is a telescope you will stop using. At 150× magnification, every vibration is magnified 150 times too, so a tripod that flexes turns each focus adjustment into a wait. The mount is where cheap telescopes save money, and it is the saving you feel every single session rather than the one you read about.
The other half of the mount question is finding things at all. Star-hopping instructions assume you can see the faint guide stars they hop between; under a suburban sky you frequently cannot, and an evening ends with nothing found. Push-to and computerised mounts exist to solve that specific frustration. They do not make anything brighter, and it is worth being clear with yourself about which problem you are paying to fix.
Expectation is the most common reason a telescope is sold a year after it is bought. Photographs taken with tracked mounts and stacked exposures are not what an eyepiece delivers, and nobody says so on the packaging. Here is the honest version.
The Moon is spectacular in anything, and it is spectacular the first time and the fiftieth. Saturn shows a distinct ring rather than a bulge from about 70 mm upward, and it is small, sharp, and genuinely startling. Jupiter shows two dark cloud belts and four moons that visibly change position within one evening. Mars is a small orange disc that only rewards you near opposition, every couple of years.
Deep-sky objects are grey. Not muted colour — grey. Your eye’s colour receptors need more light than any amateur telescope delivers from a nebula, so the Orion Nebula appears as a soft grey-green glow with structure, not as the red and blue image you have seen. Globular clusters resolve into individual stars at the edges from about 130 mm and become genuinely three-dimensional by 200 mm. Galaxies are faint smudges whose spiral arms only emerge with real aperture under a genuinely dark sky. People who understand this in advance tend to stay in the hobby; people who expect the photographs tend not to.
Sky darkness matters as much as aperture for anything that is not the Moon or a planet. A 200 mm telescope under heavy light pollution will show fewer deep-sky objects than a 130 mm telescope an hour’s drive away. That is worth knowing before deciding that the answer is a bigger tube, because a bigger tube you cannot easily transport is the one that ends up unused.
The hairline under the header is the Harvard spectral sequence — the actual rendered colours of stars from hottest to coolest. There is no green in it, because no star looks green: a blackbody peaking in green emits enough red and blue alongside it to read as white.
HOT → COOL · O B A F G K M