Every telescope is sold with a photograph on the box, and every photograph is a long exposure. Your eye is not. The simulator above shows the other thing: what a given aperture, eyepiece and sky actually deliver to a human retina, worked out from the optics rather than from the marketing.

Pick a telescope, point it at something, and read the verdict. Move the light pollution slider and watch what happens to the galaxies. Compare two apertures and see how little difference there sometimes is, how much there sometimes is, and which is which.

What will I actually see with my telescope?

Three numbers decide almost everything, and only one is printed on the box.

Aperture is the diameter of the main lens or mirror. Light grasp scales with aperture squared, so doubling aperture collects four times the light; resolving power scales with aperture directly.

Magnification is not a property of the telescope: it is focal length divided by eyepiece focal length, so it changes with every eyepiece. The huge numbers on department-store boxes are arithmetically true and practically useless, since the image falls apart long before you get there.

Your sky is the one nobody sells you, and for faint objects it matters most. The Bortle scale runs from 1, at a dark site, to 9, in a city centre, a gap worth three and a half magnitudes, more than the difference between a 60mm telescope and a 300mm one.

What each aperture is capable of

Resolution is the Dawes limit, the classic measure of how close a double star can be and still be split. Everything else assumes average two-arcsecond seeing under a Bortle 4 sky.

Look at the last two columns before you spend anything. Light grasp climbs by a factor of twenty-six across this table, and it is real: that is why the faintest star improves by nearly three magnitudes. But the smallest lunar crater barely moves, and the useful magnification hardly moves at all, because on an average night both are set by the atmosphere rather than by the telescope. Aperture buys you faint. It buys you far less fine than the advertising implies, and on a turbulent night it buys you almost none.

Object by object, aperture by aperture

Bortle 4 sky, average seeing, and each object at the magnification you would actually use for it. The planets are shown at their next favourable apparition rather than tonight, because Mars in particular swings sevenfold in apparent size and a single date would misrepresent every aperture in the table. Every verdict comes from the same contrast and resolution model the simulator runs.

The row that should change your mind is Andromeda. It barely improves across a fivefold increase in aperture, because no telescope can raise a faint object’s contrast against the sky behind it. Set the light pollution slider to Bortle 2 and that same row transforms in every column. For galaxies and nebulae, where you observe from matters more than what you observe with.

What does Saturn look like through a telescope?

Small, sharp and unmistakable. Saturn’s globe spans 15 to 20 arcseconds, the rings a little over twice that. At 50× it stops being a star. At 100× in anything from 60mm up, the rings separate from the planet, a first sighting that sells more telescopes than anything else in the sky.

Saturn keeps its colour, a pale butterscotch yellow, because it is bright enough to drive your cone cells. Almost nothing else in deep space is.

Three things decide the view, in order:

• Ring tilt. The rings open and close on a fifteen-year cycle, from wide open at 27 degrees to edge-on, when they vanish. The last edge-on crossing was March 2025, so the late 2020s see them reopening. Drag the date control above: tilt changes the view more than tripling aperture.
• Seeing. Saturn rewards magnification; turbulence takes it away. A steady 130mm can beat a 250mm on a bad night.
• Aperture. Third, not first. It decides whether the Cassini Division, the 4,700 km gap between the A and B rings, is reachable, which needs roughly 100mm, steady air and open rings.

Titan, at magnitude 8.4, sits clear of the planet like a faint field star; four more moons appear around 150mm.

Why Andromeda never looks like the photograph

The most common disappointment in amateur astronomy has a wrong usual explanation: that you need a bigger telescope.

A telescope cannot make an extended object brighter per unit area than it appears to the naked eye. It can only make it bigger.

The objective gathers far more light than your pupil, but spreads it over an image whose area grows by the same square of magnification. The two effects cancel: what remains is the ratio of exit pupil to eye pupil, at most 1. A 10-inch at 150× shows Andromeda fainter per square arcsecond than the naked eye does.

Cones see colour and need light; rods are more sensitive but colourblind. Below about 0.003 candela per square metre only rods work, and a single-channel detector cannot encode hue. Andromeda’s surface brightness sits hundreds of times below that threshold in any telescope built, so it is grey. Not grey because your telescope is too small, but because human rods do not do colour.

What you see is a bright oval core fading into a soft glow, a degree across from a dark site; dust lanes need about 200mm and dark skies. The spiral arms in the photographs are hours of exposure, and no observer has ever seen them. The one exception is the core of the Orion Nebula, bright enough to sit just above the cone threshold, which is why experienced observers with large telescopes report a green tinge in M42 and in nothing else.

Telescope magnification: how much can you actually use?

The traditional rule is 50× per inch of aperture, or twice the aperture in millimetres. It comes from the Dawes resolution limit: detail becomes comfortable to view around four arcminutes, and twice-aperture magnification lands exactly there.

What the rule leaves out is the atmosphere. Astronomers call it “seeing”: the arcsecond width a star’s image blurs to, around one arcsecond on a good night, two on average, four or worse on a poor one. Past about 100mm, seeing usually limits the view before the optics do.

In average seeing, a 60mm and a 300mm telescope reach almost the same maximum useful magnification: the bigger instrument shows fainter objects and a better image, but not ten times the detail, and on a bad night, possibly none.

Push past that limit and the image only grows larger, dimmer and softer, with no new detail. That is what “up to 675×!” on a 60mm box actually buys. Set the simulator to a small aperture and a 4mm eyepiece with a 3× Barlow to see it.

Exit pupil, the number nobody mentions

Divide aperture by magnification and you get the exit pupil, the width in millimetres of the light cone leaving the eyepiece.

If it is wider than your own pupil, your iris becomes the aperture stop and the objective’s outer ring is wasted: an 8-inch at 25× with a 6mm eye pupil behaves like a 150mm scope.

Your pupil is also not 7mm unless you are young; it narrows with age, to around 5mm by sixty, narrowing the widest useful eyepiece with it.

Rules of thumb: about 2mm suits planets, 3 to 5mm suits galaxies and nebulae, and below 0.5mm the view is dim and mushy regardless of the instrument.

Light pollution beats aperture

For stars, star clusters and planets, aperture wins. A star stays a point at any magnification, so contrast against the dimming sky improves with both aperture and power. Doubling aperture reaches 1.5 magnitudes fainter.

For galaxies and nebulae it runs the other way: aperture cannot improve object-to-sky contrast, so it only enlarges something already too faint. An 80mm refractor under Bortle 2 shows more nebulosity than a 250mm Dobsonian from a city.

If you own a telescope and want a better view of deep-sky objects tonight, the highest-value move is to drive somewhere darker. Set the simulator to Andromeda, put the light pollution slider at 7, and then walk it down to 3.

How this simulator works

Everything on screen is computed, not drawn from a picture.

Planet positions come from the JPL approximate-elements ephemeris; lunar phase, distance and libration from the truncated ELP-2000 series; Saturn’s ring tilt, Jupiter’s central meridian and its moons from Meeus’s Astronomical Algorithms. The lunar surface is NASA’s LROC mosaic, lit by the real solar angle with LOLA relief, so tonight’s terminator falls where it really falls.

The optics are modelled in the order light meets them: diffraction at the aperture, softened by a reflector’s obstruction and spiked by its spider vanes; atmospheric seeing as a shifting blur; then the eye, with sky background added and colour drained per the mesopic curve.

Deep-sky objects are built from published brightness profiles, not photographs: a long exposure has already destroyed the surface-brightness data this simulation needs, so degrading one would only yield a blurrier astrophoto, not an honest eyepiece view.

The model is calibrated against observations anybody can check: the Bortle scale’s own definitions of naked-eye visibility, the aperture at which M57 first shows, published limiting magnitudes and the resolution limits of standard double stars. Where a value is empirical rather than derived, it is fitted to those anchors rather than guessed.

Two caveats. The enlargement control scales the picture for the screen and says so; the field-of-view readout is always true. Field stars match the correct density and brightness for that patch of sky rather than a catalogue, so the count and look are right even though the individual stars are not.

Frequently asked questions

What can you see with a 130mm telescope?

A 130mm telescope resolves to about 0.9 arcseconds and reaches roughly magnitude 13 under a rural sky: enough for Saturn’s rings as a separate structure, Jupiter’s cloud belts and four Galilean moons, lunar craters to about 2 kilometres, globular clusters starting to resolve at the edges, and the Ring Nebula as a small grey smoke ring. It shows no colour except in the core of Orion, and no spiral arms.

What does Saturn look like through a telescope?

Saturn shows as a small yellowish oval, rings standing clear of the globe from about 60mm of aperture upward, at 50x or more. It keeps its colour because it is bright enough for your cone cells, unlike almost every deep-sky object. The Cassini Division needs roughly 100mm, steady air, and open rings; tilt cycles every fifteen years and matters more than aperture.

Why does the Andromeda Galaxy look nothing like the photographs?

Every photograph is a long exposure; your eye is not. A telescope cannot make an extended object brighter per unit area than it appears to the naked eye, only bigger. Andromeda’s surface brightness sits hundreds of times below the level your colour vision needs, so it is grey in every telescope ever built. What you see is a bright oval core fading into a soft glow.

How much magnification can my telescope actually use?

About twice the aperture in millimetres on a perfect night, where the old "50x per inch" rule comes from. But the atmosphere usually decides first: in average two-arcsecond seeing a 200mm telescope tops out near 100x, not 400x, and past that the image only grows larger, dimmer and softer. The number on the box is almost always far beyond anything useful.

Is a bigger telescope always better?

For stars, star clusters and planets, more aperture genuinely helps. For galaxies and nebulae it helps far less, because aperture cannot improve the contrast between a faint object and the sky behind it; only a darker sky can. An 80mm telescope under a genuinely dark sky shows more nebulosity than a 250mm from a city.

Does this simulator show real positions and dates?

Yes. Planet positions come from the JPL approximate-elements ephemeris, lunar phase and libration from the truncated ELP-2000 series, and Saturn’s ring tilt and Jupiter’s moon positions from the Meeus algorithms. The date control updates the view, so what you see for tonight is what is actually up there.

Where to go next

If you are choosing a first telescope, start with how to choose a telescope and finding the best telescope for you. For specific recommendations there are guides to the best telescopes for adults, the best reflectors for beginners, and options under $500 and under $200.

If you already know what you want to look at, read the guides to the best telescopes for viewing planets and the best telescope for galaxies together: the answers are genuinely different, for the reasons above.

And for the arithmetic on its own, without the pictures, there is a telescope magnification calculator and a field of view calculator.