What a telescope really shows

Instrument by instrument

Your eyes

After twenty to thirty minutes in the dark your pupils open to about 7 mm (5–6 mm once you are past forty or so), and the eye’s chemistry adjusts to faint light. A dark country sky then shows stars down to magnitude 6 and the Milky Way; a city sky shows magnitude 3 or 4 — from a few dozen to a couple of hundred stars. One look at a bright phone screen undoes much of that, so dim it or use the red night vision mode (the eye button at the top of every page). Learning the constellations with the naked eye is the best first step, and it costs nothing — our constellation guide shows where to begin.

Binoculars: 7×50 and 10×50

“7×50” means seven times magnification and front lenses 50 mm across. The light leaves the eyepieces in a beam 50 ÷ 7 ≈ 7 mm wide, which suits a young dark-adapted eye, and the field of view is about 7° — fourteen full Moons. A 10×50 shows a little more detail with a 5 mm beam and a field of 5–6.5°, but it shakes more in the hands.

Binoculars show far more stars than you can count, the Moon’s craters, Jupiter’s moons, the Pleiades, the Andromeda Galaxy and passing comets. Steady them against a wall, lie back in a garden chair, or put them on a photo tripod with an adapter. Skip zoom binoculars and tiny pocket models with 21–25 mm lenses. Many experienced observers give the same advice: if you don’t know the sky yet, start with binoculars. You will use them for the rest of your life.

A small refractor (60–90 mm)

A lens at the front, an eyepiece at the back, and nothing to adjust. A 70–80 mm refractor on a steady mount shows Saturn’s rings, Jupiter’s belts and moons, the phases of Venus, hundreds of lunar craters, colourful double stars such as Albireo and the bright star clusters. Faint galaxies and nebulae are not its strength. The commonest disappointment is not the optics but the mount: on a flimsy tripod the image dances at every touch. A solid mount matters more than the last few millimetres of lens.

A 6–8-inch Dobsonian (150–200 mm)

A Newtonian reflector sitting in a simple wooden box that swivels left and right and tips up and down. You aim it like a cannon. For the money it gives the most aperture of any design, which is why it is the classic first “real” telescope. A 200 mm mirror collects about 800 times as much light as your dark-adapted eye: globular clusters break up into stars, galaxies appear by the dozen under a dark sky, and the planets show real detail on steady nights.

The costs are bulk — an 8-inch tube is about 1.2 m long and the whole thing weighs around 20 kg, in two parts — and the lack of tracking: you nudge the tube along as the sky turns. At 150× a planet drifts across the field in a minute or two. The mirrors need lining up (collimating) now and then, and a big mirror needs 30–60 minutes outdoors to cool before it gives sharp views.

Schmidt-Cassegrains and Maksutovs

These “catadioptric” telescopes fold a long light path into a short tube with two mirrors and a corrector lens. A 90–127 mm Maksutov is a compact grab-and-go telescope with sharp views of the Moon, planets and double stars; a 6–11-inch Schmidt-Cassegrain, often on a computerised mount, is a versatile, fairly portable instrument for its size. The long focal length gives high magnification easily but narrow fields: the Pleiades and other big clusters do not fit. They take longer to cool, the front glass dews up on damp nights, and they cost more than a Dobsonian of the same aperture.

Smart telescopes

The Seestar, Dwarf and Vaonis instruments and others like them are cameras with a small lens on a motorised mount, run from a phone or tablet. You do not look through them. They find a target on their own, follow it, and stack dozens or hundreds of short exposures into one picture that improves while you watch. A few models add an electronic eyepiece, but that too is a small screen.

They are excellent at what photographs do well: colour images of nebulae and galaxies — nebulae even from a light-polluted town, with the right filter — with no knowledge of the sky needed, and pictures that are easy to share. Their small apertures make the planets small and soft, and you depend on apps, batteries and software updates. Some people love them; people who want to see the universe with their own eyes are often disappointed. Know which kind of person you are before choosing.

How telescopes work: refractors, reflectors and catadioptrics

Every telescope does two jobs. The big lens or mirror at the front collects light and brings it to a focus, forming a small image. The eyepiece then magnifies that image for your eye. The three families differ in how they do the first job.

Refractors: achromats and apochromats

Newtonian reflectors and Dobsonians

Catadioptrics: Schmidt-Cassegrain and Maksutov-Cassegrain

Mounts: half of every telescope

A telescope magnifies every wobble as much as it magnifies the sky. A good mount is the difference between a pleasant night and a frustrating one.

• Alt-azimuth mounts move up and down and left and right, like a camera tripod head. They are simple and intuitive. Dobsonian bases and most mounts for small refractors are of this kind.
• Equatorial mounts tilt one axis to point at the celestial pole. Once set up, you follow a star by turning a single knob, or a motor does it for you — essential for long-exposure photography. They are heavier, need counterweights and “polar alignment”, and leave the eyepiece in awkward positions. Many beginners find them confusing at first.
• GoTo mounts have motors and a small computer. After you centre two or three bright stars (or the mount does it for itself), they find thousands of objects and keep them in view. They are a blessing under city skies, where the guide stars you would use to find things are washed out, but they need power, take a few minutes to align, and you learn the sky more slowly. “Push-to” systems and phone apps — including our sky map — help you find things without motors.

Eyepieces and Barlow lenses

Magnification is the telescope’s focal length divided by the eyepiece’s: a 1200 mm telescope with a 25 mm eyepiece gives 1200 ÷ 25 = 48×. You change magnification by changing eyepieces; the telescope stays the same. A useful starting set is a low power (25–32 mm) for finding things and for big clusters, a medium one (10–15 mm) for galaxies and the Moon, and a high one (5–8 mm) for planets on steady nights.

• Apparent field. A Plössl shows a circle about 50° wide, wide-angle designs 60–70°, ultra-wides 82° or more. A wider field shows more sky at the same magnification, so you nudge the telescope less often.
• Eye relief. If you need glasses for astigmatism, choose eyepieces with 15–20 mm or more of eye relief so you can see the whole field with them on. Otherwise take your glasses off and refocus.
• Barlow lens. Fitted before the eyepiece, it multiplies the focal length, usually by two: one 2× Barlow and two eyepieces give four magnifications. Be wary of the 3× Barlows bundled with small telescopes — they mainly push you into magnification the telescope cannot use.
• Barrel size. 1.25-inch eyepieces are standard; 2-inch ones give the widest views. Eyepieces of 0.965 inches (24.5 mm) are a sign of a toy telescope.

Aperture, magnification and other myths

• “Magnifies 600 times!” Any telescope can be made to magnify any amount. What it can use is limited by its aperture — about twice the aperture in millimetres, or 50× per inch — and by the steadiness of the air. A 60 mm telescope sold on its “675×” gives a dim, swimming blob above about 120×.
• “More power is better.” On most nights the atmosphere (astronomers call it “seeing”) limits any telescope to about 150–250×. Most of the time you will observe at low and medium powers, and faint galaxies usually look best at low power.
• “Aperture is just one number.” It is the number. The light a telescope collects grows with the square of its diameter: a 200 mm mirror gathers four times as much as a 100 mm one. Sharpness — the finest detail it can separate — grows with the diameter too.
• “Bigger is always better.” The best telescope is the one you will actually carry outside. A 10-inch Dobsonian in a fourth-floor flat without a lift often ends up as furniture.
• “You can see the flags on the Moon.” No telescope on Earth can. The finest detail a large amateur telescope shows on the Moon is about a kilometre or two across; even Hubble’s limit there is roughly 100 metres. The Apollo landers have been photographed from lunar orbit, not from Earth.
• “You need a telescope to start.” Binoculars, a sky map and a few clear evenings teach more than an expensive telescope you cannot yet aim.

Budgets, in words

Prices change and differ from country to country, so here is the order in which money is usually best spent.

Observing from a balcony or through a window

• Open the window. Double or triple glazing means four to six glass surfaces, none of them optically flat. At any real magnification the image smears and doubles; even binoculars suffer. Open the window wide, or better, step out onto the balcony.
• Mind the warm air. Heat flowing out of a warm room, or rising from a sun-baked wall or concrete balcony, shimmers in front of the telescope like the air over a hot road. Close the door behind you, keep the telescope out of the flow from the room, and let it cool outside for 20–60 minutes — longer for big mirrors.
• Fight the lights. Shield your eyes from street lamps with a hood, a cap or a dark cloth over your head, and switch off the lights behind you. Choose targets that cut through light pollution: the Moon, planets, double stars and bright clusters. Nebula filters help with some nebulae, but nothing helps galaxies much.
• Know your slice of sky. A window or balcony shows only part of the sky. My view works out which part, and every list on StarsMenu can then show only what passes through it, and when.
• Keep still. Many balconies flex when you shift your weight, and the telescope shakes with them. Stand still while you look, and keep to modest magnifications.
• Know when binoculars win. Through a narrow opening a big telescope is hard to aim and often looks at a wall. Binoculars on a tripod, or a small refractor, cover more sky faster and suffer less from the turbulence near the building.
• Never look at the Sun through binoculars, a telescope or its finder without a certified solar filter fitted securely over the front. It blinds instantly and permanently. And on a high balcony, tie down anything that could fall.

First-night checklist

1. Assemble the telescope indoors and practise in daylight before the first night.
2. Line up the finder with the main telescope on something far away — a chimney or a mast several hundred metres off. Never the Sun.
3. Put the telescope outside early, 30–60 minutes before you start, so it can cool.
4. Start with the eyepiece with the biggest number — the lowest magnification and the widest field.
5. Begin with the Moon (any phase but full; the shadows near the line between day and night show the most) or a bright planet.
6. Focus slowly until stars are the smallest possible points.
7. Raise the magnification only once the image is sharp, and come back down when it blurs.
8. Give your eyes twenty minutes of darkness and use red light only; the night vision mode turns this site red.
9. Check what is up on the Tonight page: the Moon, the planets and the cloud forecast for your place.
10. Dress warmer than you think you need, and bring a chair.
11. Write down or sketch what you saw. It trains the eye faster than anything else.
12. Be patient. Most people need a few nights to get the hang of it. The sky will still be there.