Build your own telescope

Two lenses and a tube. That is the whole invention — it is what Galileo had in 1609, and it was enough to find Jupiter's moons and change everybody's mind about where we live. Yours will be worse than a shop telescope and better than Galileo's, and you will know exactly why every part of it is the shape it is.

⚠ Before anything else — never point it at the Sun

A telescope collects light from a wide opening and squeezes it into a spot smaller than the pupil of your eye. That is the entire point of one, and it is why pointing it at the Sun is so much worse than looking at the Sun with bare eyes.

It blinds instantly and permanently, and there is no pain to warn you — the back of the eye has no pain nerves. The damage is done before you know.
A home-made tube can catch fire, and the concentrated beam coming out of the eyepiece will scorch paper and skin. Try it on a scrap of card, from a safe distance, and you will never be tempted again.
Never leave it set up outside in daylight. The Sun moves. A telescope left lying on a lawn will eventually point at it on its own, and a small child may look in. Cap it or bring it inside.
The dark filters that screw into old eyepieces are not safe and can crack from the heat with your eye behind them.

The Moon, the planets and the stars are all safe. Everything on this page is for after dark. An adult should be the one who decides where the telescope is pointed until the rule is second nature.

How it works one lens makes the picture, the other magnifies it

The front lens — the objective — bends all the light arriving from a distant thing so that it crosses over and forms a small, real picture floating in the air inside the tube. That picture is genuinely there; if you put a scrap of tracing paper at that spot you would see it on the paper.

The back lens — the eyepiece — is just a magnifying glass, and you use it to look at that little floating picture very closely. That is all a refracting telescope is: something that makes a tiny picture, and something to peer at it with.

THE ONE PIECE OF ARITHMETIC

How much it magnifies is decided entirely by the two focal lengths, and nothing else:

magnification = focal length of objective ÷ focal length of eyepiece

So a 1000 mm objective with a 25 mm eyepiece gives 40×. Notice what this means: to magnify more you want a long front lens and a short back one. That is why telescopes are long.

What to get none of it is special

Part	What to use	Why
Objective lens	A cheap magnifying glass, or a spectacle lens of about +1 or +2 dioptres from an optician's scrap box. Bigger across is better; 40–60 mm is plenty.	+1 dioptre means a focal length of 1 m, +2 means 50 cm. Weak means long, and long means more magnification and less colour fringing.
Eyepiece lens	A small strong magnifying glass, a loupe, or a lens from an old pair of binoculars. Focal length 10–30 mm.	Short focal length is what multiplies. Under about 8 mm the eye relief gets so tight it is unpleasant to look through.
Tubes	Two that slide inside one another: rolled card and paper, or PVC waste pipe with a slightly narrower pipe inside. Together they need to be a bit longer than the objective's focal length.	Sliding one inside the other is the focuser. Nothing else is needed.
Matt black paint	Blackboard paint, black poster paint, or black paper glued in. Matt, never glossy.	See below — this is the step everybody skips and it matters more than the lenses.

Black inside, or it will not work the step everyone skips

A bare cardboard or white plastic tube is a disaster, and it is worth understanding why rather than being told.

Only some of the light entering the front lens goes straight to where the picture forms. Plenty of it arrives at an angle, hits the inside wall of the tube, bounces, and arrives at your eye from the wrong direction entirely. That stray light does not form part of the picture — it just lies on top of it as a grey fog. The result is an image with no black in it, and since almost all of astronomy is faint things against a black sky, an image with no black in it shows nothing.

So: paint the whole inside matt black before you assemble anything. Matt, not gloss — a shiny black surface is still a mirror at a shallow angle, which is exactly the angle the stray light arrives at.

Better still: rings of card

Real telescopes go one better. Cut two or three rings of black card with a hole in the middle a little wider than the light cone, and glue them inside the tube at intervals. Light skimming along the wall runs into their edges instead of continuing. They are called baffles, and a tube with baffles beats a tube that is merely painted.

Putting it together and finding focus

Measure the objective's focal length first. Hold it up in a darkened room and cast the image of a window or a distant lamp onto a sheet of paper. Slide the paper until the picture is sharpest, then measure lens-to-paper. That distance is the focal length, and it decides how long the telescope has to be.
Blacken both tubes inside and let them dry properly.
Fix the objective at the front of the wide tube, held square to the tube — a lens sitting at a slight angle makes a soft image and nothing you do afterwards will fix it. A ring of card either side, glued, does the job.
Fix the eyepiece into the end of the narrow tube, and slide that tube into the wide one.
Focus by sliding. Point it at something far away on the ground — a chimney, a distant tree — and slide the inner tube slowly in and out until the view snaps sharp. Mark the spot with a pencil so you can find it again in the dark.

If it is soft and washed out: stop it down

Cut a disc of black card with a hole in the middle — say 30 mm — and put it over the front of the objective. You lose some light, and the image gets noticeably sharper and loses much of its colour fringing. A simple lens is at its worst near its edges, and this simply stops you using them. It is a genuine trade and worth trying both ways.

What you will actually see and what you will not

Two things will surprise you immediately, and neither is a fault.

Everything is upside down. A simple telescope of this kind always gives an inverted image, because that little picture inside the tube is itself upside down. Turning it the right way up needs extra lenses, which cost light. Astronomers simply stopped caring — there is no up in space.
Bright things have coloured edges. A single lens bends blue light more steeply than red, so they cannot both be in focus at once. It is called chromatic aberration, and it is the reason the objective wants to be as weak and long as you can manage. Shop telescopes use two glued-together glasses to cancel most of it.

Now the good part. With something like 30× and a steady rest, this telescope will show you:

Craters on the Moon — sharpest along the line between light and dark, where the shadows are longest. Not at full Moon, when the light is flat and it looks like a plain white disc.
Four dots beside Jupiter, changing position every night. Those are the moons Galileo found with no better a telescope than yours, and seeing them move is how he knew not everything went round the Earth.
Saturn looking wrong — not round, a bit stretched. That is the rings, not yet separated. Galileo saw exactly this and could not work out what he was looking at.
The Pleiades turning from six stars into dozens.

It will not show galaxies, colours in nebulae, or anything resembling a photograph. Nothing you can hold in your hands does — that is honest, and the rest of this site tries to say so throughout.

✋ Try it yourself — measure your own magnification

You worked out the magnification from the two focal lengths. Now check it, because a number you have measured beats a number you calculated.

Find a brick wall, or anything far away with evenly repeating lines — fence panels, roof tiles, a row of bricks.
Look at it through the telescope with one eye, and at the wall directly with the other. It takes a moment, but your brain will overlay the two pictures.
Count how many bricks the naked eye sees across one brick seen through the telescope.

That count is your real magnification. Compare it with objective ÷ eyepiece. If it comes out lower than you expected, the usual reason is that the eyepiece's focal length is longer than you assumed — measure it the same way you measured the objective, by projecting a lamp onto paper.

Then try the honest test of the black paint: point it at the same wall in daylight with the inside blackened, and again with a sheet of white paper slipped inside the tube. The second one looks like the first with fog poured over it. That fog is what you removed.

Galileo's best telescope magnified about 30 times and had a lens smaller than a modern spectacle lens. What he had that mattered was the patience to look at the same four dots for weeks. That part is still available.