Observing Skills · 7 of 12

Your First Telescope

Why does a telescope’s width matter more than its magnification?

Aperture gathers the light and power only spreads it: the focal ratio and the field, the useful limit, the mount, and M13, Albireo, the Moon, Saturn and Jupiter in a small telescope.

You’ll be able to judge a telescope by its aperture, and work out its power, its field and its useful limit.

8 steps · about 6 min

Where and which night

Working out when this is best placed from where you are…

Where

Which night

Date

Time

Kept for this guide only, until you change it here — your map settings, and every other guide, stay as they are.

Print, play outside or present

Present on a big screen

Scan with a phone's camera, or open

Sources

Sky at Night, "Telescope stats explained" — light grasp is proportional to the area of the aperture; power is the focal length over the eyepiece's
Sky at Night, "Telescope stats explained" — "smaller focal lengths deliver wider fields"; the focal ratio relates the focal length to the aperture
Sky at Night, "Telescope stats explained" — "the theoretical useful limit is two times the aperture in millimetres"
Kelly, "Choosing Binoculars for Astronomy", RASC Halifax Centre (2021) — a young eye's pupil dilates to about 7 mm
EarthSky, "M13: the finest globular cluster in northern skies" — the unaided eye sees it as "a faint and fuzzy point of light"; large apertures show it best
Sky & Telescope, "Tune Up Your Telescope" — tap the scope and time the view settling: "a couple of seconds is fine, but 10 seconds is way too long"
The Planetary Society, "Five best things to see with a beginner telescope" — crater shadows, Saturn's "unmistakable" rings, Jupiter's four largest moons, and M13's hundreds of thousands of stars
The Planetary Society — "The dots are in fact the four largest moons of Jupiter"

← GUIDES

More below ↓

With a keyboard, focus the sky, aim it with the up and down arrow keys, and press Enter to drop the pin at the centre of the view; or press Show me.

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That faint smudge is , a ball of hundreds of thousands of stars. Telescope boxes print their magnification biggest. Before the sky answers: what makes one telescope show more than another?

What counts is the aperture, the width of the main mirror or lens: it gathers light by its area. A 130 mm mirror collects about 345 times what your dark-adapted pupil does. To the eye M13 is faint and fuzzy; aperture, not power, shows its stars.

Power is not built in: it is the telescope's focal length over the eyepiece's. A 650 mm tube and a 25 mm eyepiece make 26×, and the dashed ring is its field: the whole first-quarter Moon, crater shadows deep along its shadow line.

Focal length over aperture is the focal ratio: that tube is f/5. An f/10 as wide doubles the power with the same eyepiece, to 52×, and halves the field to the ring. Short focal lengths give wide fields, for clusters and star fields.

Shorter eyepieces raise the power, but stop near 2× the aperture in mm: 260× for 130 mm, a field narrower than the Moon. Past it the Moon grows bigger and blurrier, with nothing new to see. A box promising 600× from a small lens is selling a number.

A cheap telescope's weak point is its mount. Tap the tube: on a good one the view settles in a couple of seconds; on a flimsy tripod it dances for ten, and power magnifies the shake. Steady and at low power, it splits Albireo, gold and blue.

Jupiter at its next . A small telescope shows a disc and up to four points of light in a line beside it: its four largest moons. Saturn's rings are unmistakable in one too.

M13 again: light, not power, brings out its stars.

Aperture gathers light; power spreads it
Useful power: about 2× the aperture in mm
A steady mount beats a big number