How Do Telescopes Work?

Step 1 of 5

Faint means too little light reaching your eye.

A distant galaxy is not blurry — it is dim. Its light spreads out over the whole sky, and your pupil only intercepts a pinhole of it.

Step 2 of 5

A wider opening catches proportionally more.

Double the width and you catch four times the light, because area grows with the square. A 200 mm telescope collects about 800 times what your eye does.

Step 3 of 5

A lens or mirror bends the light to a point.

All the light landing on that wide surface gets bent inward and piled onto one small spot, called the focus. Concentrated light means a bright image.

Step 4 of 5

Then a small lens magnifies that spot.

The eyepiece works like a magnifying glass held over the focused image. Swapping eyepieces changes the magnification without changing how much light you caught.

Step 5 of 5

Big telescopes use mirrors, not lenses.

A giant lens sags under its own weight and can only be held at the edges. A mirror can be supported all across the back, so every huge telescope uses one.

Telescopes go on mountains and into orbit to get above the wobbling air that makes stars twinkle.