A telescope ad boasting "525x magnification!" is selling you the least important number on the spec sheet. Magnification comes entirely from the eyepiece — a small, swappable lens near where your eye goes — and it can be changed on the exact same telescope in seconds, just by popping in a different eyepiece.
This is the same 200 mm telescope the whole time — only the eyepiece is changing. Watch which number moves, and which one never does.
The number that actually defines a telescope is its aperture — the diameter of the main lens or mirror. A bigger aperture catches more photons from a faint, distant object, which is exactly what lets a telescope reveal things too dim to see at all with a smaller one. Because aperture describes a circular opening, light-gathering power scales with its area — proportional to the diameter squared, not the diameter itself.
Light-gathering power scales with the area of the aperture, not its diameter — so it grows with the square of the ratio, not the ratio itself.
A research telescope has a 10-meter mirror. An amateur telescope has a 0.25-meter (25 cm) mirror. How many times more light can the research telescope gather?
There are two ways to bring starlight to a focus. A refracting telescope uses a convex lens — light bends (refracts) as it passes through the glass. Galileo's telescopes were refractors, and so are today's binoculars. A reflecting telescope uses a curved mirror instead — light bounces off the mirror's surface to reach a focus, usually redirected by a smaller secondary mirror to a spot where an observer or instrument can access it.
Light bends (refracts) through a lens to reach a focus.
Almost every large modern research telescope is a reflector, and the reasons are practical, not about image quality:
| Refractor (lens) | Reflector (mirror) | |
|---|---|---|
| Light path | Passes through the glass — needs flawless glass all the way through | Bounces off the surface — only the front needs to be accurately shaped |
| Color problem | Chromatic aberration — different colors bend by different amounts, blurring the focus | None — reflection doesn't depend on wavelength |
| Structural support | Can only be held at the edges — large lenses sag under their own weight | Can be supported from behind across the whole back surface |
| Practical size limit | ~40 inches (the Yerkes Observatory refractor, the largest still in use) | Multiple meters — the Keck telescopes use 10-meter mirrors |
Aperture does double duty. Besides light-gathering power, it also controls a telescope's resolving power — its ability to show fine detail, or to tell two very close objects apart as separate rather than a single blur. A bigger aperture gives better resolution, for reasons rooted in the physics of light itself (diffraction) that go beyond this course.