Start here · Essential guide
What Is Aperture on a Telescope? Why Size Matters
Aperture is the diameter of a telescope's main lens or mirror. Learn why it matters more than magnification, and what size is a sensible choice.
Choosing a Telescope
Telescope Optics & Specs
6 guides, starting with What Is Aperture on a Telescope? Why Size Matters.
Fast vs Slow Telescope: Focal Ratio
A fast telescope has a low focal ratio and a slow one has a high ratio. Learn how to calculate it and how it affects viewing and astrophotography.
Focal Length of a Telescope: What It Tells You
Telescope focal length is the distance from the main lens or mirror to the focus. See how it affects magnification, field of view and eyepiece choices.
How to Find the Light Gathering Power of a Telescope
Light gathering power compares a telescope's collecting area with the eye's. Learn the formula and see worked examples for 8-inch and 10-inch scopes.
Telescope Lenses: Concave or Convex? What Each Does
Telescopes use convex lenses to gather and focus light, while concave elements appear in mirrors, Barlows and Galilean designs. Here is what each does.
Telescope Magnification Power: How Much Is Useful?
Magnification is focal length divided by eyepiece focal length, but only so much is useful. Learn limits, whether 200x is good, and how to read 40x60 labels.
Related topics
More in choosing a telescope
Telescope Types 7 guides, starting with Telescope Types Explained. Choosing a Telescope 6 guides, starting with How to Pick a Good Telescope: A Buyer’s Guide. Dobsonian Telescopes 5 guides, starting with Dobsonian Telescope for Beginners: Why It Is a Great First Scope. Eyepieces & Finders 5 guides, starting with Telescope Eyepieces Guide: Types, Sizes and Choices. Telescope Filters 3 guides, starting with Telescope Filters Guide: Which Ones Are Worth Buying.