How to read a star chart — and pick a telescope that is not a waste of money

The eyepiece math

Magnification = telescope focal length / eyepiece focal length. A 1,000 mm telescope with a 10 mm eyepiece gives 100×. A 1,000 mm telescope with a 25 mm eyepiece gives 40×. The exit pupil — the diameter of the beam of light leaving the eyepiece — is telescope aperture / magnification. For night-time observing, the exit pupil should match the dilation of your dark-adapted eye (about 6–7 mm). Higher magnification makes things bigger but dimmer, and exaggerates any shake in the mount.

What aperture actually does

Aperture (the diameter of the primary lens or mirror) controls two things: how much light you collect, and how much detail you can resolve. Both are linear in aperture, so doubling the aperture doubles both. A 200 mm telescope collects 4× the light of a 100 mm telescope and resolves details 2× as fine. Magnification does neither — it only makes the image bigger. This is why a 200 mm telescope with low magnification is more useful than a 100 mm telescope with high magnification.

Dawes and Rayleigh limits

The Dawes limit is the smallest angular separation two stars can have and still appear as two distinct points: Dawes limit (arcseconds) = 116 / aperture (mm). The Rayleigh limit is similar but slightly looser: Rayleigh = 138 / aperture. A 100 mm telescope resolves down to about 1.16 arcseconds (Dawes) or 1.38 (Rayleigh). A 200 mm telescope halves that. Most of what you see in a small telescope is limited by atmosphere, not by the optics — the "seeing" matters more than the spec sheet.

Field of view

Field of view = apparent field of view of the eyepiece / magnification. A 50° apparent FOV eyepiece at 50× gives a true FOV of 1°. That is about twice the diameter of the full moon. A wider apparent FOV is more immersive but more expensive; the wide-field eyepieces on this site's calculator will tell you the actual number for any combination.

What this site gives you

An eyepiece calculator that takes telescope aperture, telescope focal length, and eyepiece focal length and reports magnification, exit pupil, true field of view, Dawes limit, and Rayleigh limit. A lunar phase tool that tells you the current phase and the next new and full moons. A telescope collimation checker for Newtonian reflectors. A dark-sky quality planner for picking an observing site. All free, all in your browser.