The Optics of Astronomy: Magnification and Eyepieces
In amateur astronomy, a telescope's performance is not determined by its magnification, but by its aperture (light-gathering power) and focal length. Magnification is not a fixed property of the telescope; it is changed by swapping eyepieces. The telescope focal length and the eyepiece focal length work together to determine magnification.
The formula for magnification (M) is: \(M = \frac{\text{Telescope Focal Length}}{\text{Eyepiece Focal Length}}\). For example, a telescope with a 1000mm focal length paired with a 25mm eyepiece yields a magnification of 40x. If you add a Barlow lens, it multiplies the effective focal length of the telescope (e.g., a 2x Barlow doubles the focal length to 2000mm, doubling magnification to 80x).
While higher magnification makes objects look larger, it also spreads out the light, making the image dimmer. That is why understanding the relationship between magnification and exit pupil is critical for viewing dim nebulae or bright planets.