Guide to purchasing an eyepiece

It is the distance between the focus and the eyepiece lens and is measured in millimeters. The magnification provided by a telescope is a function of the focal length of the eyepiece: the smaller it is, the greater the magnification. The increases are calculated like this:

Magnification factor = telescope focal length/eyepiece focal length

Suppose a telescope has a 150mm objective and 750mm focal length, then the 20mm eyepiece will provide: 750 / 20 = 37.5x magnification , while the 6mm eyepiece will provide: 750 / 6 = 125x magnification .

The factor used will always depend on the object to be observed:

• Eyepiece for searching or observing large nebulae : factor increase from 20 to 25
• Galaxies, the Moon and the Sun : factor increase from 50 to 80
• Details about the surface of the Moon and the planets : factor increase from 120 to 150
• Details on planets and binary stars : factor increase from 200 to 300