EYEPIECE FOCAL LENGTHS
Eyepieces are sold not by their magnifications but by their focal lengths (always given in millimeters). Look for numbers such as 25 mm, 12 mm, or 9 mm on the barrel. This indicates the focal length of the miniature optics inside the eyepiece.
To calculate how much power a given eyepiece will produce on your telescope, divide the focal length of the telescope by the focal length of the eyepiece.
A 20 mm eyepiece inserted into a telescope with a focal length of 2,000 mm, for instance, would give 100 power. The same eyepiece inserted into a 1,000 mm focal length scope would give 50 power.
The shorter the eyepiece's focal length, the higher the power it gives. Any observer can use at least three eyepieces: a low power (35x to 50x), a medium power (80x to 120x), and a high power (150x to 180x) eyepiece. This means that if your scope has a tocal length of 1,000 mm, for example, you might select 25 mm, 12 mm, and 6 mm eyepieces. Low power provides wide fields for locating targets and tor panoramic views of star fields; medium power resolves clusters and double stars; and high power reveals details on the planets. Magnifications much higher than 180x are rarely ot any use—the image gets bigger but becomes blurry in the process. Also avoid zoom eyepieces— they may promise to combine a range of powers, but in practice they deliver poor images.