How to Choose a Telescope Eyepiece

Eyepieces determine the magnifying power of your telescope.

A telescope derives its magnifying power from the eyepieces you use with it. Telescopes alone produce no magnification. They simply gather light and direct it to the eyepiece. Before choosing an eyepiece, determine what magnifications you need. A low magnification eyepiece, 70x or less, works best for open clusters and nebulae. Use a medium power eyepiece between 70x and 120x for galaxies and globular clusters. Magnifications above 120x provide the best views of the moon and planets.

Instructions

1. Consult your telescope's user manual to determine its aperture and focal length. Telescope manufacturers list this information among the technical specifications.

2. Divide the focal length of the telescope by its aperture to determine its focal ratio. For example, a 1200mm focal length telescope with 203mm of aperture has a focal ratio of f/5.9. Knowing the focal ratio will help you select an eyepiece with an apparent field of view (AFOV) appropriate for your scope.

3. Purchase an eyepiece with a wide AFOV -- 68, 82 or 100 degrees -- if your scope has a focal ratio of f/6 or higher. Purchase an eyepiece with a 50-degree AFOV if the focal ratio is lower than f/6. Only the most expensive wide-field eyepieces perform well in f/5 or lower scopes. Less expensive wide-field eyepieces produce distorted stars near the edge of the field.

4. Identify the focal lengths and magnifying power of your current eyepieces. Locate the focal length printed on the barrel of each eyepiece. Divide the focal length of the telescope by the focal length of each eyepiece to calculate the magnification. For example, a 1200mm telescope provides 120x power when used with a 10mm eyepiece.

5. Identify what magnifications you need. For instance, purchase a high-power eyepiece if you already have low and medium powers. The appropriate focal length of the eyepiece depends on the focal length of your telescope. An 8mm eyepiece, for example, provides 150x magnification with a 1200mm telescope, but only 60x with a 480mm telescope. Manufacturers offer eyepieces with focal lengths ranging from 2mm to 55mm.

6. Purchase an eyepiece with an AFOV and magnification appropriate to your telescope and magnification needs.

Tips & Warnings

• Magnifications above 150x require clear skies and a steady atmosphere. Many nights out of the year will not support magnifications above 150x. Only a handful of nights allow for magnifications above 300x.

• Purchase an eyepiece compatible with your telescope's focuser. Telescopes typically use 965-, 1.25- or 2-inch focusers.

References

• Acton Astro: Choosing Eyepieces for Your Telescope
• Sky and Telescope: An Eyepiece Primer
• Analytical Scientific: Selecting an Eyepiece Set