Build an eyepiece set around three jobs: a low-power eyepiece for finding objects and wide views, a medium one for most deep-sky viewing, and a high-power one for the Moon and planets. Magnification is telescope focal length divided by eyepiece focal length; true field is apparent field divided by magnification. Choose 15 mm or more of eye relief if you wear glasses, and match barrel size (1.25 or 2 inch) to your focuser.
Eyepieces set your magnification and field of view. A thoughtful set of three or four covers almost everything.
A starter set
| Role | Exit pupil target | Example on a 1,200 mm f/6 telescope |
|---|---|---|
| Low power, finding | 4 to 6 mm | 25 to 32 mm eyepiece, 38 to 48× |
| Medium, deep sky | 2 to 3 mm | 12 to 18 mm, 67 to 100× |
| High, Moon and planets | 0.7 to 1 mm | 5 to 6 mm, 200 to 240× |
Key numbers
| Term | Formula or meaning |
|---|---|
| Magnification | Telescope focal length ÷ eyepiece focal length |
| Apparent field (AFOV) | The eyepiece’s field, often 50 to 100 degrees |
| True field | AFOV ÷ magnification |
| Exit pupil | Eyepiece focal length ÷ telescope focal ratio |
| Eye relief | Distance your eye sits from the lens |
Barrel size
1.25-inch eyepieces fit almost every telescope. 2-inch eyepieces allow wider fields at low power on telescopes with 2-inch focusers.
Filters
Moon filters reduce glare; narrowband and line filters can improve views of some nebulae. Color filters have modest effects on planets. Never use eyepiece-end filters to view the Sun; see solar viewing safety.
Understand the optics with aperture and magnification. Browse mounts and eyepieces.
Common questions
Which eyepiece should I upgrade first?
Often a quality medium or high-power eyepiece, since bundled short eyepieces are usually the weakest.
What is exit pupil?
The width of the light beam leaving the eyepiece: eyepiece focal length divided by telescope focal ratio. About 5 to 7 mm is the useful maximum.
Do I need long eye relief?
If you observe with glasses, yes; about 15 mm or more lets you see the full field.
Is a Barlow lens worth it?
A good 2× Barlow doubles each eyepiece's power and can extend a small set, but adds glass to the light path.