## A logical approach to eyepieces?Posted By Dick Jacobson |

There are four "magic" focal lengths that you should cover in your collection. I call them Maximum Field, Maximum Starlight, Maximum Definition, and Maximum Resolution. These are not specific millimeter values but depend on your telescope and your eyes.

The Maximum Field eyepiece shows the widest actual field of view possible in your scope. If you have a 2" focuser you should look for eyepieces with a Field Stop of around 40 millimeters, or 27 millimeters for a 1.25" focuser. You can estimate the Field Stop by multiplying the focal length by the apparent field and dividing by 60. The exit pupil may be too large for your eyes, but the whole point is to get as large a field as possible although some star brightness might be sacrificed.

The Maximum Starlight eyepiece shows as many stars as possible at full brightness. The exit pupil should exactly match your eyes (or maybe slightly smaller to allow for misalignment). The exit pupil of the eyepiece equals the focal length divided by the telescope's focal ratio.

The Maximum Definition eyepiece shows faint planetary detail as well as possible. Many observers agree that definition is best when the exit pupil is about 1 millimeter (25x per inch of aperture). The focal length of this eyepiece should equal your telescope's focal ratio.

The Maximum Resolution eyepiece gives the highest usable magnification, usually 50x per inch but this may be too high for telescopes larger than 10".

I like to have ratio of about 1.6 to 1 between magnifications. This meshes perfectly with a 2x Barlow if you want really close spacing.

The Maximum Field eyepiece shows the widest actual field of view possible in your scope. If you have a 2" focuser you should look for eyepieces with a Field Stop of around 40 millimeters, or 27 millimeters for a 1.25" focuser. You can estimate the Field Stop by multiplying the focal length by the apparent field and dividing by 60. The exit pupil may be too large for your eyes, but the whole point is to get as large a field as possible although some star brightness might be sacrificed.

The Maximum Starlight eyepiece shows as many stars as possible at full brightness. The exit pupil should exactly match your eyes (or maybe slightly smaller to allow for misalignment). The exit pupil of the eyepiece equals the focal length divided by the telescope's focal ratio.

The Maximum Definition eyepiece shows faint planetary detail as well as possible. Many observers agree that definition is best when the exit pupil is about 1 millimeter (25x per inch of aperture). The focal length of this eyepiece should equal your telescope's focal ratio.

The Maximum Resolution eyepiece gives the highest usable magnification, usually 50x per inch but this may be too high for telescopes larger than 10".

I like to have ratio of about 1.6 to 1 between magnifications. This meshes perfectly with a 2x Barlow if you want really close spacing.