Choosing an Eyepiece

The eyepiece design is an activity that has more than 300 years. While the classics are still in use, there are new designs that offer wider fields and more detailed images at the same time have a higher Eye Relief.

The ratio D / F of the telescope is what determines which eye to use for a perfect image. With an f / 4 only get good pictures if you have a modern eye.

Apparent FOV:

The visual field is dependent on the design of the eyepiece. The apparent field of view of an eyepiece is the angular diameter, and is expressed in degrees (°), the circle of light that the eye sees. Most eyepieces are 40 ° to 50 °.

The true field of view is the area of sky you see through the eyepiece of the telescope. The true field can be approximated using the formula:

True field = apparent field / increase

For example, if you have a telescope of 200mm diameter and 2000mm focal length with an eyepiece of 20 mm. and the apparent field is 50 °. The extension would be 100x (2000 mm. / 20 mm.). The true field is: 50 º / 100, 0.5 °, equivalent to the apparent diameter of the full moon.

The newer designs eye passed 60 °. If instead of having one eye has a 30 ° is 60 °, the result (following the example of the Moon) will be twice as big the moon.

The observe experienced eye often use 50 °, however there are many who want a wider field. But the latter is recommended for people with experience.

Eye Relief:

The eye relief is the distance from eye to eye lens. If you use glasses, you will need an eyepiece with 15mm eye relief at least.

In traditional designs eye, the eye relief is proportional to the focal length, meaning that the shorter focal length, the shorter the eye relief.