Useful Tips: How to choose right eyepieces?

Each telescope model has limited useful magnification power. When this limit is exceeded the picture starts to darken and blur. Keep that in mind when choosing an eyepiece. Magnification is calculated by dividing the focal length of the telescope by the focal length of the eyepiece. Accordingly, the focal length of the eyepiece equals the objective focal length divided by the magnification. For example, a telescope with 2000mm focal length and 20mm eyepiece will give you 100x increase.

Highest practical power of a telescope directly depends on its aperture. Large telescopes are able to collect more light, capture a broader wavefront and, therefore, produce sharper views. The magnification also determines the size of the exit pupil. The exit pupil diameter can be obtained by dividing the telescope’s aperture by its magnifications. You can also use another formula: just divide the eyepiece focal length by the telescope’s focal ratio. The exit pupil must be smaller than the diameter of the pupil of the observer’s eye; otherwise some of the light rays will not make it into the pupil. Young people have great night vision; their eyes are fully adapted to the darkness, and therefore, they have pupils about 7mm in diameter. Maximum pupil diameter decreases with age. So an average middle-aged adult has 5mm-wide pupils. On the other hand, with exit pupil diameters less than 1 millimeter so-called ‘empty magnification’ appears, meaning that the picture quality decreases very rapidly.

 

Magnification power range
Exit pupil diameter, mm
Magnification power per inch of aperture, x
Magnification power
(75-mm telescope), x
Magnification power
(200-mm telescope), x
Application

very low

4.0 – 7.0

3 – 6

10 – 18

28 – 50

Lowest usable power. Wide-field observing of deep-sky objects in dark sky.

low

2.0 – 4.0

6 – 12

18 – 36

48 – 100

General observations, locating objects, observing most deep-sky objects.

medium

1.0 – 2.0

12 – 25

36 – 75

100 – 200

Moon, planets, compact deep-sky objects, wide double stars.

high

0.7 – 1.0

25 – 35

75 – 100

200 – 280

Moon and planets (steady air), double stars, compact clusters.

very high

0.5 – 0.7

35 – 50

100 – 150

280 – 400

Planets and close double stars (very steady air).

 

 So how many eyepieces do you need? Just a few. You can use one low-power and one high-power eyepiece for a long time, but eventually you will want to extend your accessory collection by adding a few more magnifications. For instance, if you have an f/10 telescope, 25mm and 9mm eyepieces would make a wonderful starter kit, and then you might want to add some 15mm and 6mm eyepieces. If you have several different eyepieces, you can find the optimal magnification for each selected object. Starting with a minimum increase when locating objects, you can lift it up until you reach the optimum picture quality. You can also use 2x Barlow lens that doubles the magnifying power of any eyepiece. Therefore, instead of a 3mm eyepiece, you can take a 6mm one and 2x Barlow lens and get the same magnification. The number of eyepieces in your accessory collection can be minimal if you have a Barlow lens among them. Just make sure that the eyepieces’ focal lengths are not multiples of the Barlow lens’ magnification. For example, if your eyepieces are 25, 12.5, and 6 millimetres (multiples of 2), then a 2x Barlow lens will be useless; it won’t give you any additional magnifications. But if you have 25mm, 15mm, 10mm eyepieces and 2x Barlow lens, you will get three more focal lengths – 12.5mm, 7.5mm, and 5mm: practically, three additional eyepieces!