Understanding Astronomy Eyepieces

The purpose of an eyepiece is to magnify the focused image, and different eyepieces can be used to give different magnifications and fields of view. You will need to experiment with different ones to find the best one for a particular application. For example, if you want to observe a large area of sky, you will need to achieve a low magnification and a wide field of view.

Magnification

To work out the magnification that is achieved by using a particular eyepiece, use the following formula:

telescope focal length / eyepiece focal length

For example, if your telescope has a focal length of 650 mm, such as the Sky-Watcher Explorer 130P, and you use an eyepiece with a 10 mm focal length, the resultant magnification will be:

650 / 10 = 65x

If you use a Barlow lens, see below, this magnification can be doubled to 130x, or trippled to 195x.

Barrel Size

Different barrel sizes are also available, namely either 1.25 inches or 2 inches diameter. Adapters are available for both, enabling you to use a 1.25 inch eyepiece in a telescope designed for 2 inch eyepieces, and visa versa.

Focal Length

This is measured in mm, and the longer it is, the wider the field of view, but the lower the magnification. Short focal lengths are used for high magnifications, while long ones are used to obtain a wide field of  view.

Eye Relief

If you wear glasses you can get eyepieces that offer long eye relief. This means that you do not have to place your eye right up against the eyepiece in order to see something. The shorter the focal length, the shorter the eye relief.

Barlow Lens

A Barlow lens is a special type of eyepiece that is used to either double or tripple the magnification. It is inserted in between the telescope barrel and the eyepiece.

How Many Eyepieces Do I Need?

In order to cover a range of magnifications from, say, 20x up to 300x, you would typically need four or five eyepieces plus a Barlow lens. It’s also best to go for good quality eyepieces. If you buy cheap ones, you may well find that you have to go out and buy new, more expensive ones, later on.