Aside from the size of your telescope, the eyepiece plays the biggest role in determining what your view of the Universe will look like. But how exactly do they work, and what can you do to improve your views of the night sky?

What are eyepieces?

Eyepieces are a type of lens that is usually the last piece of glassware that light will travel through before entering an observers eye. The mirrors or objective lenses in telescopes focus the collected light towards a point near to the eyepiece known as the focal point. When an eyepiece is inserted into a telescope, the glass within the eyepiece is designed to magnify the image at the focal point before it reaches the observers eye. By swapping eyepieces while viewing objects through a telescope, stargazers are able to increase or decrease the magnification on the object, making it appear larger or smaller.

How to choose the right eyepiece

While there is no “perfect” eyepiece for any object in the night sky, it is useful to know what role the eyepiece plays in the telescope and the image it produces. As mentioned above, the eyepiece plays an important role in determining the magnification of the telescope- or how “zoomed-in” the image appears. The magnification factor can be calculated using this simple formula:

For example, if a 25 mm eyepiece is used in a telescope with a 1000 mm focal length, the resulting magnification would be 40x. If a 10 mm eyepiece was used instead, the resulting magnification would be 100x. This means that eyepieces with a smaller diameter will give “more zoomed-in” views of objects in the night sky. This is useful to know when stargazing, as it can help you choose the right eyepiece for the your target. Large objects, like the Moon or the Eta Carina Nebula, do not need a high magnification to view the entire object, so wider eyepieces are the best choice. Smaller objects, like individual craters on the Moon, the planets and the Jewel Box Cluster, require high magnifications, and therefore, smaller eyepieces are ideal.

Unfortunately, some telescopes are not designed for viewing small objects and even with a small eyepiece, small objects – like the planets – can appear far too tiny to make out any features. Fortunately, there is a piece of glassware that can help squeeze a little more detail out of these objects – Barlow Lenses. Barlow Lenses sit between the focal point of the telescope and the eyepiece. They are designed to increase the magnification of a telescope by increasing the focal length of the optical system. Barlow Lenses can increase the magnification factor by 2x, 3x, or even 4x! 

Be careful when using a Barlow Lens or using a high magnification eyepiece. Contrary to what you might assume, a zoomed in view is not necessarily the best view of an object. Too much zoom will lead to blurriness in the final image, often times a slightly under-zoomed eyepiece will give the best views of small objects!

Riley Johnston

Riley is an experienced astronomy guide who has been working within the astronomy community for several years. He is incredibly passionate about the night sky and holds a Bachelor’s Degree in Science, Math and Education.

Riley Johnston


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