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View from a Scope | July 25, 2021

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The Importance of Eyepieces

The Importance of Eyepieces

While researching my first telescope I have learned the importance of the individual components of the scope. One thing that interested me was the ability to swap eyepieces for your telescope.
Essentially this allows you to alter your field of view and magnification levels on the fly.

The magnification of a telescope is calculated by dividing the focal length of the telescope by the focal length of the eyepiece.

In simple terms the focal length of the telescope is the distance between the eyepiece and the lens / mirror.

If my telescope had a focal length of 650mm and my eyepiece had a focal length of 25mm

I should be able to get a magnification of

650/25 = 26

Now I could swap the eyepiece with one that has a focal length of 10mm and using the same calculation I would get a magnification of

650 / 10 = 65


Barlow Lens

There is also another device called a Barlow lens, this can be used with your eyepiece and increases your magnification, so a 2x Barlow lens would double your magnification. Using the example above you would get magnifications of 52 and 130 compared to 26 and 65.

So lower focal length of your eyepiece i.e. 5mm / 8mm give more magnification but a higher focal length i.e 25mm allows for a wider field of view and is generally recommended for stargazing.

Recommended Eyepieces

Starter telescopes generally come with a 25mm and a 10mm eyepiece, it is recommended that you purchase a good quality 2x barlow lens to double the magnification.

The most popular eyepieces seem to be Plossl eyepieces, with the recommended eyepiece being a four-element Celestron Plössl. Individually these cost between  £29 and £50, the 32mm ideal for wide sky viewing and the 6mm for lunar and planetary viewing.

However for people who wear glasses it is recommended that they go for something like a Williams Optic SPL eyepiece as it give better eye relief in the shorter focal lengths.

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