What Do Different Telescope Eyepieces Do?

Eyepieces are an incredibly important part of your telescope, but this may not be obvious straight away.

You can look through a bad eyepiece for years, and only when you look through a good eyepiece for the first time do you see what you’ve been missing.

But while it’s normal to feel deflated, or kick yourself for not discovering your amazing new eyepiece sooner, a lot of astronomers find that a lot of trial and error makes them more appreciative of what they can see through a good-quality eyepiece, as well as more appreciative of what it takes to be able to see the night sky properly.

But if you’re new to astronomy, you may be wondering how eyepieces work, and what the different types are. Well, let’s jump straight in and find out!

How do eyepieces work?

An eyepiece takes the light that’s captured and focused by your telescope and magnifies the image that is seen by your eye. To get the best view of that celestial object, the eyepiece needs to be effective and of good quality.

As light travels through the lenses of your eyepiece, a little bit of it is taken away. To make this loss of light minimal, manufacturers of eyepieces will coat the lenses with substances like magnesium or calcium fluoride.

Small refracting telescopes are often packaged with a metal tripod, a basic altazimuth mount, a finder, a couple of eyepieces and a Barlow lens that doubles their magnification. 

The mount usually comes with an eyepiece tray that sits between the tripod legs and holds the Barlow lens - which is quite useful when observing the sky!

You may be wondering how you’re going to tell the difference between your eyepieces when handling them at night. But as all eyepieces are a slightly different size depending on how powerful they are, it’s quite easy to tell them apart when you’re changing them. 

Like most items that come with small refractors however, the eyepieces are not of the greatest quality, but they do the job and if they’re put back in their boxes after every observing session and are squeaky clean, then they should be fine.

The last thing you want is to pop an eyepiece into the end of your telescope and see that it is scratched or damaged.

Different types of eyepieces

Barlow lens: Although it’s not actually an eyepiece, a Barlow lens is a fantastic piece of equipment and has optical elements that work with an eyepiece to increase the magnification. 

This is achieved by simply slotting the eyepiece into the Barlow lens and the whole contraption gets popped into where the eyepiece would go.

Depending on the Barlow lens, you can double or triple the magnification you would get from the eyepiece. This means one Barlow lens is doubly effective as one eyepiece! 

Nagler eyepiece: The most impressive attribute of the Nagler eyepiece is its huge field of view. 

Most manufacturers design their eyepieces to have a human eye’s 50° field of view, but the Nagler eyepiece has an ultra-wide 82° field of view, granting you access to more nebulae than ever!

The design of the Nagler incorporates 6 or 7 elements that are incorporated with special chemicals to increase the amount of light that travels through the eyepiece. However, these eyepieces are heavy, and so you may need to rebalance your scope.

Plössl eyepiece: Plössl eyepieces have a human eye’s field of view of around 52°, making them suitable for planetary as well as deep-sky viewing. However, they have short eye relief that becomes an issue with focal lengths of 12mm or less. 

‘Eye relief ' refers to how far your eye needs to be from the eyepiece in order for you to see the entire field of view.

The internal construction of a Plössl eyepiece consists of two back-to-back lens systems. The price also varies wildly between high-quality examples of the Plössl eyepiece, and more cheaply produced versions. 

Orthoscopic eyepiece: Until Plössls came on the scene, Orthoscopic eyepieces were the go-to eyepieces for amatuer astrnomers, and they still are a trusted item today. They’re made with a four-element optical system that provides excellent eye relief. 

Their design also reduces the amount of that light that is refracted within the system effectively. However, the field of view is only around 40° to 45° but they still make useful eyepieces for observing the Moon and planets.

Radian eyepiece: The Radian eyepiece is one of the newer types of eyepieces available right now, with a similar field of view to the Plössl. However, the main difference between the Radian and the Plössl is the eye relief, with focal lengths down to 3mm. 

If you need to wear glasses while observing, then the Radian eyepiece is an excellent choice for you, and is just extremely user-friendly for those who don’t wear glasses.

The design is suited to medium and higher magnifications in order to get the most detailed view of planets. There are 6 or 7 internal lens elements with very short focal lengths.

Eyepieces Key Terms

Below are terms you will come across often when looking for telescope eyepieces.

Eye relief: As we’ve already mentioned, eye relief tells you how far your eye must be from the eyepiece in order to see the entire field of view. A bigger distance, or longer eye relief, is useful if you wear glasses.

Exit pupil: This is the size of the image that comes out of the eyepiece. It should be close to the size of your dark-adapted pupil which is around 5mm to 7mm.

Field of view: Sometimes abbreviated to FOV, field of view is the figure that lets you know how much of the sky you can see through your eyepiece. This measurement will be given in degrees, and there are field of view calculators available online to calculate an FOV.

Power: This is another name for magnification. Remember, a telescope just captures the light. The image is magnified by the eyepiece.

Gordon Watts
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