Eyepieces – You cannot start observing with just a telescope without eyepieces.

• There is a huge number of eyepieces of many different types of construction on the amateur astronomy market.
• You’ll know terms such as apparent:
  – field of view
  – focal length
  – exit pupil
• you may still not really know which eyepiece is the right one for you.
• In order to remedy this, we have put together an overview of the different construction types with their advantages and disadvantages.

Eyepieces are a little like magnifying glasses, used to enlarge the intermediate image produced by the telescope. In principle, such a magnifying glass could consist of a single lens. However, since you’ll want to have different fields of view, an eyepiece must consist of a combination of lens elements spaced at a pre-determined distance from one another. And of course you want a comfortable eye relief and the correction of image defects. Finally, you’ll need a mount that holds the lenses. This is also referred to as the eyepiece barrel.

Plössl eyepiece

• Plössls have a wide field of view (around 52°), so they can be used successfully for planetary as well as deep-sky viewing.
• The drawback is the short eye relief that becomes an issue with focal lengths of 12mm or less.
• ‘Eye relief‘ refers to how far your eye must 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.
• There’s quite a price variation between the highest quality examples and those produced more cheaply.
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Radian eyepiece

The Radian is one of the newer types of eyepiece on the market. With a field of view comparable to a Plössl, you may wonder what the difference is. Well, one is the big eye relief, even with focal lengths down to 3mm.

This is a lifesaver if you need to wear glasses while observing, and very user-friendly for everyone else.

The design suits medium and higher magnifications in order to get plenty of detail when looking at the planets. Internally, there are 6 or 7 lens elements that have very short focal lengths.

Nagler eyepiece

The Nagler’s most impressive attribute is its huge field of view. While other manufacturers keep their eyepieces within the human eye’s 50° field of view, Naglers go the extra mile to develop an ultra-wide 82° field.

Imagine the amazing vistas of star fields and nebulae you get with that!

The design incorporates 6 or 7 elements, all coated with special chemicals to increase the amount of light that travels through the eyepiece.

The downside to some of these eyepieces is their weight, which may require you to rebalance your scope.

Orthoscopic eyepiece

Orthoscopics were the mainstay for many an amateur astronomer until Plössls took over and are still good eyepieces.

Orthoscopics are made with a four-element optical system that provides very good eye relief. The design also keeps down the amount of light that is refracted within the system very effectively.

The field of view, at only 40° to 45°, may not be as great as a Plössl, but they are still good all-rounders. They come in particularly useful for making observations of the Moon and planets.

Barlow lens

A Barlow lens is a marvellous bit of kit. It isn’t actually an eyepiece, but has optical elements that work with an eyepiece to increase the magnification.

This is achieved by a very simple process: you slot the eyepiece into the Barlow lens and the whole contraption gets popped into where the eyepiece would normally go.

Depending on the Barlow, you can double or triple the magnification you would get from the eyepiece alone.

This means that with one Barlow lens you have effectively doubled the number of eyepieces – and therefore magnifications – that you have at your disposal.

https://skyandtelescope.org/astronomy-equipment/telescope-eyepiece-guide/#:~:text=simple%20magnification%20calculator%3A-,Telescope%20focal%20length(mm)%20%3D,0,-But%20there%27s%20also