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Back Garden Astronomy - Choosing an Eyepiece
Back Garden Astronomy - Choosing an Eyepiece

Orion is proud to partner with BBC Sky at Night Magazine, the UK's biggest selling astronomy periodical, to bring you this article as part of an ongoing series to provide valuable content to our customers. Check back each month for exciting articles from renowned amateur astronomers, practical observing tutorials, and much more!

Make sure you get the best view of the night sky by using the right eyepiece

Orion Sirius Plossl 1.25-inch Telescope Eyepieces

Orion Sirius Plossl 1.25" Telescope Eyepieces

Which is more important: the telescope or the eyepiece? The telescope gets lots of attention because it's the most expensive and impressive-looking part of your setup — but without decent eyepieces, the views you get can be disappointing.

What you ideally want is a good range of eyepieces, because different focal lengths are useful for producing better views of different kinds of objects. This is due to the fact that each eyepiece will have a differing field of view and magnification, depending on the telescope used.

Size Matters

To find out what magnification you're getting with any eyepiece takes a very easy calculation — you simply divide the focal length of the telescope, which is usually printed on a label on the scope near the eyepiece end, by the focal length of the eyepiece. The focal length of any decent eyepiece will be marked in millimeters around its collar. So for example, to work out the magnification of an 800mm focal length telescope with a standard 25mm focal length eyepiece, you divide 800 by 25, which is 32. This setup will magnify objects you see in the eyepiece by a factor of 32.

For wider views of nebulae and star clusters, this is the kind of number you will want. With higher magnifications, maybe with a 10mm eyepiece, you'll get more detailed views of the planets and double stars.

As you progress in astronomy, you will undoubtedly start to experiment with the different views that a range of eyepieces can offer. So make sure you don't underestimate these small, seemingly insignificant bits of astro equipment!

Know Your Eyepieces

There are four main types of eyepiece; a Barlow will increase their magnification.

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.

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 six or seven lens elements that have very short focal lengths.

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 six or seven 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.

These were the mainstay for many an amateur astronomer until the Plössls took over, but Orthoscopics are still good little eyepieces. They're 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 Plösl, but they are still pretty good all-rounders. They come in particularly useful for making observations of the Moon and planets.

Double up with a Barlow lens
This 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 basically 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.

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Date Taken: 10/07/2016
Author: BBC Sky at Night Magazine
Category: Accessories

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