For greatest magnification, which of these two lenses should be the eyepiece and which the objective

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The importance of eyepieces to your telescope can take a long time to realise. To say an eyepiece can make or break an astronomer is going too far, but when you look through a good eyepiece for the first time you realise that what you’ve been observing for all those years could have looked much clearer.

Some astronomers are glad they went through the years of wrestling with low-quality eyepieces, as it gives them an appreciation of what it takes to see the night sky properly.

How do eyepieces work?

An eyepiece works by taking the light that’s captured and focussed by your telescope and magnifying the image that is seen by your eye. The eyepiece needs to do this effectively if you’re to get a really good view of that celestial object.

As light passes through the lenses in your eyepiece, a little bit of it is taken away. To minimise this loss of light, manufacturers coat the lenses with substances like magnesium or calcium fluoride.

Small refracting telescopes sold at a reasonable price are frequently packaged with a metal tripod, a basic altazimuth mount, a finder, a couple of eyepieces and a Barlow lens that will double their magnification.

The mount often comes with an eyepiece tray that sits between the tripod legs and holds the Barlow lens – always a useful feature when observing.

As all eyepieces are a slightly different length depending on their power, it becomes quite easy to feel which is which in the dark when you want to change which one you're using.

And their owners carry out many, many happy observing sessions with that first telescope and its eyepieces.

Like most that come with small refractors, or indeed reflectors, the eyepieces are not of the greatest quality, but get the job done, especially if you put them back into their little boxes after every observing session and make sure they stay scrupulously clean.

The last thing you want to do with these seemingly insignificant yet important devices that you pop into the end of your telescope is to get them scratched or damaged.

Failing to do this means you’ll have to replace them sooner, yet this could mean realising earlier how much you have been missing!

What are the different types of eyepiece?

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.

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

These were the mainstay for many an amateur astronomer until 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 Plössl, but they are still pretty good all-rounders. They come in particularly useful for making observations of the Moon and planets.

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 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.

Buying a telescope eyepiece

Your choice of eyepiece will determine the magnification and the size of the field of view that your telescope will deliver.

You can pay anything from around £30 up to £400 for a good eyepiece, and an item with the latter sort of price tag is something you’ll definitely want to look after.

Eyepieces and telescopes are denoted by their focal lengths. Together, the two focal lengths produce a magnification, calculated as the telescope’s focal length divided by the eyepiece’s focal length (for more on this, read our guide to telescope stats).

The take-home message from this is that the shorter the focal length of the eyepiece, the higher the magnification you will have.

But it’s important to realise that good observing is not all about magnification. In fact, too high a magnification can spoil the view.

Different celestial objects are best seen at different magnifications, which is why most observers have a range of different eyepieces.

Typically, a collection of four – 6mm, 10mm, 15mm and 25mm – will cover most observing requirements.

Another important attribute is the eyepiece’s apparent field of view (AFOV), which will normally be marked on the body along with its focal length.

Values range from 45˚ to over 100˚, but the eyepiece’s true field of view with a given telescope can be worked out with another simple calculation: apparent field of view divided by the magnification.

Also important is the build quality of your eyepiece and particularly its lens elements. Quite exotic glass is a prerequisite.

However, as eyepieces commonly have 4 or more individual lens elements within them, non-reflective coatings are also vital, to cut down reflections between the lens surfaces that would otherwise reduce the contrast and spoil the view.

The quality of these non-reflective coatings is an important factor in the choice of eyepieces.

Sometimes, additional steps are also taken to increase contrast – such as ensuring that the inside of the eyepiece body is a very matt black.

Some manufacturers also blacken the edges of individual lens elements to further reduce reflections.

The best eyepieces will be the ones that say they are ‘fully multi-coated’, though ‘multi-coated’ eyepieces are still good.

Try to avoid eyepieces that are described as ‘fully coated’ or just ‘coated’. One way to test the coatings is to fix a black cap on the bottom of your eyepiece and look down the barrel in daylight. The darker the glass looks, the less light is lost and the better the eyepiece.

The better little cylindrical eyepieces are manufactured to an exceptionally high standard.

Some have multiple glass lenses inside that fit together to give you a beautifully crafted accessory that will last and last.

The diameter of an eyepiece gives some indication of how well it’s built. If the barrel measures just under an inch in diameter (and most eyepieces are described in imperial units) then it’s most likely been given away with one of the cheaper telescopes.

But in truth, neither the telescope nor the eyepiece will be with you for the long-term.

Most decent telescopes for beginners have a 1.25-inch eyepiece barrel. When you get up to the really good, and expensive, stuff, though, it’s two-inch barrels all the way.

Terms to know when buying an eyepiece

Exit pupil

This is the size of the image that comes out of the eyepiece. Ideally it should be close to the size of your dark-adapted pupil – around 5mm to 7mm.

Eye relief

This tells you how far your eye must be from the eyepiece in order to see the entire field of view. A bigger distance (called longer eye relief) is useful if you wear glasses.

Field of view

This is sometimes abbreviated to FOV, and is the figure that lets you know how much of the sky you can see through your eyepiece. This measurement is given in degrees. Calculate an FOV before you buy with our field of view calculator.

Power

This is just another name for magnification. A telescope just captures the light – it is the eyepiece that magnifies the image.

6 of the best telescope eyepieces

1

Celestron are a renowned company when it comes to the manufacture of telescopes and astronomy accessories. This 8-24mm eyepiece enables you to select from a range of different magnifications by rotating the barrel while observing. This enables you to choose which is best suited for the celestial object in your field of view.

It's a great option for practical astronomers who hate having to fumble in the dark for different eyepieces in order to swap magnifications.

A nice touch also is that you can start on the lowest - 24mm - to centre your target, then rotate to achieve the desired view.

A folding eyecup means the eyepiece can be used with or without spectacles, rounding this off as a nice eyepiece package suitable for lunar and planetary observing, as well as for deep-sky targets.

2

• Buy now from Altair Astro

Altair’s Ultraflat collection is designed to help correct field curvature, which causes stars to elongate towards the edge of the field of view. These eyepieces are presented in a satin green anodised finish with a bright stainless steel barrel and black rubber grip and eyecup.

The focal length of each is visible in white on green, enabling them to be seen with a red torch.

The collection includes 10mm, 15mm, 18mm, 24mm and 30mm, making them suitable for a wide range of observations.

We got beautiful views of the Pleiades using the 24mm, while the 10mm and 15mm eyepiece provided tantalising glimpses of Mare Humboldtianum on the northeastern limb.

Read our full Altair Ultraflat eyepieces review.

3

For those used to using a Plössl eyepiece, the ultra-wide field of view offered by the Omegon Panorama II eyepieces will provide a whole new observing experience. You’ll be able to see detail across a much wider expanse of sky.

Omegon’s eyepiece collection consists of oculars with focal lengths of 5mm, 10mm, 15mm and 21mm, supplied in a box and finished in gloss black with two red stripes.

Two rubber grips help with handling, even when wearing gloves. Dust covers are included for both ends, as well as a microfibre cloth for cleaning.

Weighing from 322gm to 709gm, the eyepieces have a satisfying heft to them.

We enjoyed some wonderful views of the Moon, including the Lunar X and Lunar V. We used the 5mm eyepiece to observe Montes Apenninus followed by Rupes Recta.

We trained the 21mm and 15mm eyepieces on the Pleiades star cluster and were rewarded with a sparkling, sharp view through both.

Read our full Omegon Panorama II eyepieces review.

4

The Celestron Luminos is prized by Celestron as one of their latest releases: a new range of wide-angled eyepieces offering an 82 degree field of view. Each eyepiece in the Luminos range is multi-coated and designed to correct those chromatic aberrations that can often spoil our views of the night sky.

They also come with retractable eyecups - something of a must for telescope eyepieces these days - making them usable with or without spectacles.

The Celestron Luminos eyepiece package includes dust caps and a cleaning cloth.

5

This 9mm eyepiece is medium-power but it offers an immense 120° apparent field of view. You might say this is an instrument that delivers an observing 'experience' than a simple view.

Get your eye in the optimal position, 12.5mm from the lens surface, and you can gaze in all directions around the view. You can even look sideways into the eyepiece to take in the extreme edge of the field.

We observed the Ring Nebula and the eyepiece really made it stand out. The Hercules Globular Cluster looked amazing.

Observing the Andromeda Galaxy through a 4-inch refractor felt like observing through a low-power eyepiece on a much larger scope. Using this eyepiece is an amazing experience. Highly recommended.

Read our full Explore Scientific 9mm 120° eyepiece review.

6

Buy now from Orion Telescopes, B&H

The view through the DeLite eyepiece series is excellent. The range includes eyepieces with focal lengths of 7mm, 11mm and 18.2mm.

There's the option to physically adjust the distance of your eye from the eye lens, whether you're a spectacle-wearer or not. This means you can easily place your eye at the correct eye relief.

Each eyepiece in the range has the same 62° field of view. Stars looked sharp through the DeLites and showed little or no false colour.

Likewise, we achieved beautiful views of the Moon, including intricate detail on the terminator. Crater Hevelius and crater Grimaldi were quite amazing.

The high contrast delivered by the 18.2mm eyepiece was superb, but our favourite view was through the 7mm. These are fantastic eyepieces.

Read our full Tele Vue DeLite eyepiece range review.

How to keep your eyepiece collection organised

Having a range of eyepieces outside with you allows you to observe a greater number of different objects during an observing session. You certainly don’t want to risk losing your dark-adapted vision by having to go inside to collect a new eyepiece.

It helps to start with a longer focal length eyepiece to locate your object and then use shorter focal lengths until you reach the optimum magnification for each object.

With this in mind, placing your eyepieces in focal length order is the obvious way to proceed.

Many telescope mounts have eyepiece receptacles built into them, but there are rarely enough holes for most observers’ collections.

However, a simple eyepiece sorter can be made using a plastic food container filled with ‘pick and pluck’ foam suitably crafted to house your eyepiece collection.

Alternatively, you could follow our guide to building your own rotating eyepiece turret.

If you're on the hunt for your first eyepiece or an upgrade, browse all of our online eyepiece reviews.