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?
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 6 or 7 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 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.
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.
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.
Which eyepiece should I buy?
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.
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.
Terms to know when buying an eyepiece
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.
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.
This is just another name for magnification. A telescope just captures the light – it is the eyepiece that magnifies the image.
If you’re on the hunt for your first eyepiece or an upgrade, browse all of our online eyepiece reviews.