{"id":28611,"date":"2022-01-14T13:50:08","date_gmt":"2022-01-14T13:50:08","guid":{"rendered":"https:\/\/www.skyatnightmagazine.com\/?p=28185"},"modified":"2022-01-14T14:08:16","modified_gmt":"2022-01-14T14:08:16","slug":"a-guide-to-telescope-eyepieces","status":"publish","type":"rss_feed","link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/rss_feed\/a-guide-to-telescope-eyepieces\/","title":{"rendered":"A guide to telescope eyepieces"},"content":{"rendered":"

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By Iain Todd\n \t\t<\/p>

Published: Friday, 14 January 2022 at 12:00 am<\/p>

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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\u2019ve been observing for all those years could have looked much clearer.<\/p>\n

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.<\/p>\n

\""An\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
An eyepiece sits in a telescope\u2019s focuser, held tightly by a little screw.<\/span><\/figcaption><\/div>\n

How do eyepieces work?<\/strong><\/h2>\n

An eyepiece works by taking the light that\u2019s 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\u2019re to get a really good view of that celestial object.<\/p>\n

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.<\/p>\n

\""A\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
A telescope collects light. It\u2019s the eyepiece that magnifies the image that reaches our retinas<\/span><\/figcaption><\/div>\n

Small refracting telescopes sold\u00a0at a reasonable price are frequently packaged with a metal tripod, a basic altazimuth mount, a\u00a0finder, a couple of eyepieces and a\u00a0Barlow lens<\/a> that will double their\u00a0magnification.<\/p>\n

The mount often comes with an eyepiece tray that sits between the tripod legs and holds the Barlow lens \u2013 always a useful feature when observing.<\/p>\n

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\u2019re using.<\/p>\n

And their owners carry out many, many happy observing sessions with that first telescope and its eyepieces.<\/p>\n

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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.<\/p>\n

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.<\/p>\n

Failing to do this means you\u2019ll have to replace them sooner, yet this could mean realising earlier how much you have been missing!<\/p>\n

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What are the different types of eyepiece?<\/strong><\/h2>\n

Pl\u00f6ssl eyepiece<\/strong><\/h3>\n
\""Plossl\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>\n

Pl\u00f6ssls have a wide field of view (around 52\u00b0), 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. \u2018Eye relief\u2019 refers to how far your eye must be from the eyepiece in order for you to see the entire field of view.<\/p>\n

The internal construction of a Pl\u00f6ssl eyepiece consists of two back-to-back lens systems. There\u2019s quite a price variation between the highest quality examples and those produced more cheaply.<\/p>\n

Radian eyepiece<\/strong><\/h3>\n
\""radian\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>\n

The Radian is one of the newer types of eyepiece on the market. With a field of view comparable to a Pl\u00f6ssl, you may wonder what the difference is. Well, one is the big eye relief, even with focal lengths down to 3mm.<\/p>\n

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.<\/p>\n

Nagler eyepiece<\/strong><\/h3>\n
\""nagler\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>\n

The Nagler\u2019s most impressive attribute is its huge field of view. While other manufacturers keep their eyepieces within the human eye\u2019s 50\u00b0 field of view, Naglers go the extra mile to develop an ultra-wide 82\u00b0 field. Imagine the amazing vistas of star fields and nebulae you get with that!<\/p>\n

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.<\/p>\n

Orthoscopic eyepiece<\/strong><\/h3>\n
\""orthoscopic\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>\n

These were the mainstay for many an amateur astronomer until Pl\u00f6ssls took over, but Orthoscopics are still good little eyepieces. They\u2019re 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.<\/p>\n

The\u00a0field of view, at only 40\u00b0 to 45\u00b0, may not\u00a0be as great as a Pl\u00f6ssl, but they are still\u00a0pretty good all-rounders. They come\u00a0in\u00a0particularly useful for making observations\u00a0of\u00a0the Moon and planets.<\/p>\n

Barlow lens<\/strong><\/h3>\n
\""Barlow\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>\n

This is a marvellous bit of kit. It isn\u2019t 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.<\/p>\n

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 \u2013 and therefore magnifications \u2013 that you have at your disposal.<\/p>\n

\""Altair\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
Altair Astro\u2019s Ultraflat eyepieces are designed to correct field curvature, an aberration that results in stars towards the edges of the field of view taking on an elongated shape. Do they work? Read our full Ultraflat eyepieces review<\/a>.<\/span><\/figcaption><\/div>\n

Buying a telescope eyepiece<\/strong><\/h2>\n

Your choice of eyepiece will determine the magnification and the size of the field of view that your telescope will deliver.<\/p>\n

You can pay anything from around \u00a330 up to \u00a3400 for a good eyepiece, and an item with the latter sort of price tag is something you\u2019ll definitely want to look after.<\/p>\n

Eyepieces and telescopes are denoted by their focal lengths. Together, the two focal lengths produce a magnification, calculated as the telescope\u2019s focal length divided by the eyepiece\u2019s focal length (for more on this, read our guide to telescope stats<\/a>).<\/p>\n

The take-home message from this is that the shorter the focal length of the eyepiece, the higher the magnification you will have.<\/p>\n

\""Telescope\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
Get to know your telescope stats, as they will fare you well when selecting an eyepiece.<\/span><\/figcaption><\/div>\n

But it\u2019s important to realise that good observing is not all about magnification. In fact, too high a magnification can spoil the view.<\/p>\n

Different celestial objects are best seen at different magnifications, which is why most observers have a range of different eyepieces.<\/p>\n

Typically, a collection of four \u2013 6mm, 10mm, 15mm and 25mm \u2013 will cover most observing requirements.<\/p>\n

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A good selection of eyepieces will serve you well and give you options depending on what you want to observe.<\/span><\/figcaption><\/div>\n

Another important attribute is the eyepiece\u2019s apparent field of view (AFOV), which will normally be marked on the body along with its focal length.<\/p>\n

Values range from 45\u02da to over 100\u02da, but the eyepiece\u2019s true field of view with a given telescope can be worked out with another simple calculation: apparent field of view divided by the magnification.<\/p>\n

Also important is the build quality of your eyepiece and particularly its lens elements. Quite exotic glass is a prerequisite.<\/p>\n

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.<\/p>\n

The quality of these non-reflective coatings is an important factor in the choice of eyepieces.<\/p>\n

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A basic understanding of how the human eye works helps explain the function of eyepieces So, what you need is a selection of eyepieces to match each of your different observing objectives.<\/span><\/figcaption><\/div>\n

Sometimes, additional steps are also taken to increase contrast \u2013 such as ensuring that the inside of the eyepiece body is a very matt black.<\/p>\n

Some manufacturers also blacken the edges of individual lens elements to further reduce reflections.<\/p>\n

The best eyepieces will be the ones that say they are \u2018fully multi-coated\u2019, though \u2018multi-coated\u2019 eyepieces are still good.<\/p>\n

\""Vixen\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
The Vixen SSW eyepieces incorporate a fully multicoated, seven-element design. Read our full Vixen SSW review<\/a>.<\/span><\/figcaption><\/div>\n

Try to avoid eyepieces that are described as \u2018fully coated\u2019 or just \u2018coated\u2019. 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.<\/p>\n

The better little cylindrical eyepieces are manufactured to an exceptionally high standard.<\/p>\n

Some have multiple glass lenses inside that fit together to give you a beautifully crafted accessory that will last and last.<\/p>\n

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A coated eyepiece lens<\/span><\/figcaption><\/div>\n

The diameter of an eyepiece gives some indication of how well it\u2019s built. If the barrel measures just under an inch in diameter (and most eyepieces are described in imperial units) then it\u2019s most likely been given away with one of the cheaper telescopes.<\/p>\n

But in truth, neither the telescope nor the eyepiece will be with you for the long-term.<\/p>\n

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\u2019s two-inch barrels all the way.<\/p>\n

\""The\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
The three most common designs of eyepiece you\u2019ll come across as an observer: an orthoscopic (left), a Pl\u00f6ssl (middle) and a wide angle (right). Image Credit: The Secret Studio<\/span><\/figcaption><\/div>\n

Terms to know when buying an eyepiece<\/strong><\/h2>\n

Exit pupil<\/b><\/span><\/h3>\n

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 \u2013 around 5mm to 7mm.<\/p>\n

Eye relief<\/b><\/span><\/h3>\n

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.<\/p>\n

Field of view<\/b><\/span><\/h3>\n

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<\/a>.<\/p>\n

Power<\/b><\/span><\/h3>\n

This is just another name for magnification. A telescope just captures the light \u2013 it is the eyepiece that magnifies the image.<\/p>\n

6 of the best telescope eyepieces<\/strong><\/h2>\n
1<\/span>

Celestron 1.25 inch \/ 8-24mm Zoom Eyepiece<\/h3>\n<\/div>

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\""Celestron\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>\n
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