Telescope Eyepiece Fundamentals
There is no faster way to degrade your telescope's optics than to pair it with a low- quality bargain-bin eyepiece. Choosing the right eyepieces for your telescope goes beyond just basic magnification. Poorly made telescope accessories and eyepieces can reduce a crisp, beautiful image to a sad, fuzzy mess. There are a number of factors that go into making a great telescope eyepiece, but choosing the right eyepiece for your telescope also requires some forethought and consideration. Before we list our favorite telescope eyepieces for sale, we’ll cover some of the basics you need to know to make an informed decision.
How to Choose the Best Eyepieces for Your Telescope
Whenever possible, you should choose an eyepiece with an optical quality that matches or exceeds your telescope. You have invested in the optics of your telescope, don't cheapen your observing experience by skimping on the eyepieces that will bring your view into focus. Keep in mind that the final image you see depends on several factors, including some things you can’t control (like the seeing conditions) and others you can, such as the optical quality of each component in your optical chain.
Here are some terms you should familiarize yourself with before buying telescope eyepieces online eyepiece shopping:
Focal Length
The most important measurement of an eyepiece, the focal length is the distance that light travels through the eyepiece to reach your eye, but most importantly, it is the determining factor in the resulting magnification of an image through your telescope. Keep in mind that the higher the focal length of the eyepiece the lower the resulting magnification will be. See the "Eyepiece Math" section below.
Apparent Field of View
Commonly called out as AFOV in degrees. This is the calculated angular diameter of light you can see through the eyepiece. Anything less than 40 degrees is considered narrow and is best suited to observing the planets or the moon. 80 degrees and above is considered wide. A wide field of view provides a more immersive visual experience, especially for large deep sky objects.
True Field of View
The angle of the sky that your eyepiece will produce when paired with your particular telescope also represented in degrees.
Barrel Size
The barrel of an eyepiece is the section that slides into the focuser or diagonal, and it generally comes in one of two sizes: 1.25" or 2". Sure, there is such a thing as a .965" eyepiece, but they are getting scarce since most telescope manufacturers around the world switched over to the standard quite some time ago, which is, you guessed it, 1.25".
Eye Relief
The distance your eye must be from the eyepiece to view a focused image. Too short and eyelashes will brush the glass, too long and you might find yourself bobbing and weaving to find the image. Comfortable eye relief is a matter of personal taste, though most will find eyepieces with 10-20mm easy to use. Eyeglass wearers will appreciate a longer (more than 15mm) eye relief if they like to keep their glasses on while observing.
Telescope Eyepiece Magnification Formula
Simple Math For Determining The Focal Length For Your Telescope Eyepiece
An eyepiece is defined by its focal length, apparent field of view, and barrel size, but how do you know whether it's right for your telescope? First of all, you need to know the magnification that will result if you pair an eyepiece you are considering with your telescope. The math is easy division; take your telescope's focal length in millimeters and divide it by the focal length of the eyepiece, also in millimeters. For instance: a telescope with a focal length of 500mm divided by a 10mm eyepiece will yield 50X magnification. Simple.
Another way to tackle it is to figure out a magnification you’d like to have available for your telescope and then pick an eyepiece that would provide that magnification. This can be achieved by dividing the telescope’s focal length by the desired magnification to solve for the eyepiece focal length you would need to achieve that magnification. For example: If you have an 8" f/10 SCT, it has a focal length of 2032mm. Let’s say you’d like an eyepiece that gives you 250X. 2032 divided by 250 is about 8, so you know an 8mm eyepiece would get you close to that desired 250X. Easy!
Before you sit down with your calculator and come up with a set of eyepieces that will be perfect for your telescope, there are a couple of things to consider when it comes to magnification. The physical limits of a telescope and the sky above hinder us from increasing magnification without restriction. A good rule of thumb is to keep maximum magnification to approximately 40X or 50X for every inch of aperture up to about 300X for average seeing conditions. If you are lucky enough to observe in excellent, clear and dry skies you could push this limit to 400X or even 450X, but that is not normally the case. Why? Because you're not just magnifying the object you are looking at, you are magnifying everything between your telescope and that object. In most locations, this means a lot of pollutants, moisture, and atmospheric turbulence. Imagine zooming into a photo on your computer, the image remains sharp up until a point and then it becomes distorted and blurry. The "stuff" that gets between your telescope and your target (celestial or terrestrial) does that same thing to your view if you magnify the image beyond what the conditions can tolerate. Terrestrial viewing has so much atmosphere, heat, dust, and pollutants to deal with that magnification can rarely go beyond 60X! That is why most any spotting scope you’ll find on the market has a max magnification of 60 times as far as your eye can see. Now you know why.
PS: For years, the general rule of thumb for maximum magnification per inch of the aperture was 50X - 60X, which is higher than we stated above. Problem is, this generous guideline assumes better seeing conditions than most people have to contend with today. In our experience, this has disappointed many an amateur astronomer who found that their high magnification eyepiece didn’t come to focus 95% of the time. We are not saying that your conditions might support higher power than normal, it might. However, we will stick with our 40X - 50X up to 300X max for most people on most nights.
Now that you know the math needed, let’s use our 8" SCT example again. 8 inches means we technically have a magnification limit of 400X, but let's say you live in the suburbs at a low elevation and the seeing conditions are just so-so. We stick with our 300X limit. For a high power eyepiece (2032/300) you could go with a 6mm or 7mm eyepiece. For a medium power eyepiece (approximately 150X) a 13mm or 14mm eyepiece would do. For a low power eyepiece (about 75X), which are great for finding and centering or observing very large and close objects like the Moon or Sun, an eyepiece between 25mm and 30mm would work well. Fill in from there as your budget allows and to support the types of objects you love to observe most.
Determining True Field of View
But what about that field of view, you may ask? And what's the difference between Apparent Field of View (AFOV) and True Field of View (TFOV)?
Apparent field of view is the width or angle of the view through the eyepiece before it's paired up with a telescope. This is the measurement that is advertised for most eyepieces…82º, 68º, etc. True field of view is the width of the view when it's paired with a telescope, and it’s called "true" because you won’t be doing much observing with just an eyepiece alone! True field of view is found by dividing the apparent field of view of the eyepiece by it's resulting magnification. The number you get from this equation is a much smaller measurement in degrees that relate to the apparent size of objects in the night sky.
Revisiting our 8" SCT…when we use a 7mm eyepiece with an 82-degree apparent field of view in our telescope, it will yield a magnification of approximately 290X. So take the apparent field of view of 82 and divide it by 290 (the resulting magnification) and you’ll get .28 degrees. The full Moon is about 0.5 degrees, so you would see about half of the moon's disc with this eyepiece choice. If you wanted to see the entire full moon at once, you would need a lower power eyepiece and/or a wider apparent field of view. In the end, the width of view is really a matter of personal choice.
Common Telescope Eyepiece Designs
Plossl: The most ubiquitous eyepiece design you'll come across and for good reason. The Plossl is a good all-around eyepiece design, is well suited for generic use and is relatively inexpensive to make. This is why many telescopes come standard with at least one Plossl eyepiece. This doublet design has an apparent field of view of 50-degrees or wider but can suffer from short eye relief making them difficult to use for some eyeglass wearers.
"Wide": A generic classification attached to a huge number of specialized designs produced by companies such as Celestron, Explore Scientific and Meade. Typically anything with more than 5 lens elements and an apparent field of view larger than 80 degrees could be dubbed a "wide" eyepiece design, but there are also 68- and 72- degree eyepieces that have been designated as "Wide" as well. You'll find wide eyepieces cover a large number of brand names.
Zoom: An eyepiece with a variable focal length. Zoom eyepieces are well suited for outreach use, terrestrial observing and solar observing thanks to the convenience of a range of magnifications without requiring a physical change of eyepiece. In instances where you need to quickly reduce and increase magnification repeatedly, zoom eyepieces will save you from having to switch out eyepieces in rapid succession.
Orthoscopic: Also called Abbe eyepieces, Orthoscopics have a narrow field of view but produce nearly perfect images thanks to a simple but effective lens configuration. Orthoscopic eyepieces are best for lunar and planetary observing.
Tele Vue Proprietary Designs: Al Nagler, of the eyepiece manufacturer Tele Vue, has designed several eyepieces, the most famous of which is the Nagler. Nagler eyepieces have 8 elements that are fully corrected for astigmatism and chromatic aberration and have a wide 82-degree field of view. The Nagler has spawned new or modified designs since its creation in 1979, including the Ethos, which offers up to 120 degrees AFOV, and it's more compact offspring, the Delos, which is a 72- degree eyepiece with a long eye relief of 20mm. While well corrected and excellently made, eyepieces containing a large number of elements are heavier than other designs. A focuser and mount that can handle the additional weight are necessary.
Caring for Your Eyepieces
Once you've selected your eyepieces and are investing in those great views, you want to keep them clean and safe. Screw-top bolt style cases are the easiest way to add the first layer of protection to your eyepieces. Eyepiece bolt-style cases by StarDust Covers are made from a specialized plastic that will not off-gas or stick.
When you're traveling you will want an additional level of protection for your eyepieces and we recommend soft-sided cases designed by Explore Scientific and Tele Vue: Explore Scientific or Tele Vue.
Keeping telescope eyepieces clean does need to be done with care. Be sure to use cleaning fluid that's meant for the delicate coatings on lenses and soft, lint-free brushes and cloths. Never apply cleaning fluid directly to your optics (unless you have a waterproof model); instead, put some fluid onto a soft, lint-free cloth and rub gently. A LensPen is great to have on hand for a quick dusting and removing the occasional spot or watermark.
Cleaning fluid and cleaning kits offer you a number of different tools all in one package. With the Apertura 20 piece Cleaning Kit offering the most value, with Celestron's Cleaning Kit coming in next. Remember, be gentle and only do as much as absolutely necessary to keep dust, fingerprints and eyelash marks from building up on your optics, and always blow or brush off loose particles before cleaning the optical surface with a cloth or other cleaning accessory that requires pressure on the glass to do its job. Even the most careful cleaning with top-notch products will, over time, erode your coatings, so be gentle and clean as little as possible. This holds true for optics of any type…camera lenses, binoculars, telescope mirrors, you name it.
Recommended Telescope Eyepieces
Members of our customer service team recommend eyepieces to amateur astronomers of all levels each and every day, and we take their needs into consideration before making those recommendations. For instance, Tele Vue makes some of the best eyepieces in the world, hands down, but they wouldn’t be our first recommendation for customers that have budgetary constraints or beginner telescopes. Luckily, there are several eyepiece brands and lines within those brands that will give excellent results for telescopes and amateur astronomers of all kinds, and through personal experience and tons of feedback, we know which those are!
In no particular order, here are High Point’s recommended eyepieces:
Tele Vue Ethos 100-110 Degree Eyepieces
Comments
â—Ź Highly corrected across the entire 100/110 degree field of view on all scopes
â—Ź Excellent contrast, clarity
â—Ź Images are brighter than other Tele Vue eyepieces
â—Ź 15mm of eye relief on these eyepieces is good given their amazingly wide field of view
Other Considerations
â—Ź This is a high-end eyepiece and may be price prohibitive for some
â—Ź Ethos eyepieces are heavy and need a good focuser to support them
â—Ź The eye relief may be too short for those who wear glasses while observing
Tele Vue Delos 72 Degree Eyepieces
Comments
â—Ź 20mm eye relief that can be adjusted with the built-in eye guard
â—Ź Fantastic contrast and clarity while still maintaining a 72-degree field of view
â—Ź Basically an Ethos with better eye relief and a smaller field of view
Tele Vue Nagler 82 Degree Eyepieces
Comments
â—Ź Game changers in 1980 and still hold up incredibly well to those that have followed in their wake
â—Ź Widest range of top-quality eyepieces, from 3.5mm to 31mm
â—Ź Known for having flat fields and high contrast as well as great clarity
● Backed by Tele Vue’s excellent quality control
Explore Scientific 82 Degree Eyepieces
Comments
â—Ź All focal lengths are waterproof/argon purged
â—Ź Excellent apparent field of view at 82-degrees
â—Ź Great contrast and very well corrected
â—Ź This series has a good selection of focal lengths to choose from
â—Ź No undercut, which makes swapping eyepieces fairly easy
â—Ź Great price for a waterproof, 82-degree eyepiece
Other Considerations
â—Ź Eye relief on some focal lengths may be too short for eyeglass wearers
Explore Scientific 68 Degree Eyepieces
Comments
â—Ź All focal lengths are waterproof/argon purged
â—Ź Really sharp and clear across the entire field of view
Other Considerations
â—Ź Limited to medium/low focal length ranges
Baader Morpheus 76 Degree Eyepieces
Comments
â—Ź High Contrast, true colors equal high color fidelity
â—Ź Great edge correction
â—Ź 76-degree field of view is very comfortable
â—Ź A ton of eye relief - great for those who wear eyeglasses
Other Considerations
â—Ź Generous eye relief may affect proper head placement
● Not a true 2” eyepiece, but that is arguably not a problem given the focal lengths
Baader 8-24 Zoom Eyepiece
Comments
â—Ź In our opinion, the best zoom eyepiece on the market
â—Ź Aberration-free and sharp throughout the entire focal range
â—Ź Minimal re-focusing between each focal stop range
â—Ź Included focal stop range markings are easy to see at night
Other Considerations
● The price is higher compared to other zoom’s, but we think the Baader is worth it
● Field of view gets smaller at higher powers, but that is expected with all zoom’s
Celestron X-Cel LX Eyepieces
Comments
â—Ź For the money, these perform considerably better than expected
â—Ź High contrast, good build quality, blackened edges
â—Ź Better correction towards the edge of field than standard Plossl eyepieces
â—Ź Large focal length range
Apertura Super Wide Angle Eyepieces
Comments
â—Ź 68 degree at a great price
â—Ź Decent eye relief across the entire range
â—Ź Great all purpose eyepieces, better than stock Plossl eyepieces that are generally included with most telescopes
Other Considerations
â—Ź For seasoned observers, longer focal length versions will be lacking in edge correction
â—Ź 10mm version difficult to use with glasses
● Contrast isn’t as high as top eyepieces but is still good for the price
