Eyepieces are an important thing. Though a telescope’s objective is responsible for forming the image, the image cannot be inspected without an eyepiece. Fortunately, due to their relatively small size, eyepieces of sufficient quality are generally much cheaper than suitable telescopes. But that does not stop some designers from producing luxury eyepieces that can cost more than some entire telescopes! In this article, we will learn about budget-friendly eyepieces and some of the more expensive eyepiece options, and what the difference between these is.
Telescopes are image-forming systems. They take light from an object at a great distance and focus it at the focal plane, which lies usually somewhere in the focuser of your telescope. Those who use cameras with removable lenses (DSLRs, astro imagers, etc.) can put the camera sensor or film directly at the focal point of the telescope and get an image, effectively using the telescope as a camera lens. However, since your eye has optics of its own, we need to find a way to make the converging rays of light at focus diverge again so your eye lens can focus the image onto your retina. That is where the eyepiece comes in. When using telescopes visually, you absolutely must use an eyepiece to see an image.
Your telescope probably came with some eyepieces, but there’s a good chance that they are insufficient. They may be low quality or simply provide too narrow a range of magnifications. In many cases, the included eyepieces are enough to get you started, but as you gain more experience, you will eventually want to expand your eyepiece collection.
In this article, we will discuss how to choose telescope eyepieces based upon magnification, we’ll look at the pros and cons of different eyepiece optical configurations, and we’ll discuss some of the best telescope eyepieces that I have experience with. But first, we need to discuss some optical theory and use some simple equations.
Know Your Telescope Specifications First
You’ll need to know the Aperture and Focal Length of your telescope. This is usually printed on a label near the focuser, and the specs should be available online or in the manual as well. The focal length is not always the same as the length of the telescope! The aperture is required to determine what magnifications to try, and the focal length will tell you what eyepiece to get to provide that magnification.
Focal Length in mm/Aperture in mm = Focal Ratio
For example, a 6″ (150mm) Dobsonian with a 1200mm focal length would have a focal ratio of f/8. An 80mm/400mm Short-Tube 80 refractor would have a focal ratio of f/5.
The focal ratio of the telescope is an important figure when it comes to choosing eyepieces. A “slow” telescope refers to a telescope with a high focal ratio, while a “fast” telescope refers to a telescope with a small focal ratio.
In general, telescope aberrations are worse when the focal ratios are faster. Short-focus refractors and newtonians can have spherical aberration, field curvature, and coma, all of which can blur or distort the image. In addition, short-focus refractors have chromatic aberration (false color fringing). And fast telescopes are harder to collimate (optically align). Meanwhile, due to their shallower curve, long-focus newtonians won’t have much spherical aberration, even if they use spherical mirrors. Long-focus refractors can have almost no chromatic aberration since the glass doesn’t have to bend the light so much.
Eyepieces may also perform differently in fast and slow telescopes. Many simple or cheap eyepiece designs can’t handle the wider light cones in a fast telescope and may show false color fringing, spherical aberration, or off-axis astigmatism and coma. However, the same eyepiece may perform spectacularly in a slow telescope.
These are usually specified by the eyepiece manufacturer.
- Apparent Field of View: this is the size of the field circle–the visible image. A narrow field of view (less than 40 degrees) could feel like looking down a soda straw, whereas a wide field of view (more than 80 degrees) can feel like a space-walk where the edges of the optics seem to disappear. A middle-range eyepiece (50–70 degrees) provides a comfortably large field of view, which you can easily see all of at the same time.
- Barrel Diameter: Eyepieces come in basically three sizes: 0.965”, 1.25”, and 2”. The 0.965” format is obsolete–the only telescopes that use these accessories are either cheap toy telescopes or vintage/antique models. 1.25” and 2” are the modern standards. The advantage of a 2-inch eyepiece is that you can get eyepieces with wider fields of view. 1.25” eyepieces are usually cheaper, and since 1.25” focusers are cheaper, 1.25” accessories remain ubiquitous, even for telescopes with 2” focusers.
- Eye Relief: Implies how far away from the glass you should hold your eye in order to see the full field of view of the eyepiece. Short eye relief can be annoying because it may require almost touching your eye to the eyepiece, or touching your face to it and cause the telescope to shake slightly. Short eye relief eyepieces may also be unusable for individuals who need to wear glasses to correct for astigmatism. 8–10 mm of eye relief is about the minimum you can comfortably use. For a given eyepiece design, eye relief scales with focal length, so short-focus eyepieces have short eye relief. (Many designs use integrated barlows to keep the eye relief long, so this doesn’t always apply.)
- Focal Length: The most important specification, which determines the magnification, is the focal length of the eyepiece, invariably expressed in millimeters (mm).
Understanding Eyepiece Equations
Magnification = Telescope Focal Length/Eyepiece Focal Length
This very simple equation shows us that the longer the eyepiece focal length, the lower the magnification. For a given telescope, a 30mm eyepiece will give way lesser magnification that a 5mm eyepiece.
A telescope’s aperture also constrains the useful range of magnifications. The larger the aperture, the less the incoming light waves are rippled by the opening, and the more resolution the telescope has. If you use the right combination of barlows and eyepieces, any telescope can be brought to any magnification, but the aperture constrains the useful magnification.
Maximum Useful Magnification = 2x per mm of aperture = 50x per inch of aperture
We can also rearrange this formula to get the min useful eyepiece focal length for a given telescope focal ratio.
Highest useful power: Minimum Useful Eyepiece Focal Length (mm) = Telescope Focal Ratio / 2
This is a rule of thumb–you can go a little over 2x if you have a high-quality telescope and good atmospheric conditions. On an average night, you’ll be limited to perhaps 200x by turbulence in the air, whereas bad nights might limit you to 100x or less. Even on spectacularly still nights, atmospheric turbulence will limit even the largest telescopes to a useful power of perhaps 400x. If you use a magnification higher than 2x per mm, the image will be dim and fuzzy, and the telescope may be too shaky.
Exit Pupil = Aperture in mm / Magnification
An exit pupil is the size of the beam of light that comes out of the telescope. If the beam of light is larger than your eye pupil, then you’ll lose some aperture as some of the light will hit your iris. A typical young adult has a 7mm pupil under dark conditions, but it constricts with age. In bright conditions, your pupil will constrict to about 2 mm. This is why you can’t have a 1x magnification with a big telescope–you couldn’t fit all the light into your pupils without also magnifying the image to shrink the exit pupil.
Min Useful Magnification = (1/Eye Pupil Diameter in mm) per mm of aperture.
Or, assuming a 7mm pupil diameter:
Min Useful Magnification = 0.142x per millimeter of Aperture = 3.6x per Inch of Aperture
We can also rearrange this formula to get the max useful eyepiece focal length for a given telescope focal ratio.
Lowest useful power: Maximum Useful Eyepiece Focal Length (mm) = Telescope Focal Ratio / 0.142
So if we know our telescope has a focal ratio of f/10, we know we’ll never need an eyepiece with a shorter focal length than 5mm. But at f/4, we’d need a ludicrously short focal length 2mm eyepiece to reach high power.
Identifying Eyepieces You Already Have
Your telescope probably came with eyepieces, and it would be a good idea to take a moment to figure out what eyepieces you already have. Finding the focal length is easy. The focal length in millimeters is almost always written somewhere on the eyepiece. This is the most important piece of information, as it determines the magnification when used with a given telescope. The type of eyepiece is also important, as it is a shorthand for the general level of quality of the eyepiece.
An eyepiece with an “H,” “R,” or “SR,” in front of the focal length (i.e., H20, H12.5, SR4, etc) is usually a very cheap and often borderline unusable eyepiece with just two glass elements. H stands for Huygens, R for Ramsden, and SR for Symmetrical Ramsden. They have short eye relief, so you have to stuff your eye uncomfortably close to the lens. They have narrow fields of view, and they’re loaded with distortions and false color fringing in all but the slowest of telescopes. These eyepiece designs were invented before multi-coatings were available, and during a time when telescopes regularly had focal ratios of f/15 or longer.
A Kellner eyepiece has three lens elements and is a step up from the two-lens-element eyepieces. Kellners are indicated with a “K” (i.e., K20 or K10). They’re sometimes identified instead as “Modified Achromat” (MA), which is effectively the same design. These are good telescope eyepieces and have good eye relief, although their fields of view are limited.
Many telescopes above the budget price category include Plössl eyepieces (sometimes abbreviated P or PL, but usually not abbreviated.) These have four lens elements and can provide a comfortable 50 degree or so field of view. But they’re short on eye relief at higher powers.
Some telescopes come with Erfles, which are wide-field eyepieces with 60-70 degree fields of view, but with some problems which make them unsuitable for high powers.
Telescopes occasionally come with other unidentified eyepiece designs. Check the manual or website specifications for their optical design, eye relief, and focal length.
Different Eyepiece Designs and Recommendations
When searching for new eyepieces, there are a couple of designs to look for. Keep in mind that simple eyepiece designs have eye relief that scales with focal length, so short-focus simple eyepieces are often uncomfortable to use.
Plossls are good all-round eyepieces made of four lens elements in two symmetrical groups. (Technically, a ‘real’ Plossl is asymmetric, but virtually all commercial Plossls are Symmetrics–there’s not much use in getting hung up on the specifics.) They’re usually sharp most of the way to the edge of the field of view, and they have 50 or 52 degree fields of view. A 32mm Plossl provides the widest useful field of view in a telescope with a 1.25” focuser and the widest useful exit pupil at f/5, so for most telescopes, it’s a good choice for a low-magnification sky-sweeper eyepiece. The higher powers are optically good, but the eye relief gets uncomfortable below 15mm and unusable below 10mm. They can be found from all the major astronomy suppliers under various brands, and random sellers on Amazon, and they’re all good. Svbony’s plossls are the budget option and are functionally identical to the ones sold by Celestron and Orion at higher prices.
Kellners are a budget option with three lens elements and usually come with cheap but quality beginner telescopes. They have fields of view of between 40 and 50 degrees, so they can feel somewhat claustrophobic, but they’re usually sharp and free from false color fringing. Their eye reliefs are also longer than Plossls, so a 10mm Kellner is usually a little easier to use than a 10mm Plossl.
- Konig and Konig-derived, such as the RKE and “Super”
The Konig is a 3-element eyepiece with a different layout to the Kellner, which provides for an even wider field of view. Konigs often make it up to 60-70 degree fields of view, though they may have edge distortions. Konigs, RKEs, and “Super” wide-angle eyepieces from Skywatcher/Celestron are not typically sold on their own, rather they come with beginner telescopes and are usually good enough to not require an immediate upgrade. Genuine Edmund Scientific branded RKEs (Rank-Konig-Erfles or Rank-Kellner-Eyepiece… the actual meaning of the acronym is debated) are considered quite valuable, as the 28mm has a famous “floating stars” effect. As far as I can tell, the floating stars effect is more of a gimmick than something genuinely useful.
Beyond the “simple” eyepiece designs, there are a number of more unique designs sold by various sellers (often rebrands of one another). These affordable designs usually include a built-in Barlow lens so that they can retain long eye relief at short focal lengths, making them good for planetary viewing.
- Goldline or Redline (66-degree Ultra-Wide-Angle)
These budget eyepieces come in 20mm, 15mm, 9mm, and 6mm variants. They have very wide fields of view. They’re all pretty good, although the 20 and 15 have worse off-axis astigmatism. The 15mm and 20mm are essentially modified Konigs, and the 9mm and 6mm have a built-in Barlow lens. The 9 and 6 work great in long-focus telescopes (f/8 or higher). In my f/5 telescopes, I’ve found that the 6mm goldline simply cannot cope with the broad light cone and ceases to show good images either on axis or off axis. However, in an f/10 telescope, the Goldline performs superbly and remains sharp right up to the edge. Goldlines seem to mostly be found by random sellers on Amazon, including Svbony. The Redline variant is identical, just with a rubber grip on the barrel. Don’t confuse these with the Svbony Aspherics series, which also have gold bands and come in different focal lengths–these aren’t as good.
- Starguider Dual ED Eyepieces
This is a big series of high-quality 60-degree eyepieces which come in focal lengths ranging from 3.2mm to 25mm. They are all good and fairly sharp to the edge, and they have consistent performance. Just about the only thing telling these eyepieces apart is the color of the band and the label; they have about the same eye relief and the same field of view for each unit. We use these eyepieces for public viewing at the observatory since their solid rotating eye-cup is easy to sanitize during the pandemic. These can be found at Agena Astro, but a few other clone versions can be found from other sellers.
- 58-degree Planetary Eyepieces
These are some of my absolute favorite affordable, high-quality eyepieces. I have the 20mm and the 7mm, but they come from 2.5mm to 25mm in focal lengths. They seem highly comparable to the Dual ED eyepieces (to theextent that I almost wonder if they have the same optics but different barrels). They have extendable barrels that twist the eye cups up and down depending upon whether the user is wearing glasses or not. The eye relief is good and usually comfortable, but the design of the eyepiece does sometimes make me feel like it’s shorter than it really is. The shorter focal lengths are sharp right up to the edge, even in faster telescopes. They have a little bit of internal reflection, but it’s really not bad.
There are also “luxury” eyepieces, which are usually much more expensive and have much wider fields of view and remain sharp even at the edge of these ultra-wide fields of view. These are complex, heavy, expensive designs that are probably not a good choice for a beginner, just because they’re so expensive. They boast a view somewhere between looking out of a spacecraft window and actually being on a “spacewalk.” And I’m warning you: once you’ve experienced an 80-degree field of view eyepiece, Plossls start to feel kind of claustrophobic.
TeleVue – The Luxury Brand of Eyepieces
TeleVue is a brand that has built a reputation over the past several decades as the gold standard in eyepiece design. As a rule, you can expect eyepieces to have wide fields of view, minimal distortions, and be sharp right up to the edge of the field of view, even in fast telescopes. They have a set of expensive Plossls, too, though I wouldn’t buy into the hype—they may be slightly sharper, but you can get most of the performance from budget brands.
- Nagler: 82-degree apparent field of view, decent eye relief. The 16mm is the only Nagler I personally own, and it’s very good, but the 16mm has a short eye relief since it is a scaled down version of the much larger 31mm eyepiece. The Naglers come in a few different types depending upon the focal length and barrel size, and each of these has a slightly different feel, but they all have the same apparent field.
- Ethos: 100-degree apparent field of view. These are enormous and extremely expensive, but their field of view is so wide that you can’t see all of it at once.
- Delos: 70-degree afov. These are not quite as impressive, but they’re built to have long eye relief for maximum comfort.
- DeLite: 62-degrees. By TeleVue standards, this is almost claustrophobic, but they’re cheap and they have long, comfortable eye relief. They’re also perhaps the most consistent batch of eyepieces.
- Panoptic: 68-degrees afov, and designed for the widest fields of view in a given barrel.
TeleVue Clones from Explore Scientific and Other Brands
For each of the TeleVue series, there are some slightly cheaper, slightly less well-corrected, but still very good ‘clone’ models by various manufacturers and brands.
Explore Scientific has 52, 62, 68, 70, 82, 82 long-eye-relief, 92, and 100 degree ranges, and even a whopping 120 degree eyepiece. The general quality of these is good. The 52-degree series is comparable to Plossls, but with long eye relief in the short-focus varieties. They have good optics, minimal internal reflections, high contrast, and they are argon purged to provide water/dew proofing. However, they may not be quite as sharp at the edge in fast scopes as their TeleVue equivalents. I currently own an 11mm 82-degree and it’s now one of my favorite eyepieces for viewing the Moon, and it takes a barlow nicely. The 14mm 82-degree would be good for deep-sky objects and comes close to maximizing the true field of view in a 1.25” format.
Meade, APM, Astro-Tech, Lunt, and William Optics have clones of the same set of Ultra-Wide-Angle and eXtra-Wide-Angle angle eyepieces in a few different ranges of focal lengths. There’s an 82 degree series and a 100 degree series, both of which are very affordable and high quality, although the 100 degree series is definitely pretty soft at the edges at f/5 (but works great at slower focal ratios). I personally own a 20mm 100-degree eyepiece, which is absolutely incredible to use in my 10” Dobsonian despite the softer edges. It’s truly immersive, as a 100-degree eyepiece should be. But that makes even an 82-degree eyepiece feel slightly claustrophobic.
Celestron’s 82-degree series, the Luminos, is alright, though very short on eye relief, and there have been some complaints of optical quirks, such as a bright ring around the edge of the field in certain models.
This is only an incomplete look into certain eyepiece brands that I am more familiar with. There are certainly other eyepieces of quality build and optics worth looking into (literally and figuratively). Despite the immersiveness of extreme wide-angle eyepieces, it’s important to remember that wide fields of view are indeed a luxury; they’re not necessary to get good, stunning views of deep sky objects and planets. I have had great fun hunting deep sky objects with Plossls and Kellners–you don’t need ultra-wide fields, so don’t be too disappointed if you can’t afford luxury brands. Generally, it’s better to build up a useful, cheaper collection of eyepieces to figure out which magnifications you end up using most, and then after a while, you can replace those focal lengths with better, wider, sharper eyepieces as you see fit.
A Note on Eyepiece Kits
You should generally avoid kits full of a set of eyepieces and filters. A lot of the time, they include stuff you don’t really need or want, like short-focus Plossls with impossibly tiny eye relief. Sometimes the kits have lower quality eyepieces. It’s generally better to buy eyepieces on their own, so you can build up a collection to suit your specific use case.
Replacing your Eyepieces
Depending upon the pricing, your telescope may have come with some pretty bad eyepieces. Consider getting new eyepieces to replace the ones your telescope came with. Huygenian and Ramsden eyepieces have very narrow fields of view and distorted images, so if you have an H20 or similar, you may want to upgrade it for a Kellner or Plossl. If you have Plossls already, you can probably wait for a while longer before you upgrade.
The Best Wide Field Eyepieces
If you’re interested in searching for deep-sky-objects, you’ll want to have an eyepiece that maximizes the field of view first and foremost, since it is at the lowest magnification that objects appear at their brightest, and a wide field of view makes it much easier to search for DSOs.
If you’re using a 1.25” focuser, a good budget option is a 32mm Plossl. There are other designs with a wider apparent field of view and the same true field of view.
Note that 40mm Plossls are also available, but they have the exact same true field of view as a 32mm Plossl and a smaller apparent field of view. 40mm Plossls are only really useful if you have a telescope with a long focal ratio (f/10 or longer) and you want a bit more surface brightness than a 32mm Plossl would allow.
If your telescope has a 2” focuser, you can get even wider eyepieces. 40mm and 50mm Plossls become available, but those are only useful at long focal ratios. There are some ultra-wide-field designs with 60 degree apparent fields of view or wider, which would work splendidly in 2” focusers. Some telescopes come with suitable 2” eyepieces, such as the 30mm SuperView that comes with the Apertura AD-series of Dobsonians.
Some refractor telescopes have 2” focusers, and some SCTs and Maksutovs have 2” ports, but they require an additional 2” diagonal to actually use 2” accessories. Refractors can use 2” diagonals that slot into the focuser like a regular accessory, whereas SCTs and Maksutovs usually need a special diagonal that screws on like a visual-back. Newtonians have the diagonal built in, so when they have 2” focusers, they’re usually usable on their own or with a simple extension tube.
The Best High Magnification Eyepieces
The exact meaning of a ‘high magnification eyepiece’ depends upon the telescope and the target you’re looking at. Typically, a high magnification eyepiece is one that has a focal length in millimeters equal to or shorter than the focal ratio, so at f/5, a 5mm is high power and at f/10, a 10mm is high power. However, the actual focal length matters too. A 5mm eyepiece in the f/5 Short Tube 80 refractor provides only 80x –enough for Saturn’s rings but not for the Cassini division in the rings, whereas a 5mm eyepiece in the f/5 10” Dobsonian provides 240x, genuinely enough magnification to study planetary features.
A good rule of thumb is to try an eyepiece with a focal length equal to the focal ratio of the telescope, and see if it’s good and sharp. You can even do a Star Test to figure out the general quality of the optics if you’re so inclined. If it looks sharp and doesn’t seem to show a blurrier image than a low power eyepiece, then you can try a focal length that is half or 75% of the focal ratio of the telescope. (Or use the same eyepiece but with a 2x Barlow).
When studying planets, you’ll want an eyepiece capable of viewing them at about 200x magnification, assuming your telescope can handle it. (Quality 4” refractors and 4.5” reflectors can reach 200x cleanly). If you have a Dobsonian with a 1200mm focal length, I recommend starting with a 5 or 6mm eyepiece, whether you have a 6”, 8”, or 10”. If you have a tabletop Dobsonian, you’ll need to look into one of the more expensive very-short-focal-length designs of 2.5mm to 3mm.
Filling in the Gaps
You probably started with a 25mm and a 10mm eyepiece, or perhaps a 30mm and a 9mm, or a 20mm and a 10mm. These are low-power and medium-high-power eyepieces, generally. If you’re a beginner interested in planets, you probably want your first eyepiece to be a higher power one. But if you’re interested in deep-sky-objects, consider getting a mid-range eyepiece instead. Something in the middle of your first two eyepieces, so perhaps a 15mm eyepiece in one of the budget wide-field varieties. For a long time, a 15mm Goldline was my workhorse for actually viewing deep sky objects once I’d found one with a 32mm Plossl. If you started with just one eyepiece, perhaps a 25mm Plossl, don’t skip the 8-12mm range when going for high powers–these can be really useful for comfortably splitting double stars, seeing granularity in globular clusters, and overviewing the Moon in high detail, all without getting too dim for deep sky viewing. You very rarely need ultra-high magnification for deep sky objects, they just get too dim.
Several eyepiece designs exist that allow you to dial in any magnification you want by twisting the eyepiece body. These zooms (“zoom” in optics refers to devices which change their magnification or focal length; that is, it is not a synonym for “magnify”) typically have a range of magnifications, allowing you to go between a low power for wide fields of view, and up to 2x or 3x times higher power for more close examination.
Zoom eyepieces do have several drawbacks due to their complicated designs, which don’t allow for the highly precisely corrected optics of single-focal-length eyepieces. They very often have narrow fields of view, off-axis distortions such as astigmatism and coma, and aren’t as sharp as a typical eyepiece. The main advantage is that you can get nearly the full range of magnifications with one eyepiece, or perhaps one eyepiece and a Barlow, and maybe a second eyepiece with a wide field of view for low-power sweeping. This makes them ideal for travel telescopes, where you need to pack light; or for grab-n-go telescopes, where you want the telescope to be ready at a moment’s notice.
The field of view of a typical zoom eyepiece is about 40-50 degrees or less, usually less at the longer focal length end. This means that you end up changing the True Field of View slower than the magnification, so zooming out doesn’t really give you a super-wide field of view compared to the higher power. This can be limiting and means you’ll probably still want a wide-field eyepiece for locating objects.
Zooms also tend to have long eye relief and reasonable fields of view when used at their highest powers, so if they’re sharp enough, they may be good planetary eyepieces.
I have found zooms to be somewhat useful when looking at Deep Sky Objects, and a suitably sharp zoom would work well for Planets and the Moon. I tend to use high powers to look at Globular Clusters and Planetary Nebulae (against the good beginner’s advice to always use low powers when examining DSOs). A zoom allows you to find a good middle ground between a dim high power image, and a bright small image. A zoom can allow you to choose just the right magnification for looking at planets and the Moon, depending upon how the seeing conditions change and shift.
Zooms can have mechanical issues in cold weather.
Svbony carries three zooms of different designs in different focal length ranges to choose from: a 10-30mm, a 7-21mm, an 8-24mm, a 7.2-21.4mm wide-angle with a 65 degree AFOV at max power, and a 9-27mm with a 60-degree AFOV at max power. The reviews of these seem to be good.
Often considered the “gold standard” for zoom eyepieces, the Baader Hyperion Universal Zoom Mk4 8-24mm is one of the highest quality zoom eyepieces and enjoys a luxury price point. But despite its sharp image, good consistent eye relief, and sturdy mechanism, the apparent field of view is narrow by luxury eyepiece standards, and the price is approaching that of an entire kit of budget/mid-range eyepieces.
When utilised with a well thought out set of eyepieces, barlow lenses are an optional piece of equipment that can be used to both double your available set of magnifications and double your highest magnification. They are not eyepieces on their own, but they act as magnifiers for any eyepiece you put into them. Your telescope may have come with a cheap plastic barlow–but these are usually low quality and not worth using. You’ll want to make an informed choice about how to choose a high-quality Barlow that improves the image rather than just makes it blurrier. To learn more, check out our Best Barlow Lenses article for more information.