by Zane Landers
If you plan on looking through your telescope, eyepieces are pretty important.
Some people have eyepieces that can cost significantly more than their telescope itself. Eyepieces are something you almost always need to buy aftermarket to go with your telescope, as few telescopes include a full, adequate set of eyepieces to start with. While eyepiece selection can be somewhat subjective – especially at the more costly end – getting good eyepieces is important for enjoying your astronomy experience and getting the best views through your telescope.
Eyepieces take the image provided by your telescope at the focal plane and magnify it into something you can view. Different eyepieces provide different magnifications and fields of view. Low powers are best for finding targets and viewing larger ones, while high power is great for teasing out fine detail. The higher the magnification, however, the dimmer and harder to focus the image, so there’s a limit to how much magnification you can use, or want to use. At the same time, too-low magnification can reduce the contrast and resolving power to a point that it’s hard to see what you’re looking at. We’ll go into this in a bit.
Low powers are best for finding targets and viewing larger ones, while high power is great for teasing out fine detail.
Any good telescope will take eyepieces without needing to use any tools or non-standard adapters. Most good eyepieces are threaded for standard filters at the bottom of the barrel and fit into your telescope by tightening a thumbscrew or two at the back of your focuser or star diagonal.
Choosing eyepieces is somewhat dependent on understanding focal ratio, focal length, and how telescopes work, so we’d recommend reading our article on choosing a telescope first as a primer before you think about eyepieces.
Field of View
The apparent field of view of an eyepiece is how far you can see side to side, measured in degrees. Back in the early days of widespread amateur astronomy in the 1950s, 40-50° was considered extremely wide. Nowadays, the cheapest acceptable eyepieces usually have an apparent field of view of around 50°. Most people enjoy fields of view subtending at least 65°, and almost any well-equipped astronomer has eyepieces with 80° or more. Wider fields of view allow you to more easily fit objects at higher magnifications, view targets for longer without motorized tracking, and just produce a more immersive and pleasant view overall.
Eyepiece designs go back to the first single-lens designs invented by Galileo and Kepler. Today, eyepieces usually have 4 lens elements or more and follow essentially the same handful of designs. The goal of any eyepiece design is to have good correction towards the edge of the field of view. Due to the way light enters the eyepiece at steep angles the further you get from the center of the field, designers of eyepieces have been in a constant battle of trying to achieve the sharpest views possible out to the very edge without compromise, especially with faster f/ratio telescopes. As a general rule, the faster focal ratio your telescope is, the more demanding it is on eyepieces.
The faster focal ratio your telescope is, the more demanding it is on eyepieces.
The other factor besides sharpness itself and apparent field is eye relief. A short eye relief eyepiece requires jamming your eyeball into the field lens to see anything at all. Good eyepieces have big lenses to look through and enough eye relief that you may even be able to use glasses.
Huygens and Ramsden eyepieces use 2 convex lens elements and produce very narrow fields of view, usually 20-35°. They are uncomfortable to use and produce poor results with anything faster than f/8 to f/10. These are usually the junk supplied with beginner telescopes. Ramsdens can be on the verge of acceptable, but generally aren’t something we’d recommend using.
The Kellner is essentially an upgraded Ramsden with a field of view typically spanning 40-50°. They are a lot sharper and more comfortable than a Ramsden, but still have some issues with sharpness even at the center of the field, and with fast instruments the view is not the best
The RKE or “Reverse Kellner” is commonly mislabeled as a Kellner, but performs quite a bit better, with slightly better correction and providing more eye relief.
The Plossl is an improved version of the Kellner with 4 or 5 lenses; most cheap ones are “Symmetrical” Plossls with two identical cemented pairs of lenses. They have fields of 45-60°. Short focal length Plossls tend to have poor eye relief as with Kellners and many other cheap types.
Erfle, Konig, Bertele, and other simple “wide-angle” designs with fields of 60-80° are older designs with 5-6 lenses that work best with slower scopes. Few work well below f/5 or f/4.5 due to aberrations towards the edge of the field. The exception is the Panoptic design from Tele-Vue which has been cloned by some manufacturers and works well at f/3.5 or above.
The Nagler design provides an 82° field of view with good correction all the way down to below f/3 using 7-10 lenses. Various copycats exist.
The Ethos design provides a 100-120° field of view with good correction even at f/2.5, using 8-12 lenses.
If you’re unable to find out the specs of an eyepiece directly, the name of the eyepiece might give you a clue:
Most “planetary” or “super” eyepieces are Plossls or Kellners, maybe with an extra lens element or two added.
“Wide-angle” generally refers to 60-70 ° of apparent field. These are usually some variant on the Erfle or Bertele design.
“Super-wide angle” generally refers to 65-75 ° of apparent field. These are also some variant on the Erfle design most of the time.
“Ultra-wide angle” generally refers to 80–90 ° of apparent field. These eyepieces are either Naglers or copycats for the most part.
“Hyperwide”, “extra wide angle” or “mega-wide angle” generally refers to eyepieces with a field of view of around 90-100 ° or more. Most of these are some variant on the Ethos.
Since most mass-produced eyepieces are copies of somebody else’s design – either the more “classic” variants or something from Tele-Vue, the bulk of the wide-angle eyepieces tend to have standardized fields of view of 68, 72, 82, 100, or 110 °, to within a few ° of rounding error.
0.965” barrel diameter eyepieces are usually found on smaller and older telescopes, and are seldom made today. Most telescopes that take 0.965” eyepieces should probably be avoided unless you’re a collector. Small refractors can be more expensive to retrofit than it’s worth; reflectors actually need physical modifications to reach focus with newer oculars, and quality 0.965” eyepieces are hard to come by and limited in selection.
1.25” eyepieces are probably the most common size. Any good telescope should at least be able to handle 1.25” eyepieces either directly or with a step-down adapter from a larger size. 1.25” eyepieces will also fit in a variety of plumbing fittings if you are building your own telescope, which is rather convenient.
2” eyepieces are usually meant for larger telescopes; most good telescopes over 6” in aperture have a 2” focuser or can be easily retrofitted for one.
3” eyepieces are seldom seen and are mainly meant for very large, long focal length telescopes, usually large refractors or catadioptrics.
The main benefit of bigger barrel diameters is the ability to gain a larger field of view. This is why most 2” eyepieces are longer in focal length than, say, 15mm or so. Even the widest field of view eyepieces with 100° or more tend to be 1.25” diameter or at least compatible with 1.25” focusers if they are at shorter focal lengths, as the field of view they contain subtends a smaller diameter than a 1.25” barrel. So with any telescope that accepts 2” accessories, your eyepiece collection is likely to consist of a mix of 1.25” and 2” oculars, which you can easily switch between using an adapter – or the eyepieces themselves have a dual-barrel capable of fitting in either size eyepiece holder.
The “gold-line” and “red-line” eyepieces offered by a variety of generic sellers such as SVBONY (or Orion, as the Expanse) work best at f/5 or above, being modified Erfles. The apparent fields range from 65° to 71° depending on the focal length. The 9mm is the best and works well down to f/4; the 20mm is susceptible to edge-of-field aberrations with faster scopes, and the 6mm can be annoying with regards to eye placement. You can buy either a set of all of them or individual units.
The various generic TMB-style “planetary” eyepieces offer 58° fields of view and work best at f/5 or above.
The generic 23mm aspheric eyepiece features a plastic lens element, which is prone to scratching, but works great down to f/4.5 and has a wider field of view than a Plossl at about 63°. The 10mm and 4mm aspherics offered as a set with this eyepiece are basically unusable, however.
The Apertura, GSO, Celestron Omni and Orion Sirius Plossls are well-made and well-corrected but have somewhat narrow fields of view at 52°. Cheaper Plossls are often nearly as good but may not have as good quality control.
The Long Perng planetary eyepieces are very comfortable to look through and sharper than the TMB designs, but have narrow fields of view at only 50°.
Vixen’s NPL line of Plossls are a bit better than the cheaper brands, and have twist-up eyecups.
Kokusai’s Orthoscopics are extremely sharp and work at any focal ratio but have narrow fields of view, below 50°.
Astro-Tech’s UWA eyepieces are well-corrected down to f/3, easy to look through, and offer an 82° field of view.
Meade’s PWA eyepieces offer an 82° field of view with good correction down to f/4.
Baader’s Hyperion eyepieces are well-corrected down to f/4, easy to look through, and a variety of accessories are available for them. They have a wide field of view of 68°.
Celestron’s Luminos eyepieces offer a huge 82° field of view, but don’t do the best below f/5.
The 10mm Masuyama is sharp and has a gigantic 85° field of view but performs best in longer scopes.
Vixen’s SLV eyepieces are extremely sharp modified Plossls with loads of eye relief and a 50° field of view.
Tele-Vue’s Plossls are the best in their class but aren’t known for comfort nor a wide field.
Astro-Tech’s 28mm UWA is well-corrected down to f/3, easy to look through, and offer an 82° field of view.
Explore Scientific’s 40mm 52° is really only usable at f/6 or above but works well.
Celestron’s 56mm Omni Plossl is meant for scopes with focal ratios of f/8 or above; it provides too low of a power for anything shorter.