A Beginner's Guide to (Budget) Eyepieces

This post is copied from a Reddit post I recently made on r/telescopes. 

First, a message of scope, so to speak. I am a budget astronomer, as such I have little experience with expensive specialized eyepiece designs. The most I've paid for an eyepiece is about 50 dollars. However I think most people who want to know about beginner eyepieces would probably be looking for a guide to affordable eyepieces anyway, so I hope this remains useful.

All telescopes, when used visually, must have an eyepiece to use. It is not uncommon for people to try their new telescope (or old telescope found in the attic) without the eyepiece, and they will be doomed in that attempt. All they’ll see is the mirrors or lens of the telescope. A camera can be placed at prime focus (the focal point of the telescope), but in that case the telescope is replacing a lens that the camera would otherwise have. You’re not allowed to remove the lens in your eyeball, so you must examine the image formed by a telescope at the focal point using an eyepiece. The eyepiece is fit into the telescope’s focuser, and the focuser is then operated to move the eyepiece back and forth near the focal point of the telescope until the image comes into sharp focus.

Refractors and Cassegrain Reflectors usually use a Star Diagonal to reflect the image up at a 90° angle to make it easier to use when pointed high in the sky. The Star Diagonal fits in the focuser, and then the eyepiece fits in the diagonal. A very cheap telescope is likely to have a poor quality Star Diagonal. If you’re getting bad images, you might try the eyepiece in the focuser without the diagonal. If it’s a better image, you need to get a new diagonal. Sometimes a prism is used instead of a mirror in a Star Diagonal. Prisms may produce some odd effects, such as a spike going through stars. A 45° prism diagonal is also not ideal for viewing high in the sky.

In addition to eyepieces, I’ll touch on Barlow Lenses, why you might want to use them, and why you might not.

An eyepiece does not provide a fixed magnification in every telescope. Different telescopes will have different focal lengths, and so the same eyepiece might provide a different magnification in one telescope than in another. You need to know both the Aperture and the Focal Length of your telescope to choose eyepieces. This information is usually printed on the telescope tube somewhere.

The Important Numbers of an Eyepiece

• Focal Length (f): The absolute most important number, as it (along with the focal length of the telescope) determines the magnification of the instrument. This is almost always written on the eyepiece barrel and given in millimeters (mm).

• Eye Relief: The distance from your eye to the exit pupil of the eyepiece, where the full field of view becomes visible.

• Apparent Field of View (AFOV): The angle from one side of the eyepiece's field, to your pupil, to the other side, or how big the eyepiece's field of view appears when you put your eye in the exit pupil. Along with magnification, this determines the True Field of View (TFOV) of the instrument in use, or how much of the sky you can see. Wider fields of view are also generally prettier and give the impression of looking out a window.

• Barrel diameter: The size of the chrome barrel which fits into the focuser of the telescope. For beginner eyepieces we will only be considering 1.25" and 0.965". 1.25" is the standard for modern eyepieces, but you'll find 0.965" eyepieces in very low quality telescopes and in vintage telescopes. For refractors with 0.965" focusers, special adapter star diagonals can be acquired which allow for the use of 1.25" eyepieces. The next step up is 2" eyepieces, but these tend to be expensive specialist eyepieces and, even if you have a telescope with a 2" focuser, you probably have a 1.25" adapter to use with it. The main reason to use a 2" focuser is for very wide fields of view, as the AFOV is restricted by the barrel diameter.

Formulas for Magnification of a Telescope

Magnification of a telescope is given by:

Magnification = Focal Length of Telescope / focal length of eyepiece.

The maximum useful magnification of a given telescope, assuming its optics are diffraction-limited and you're in excellent viewing conditions, is approximately:

Max Useful Magnification = 2x Per mm of Aperture = 50x per Inch of Aperture.

Minimum Useful Magnification is determined by exit pupil. If you go for a very very low magnification, the exit pupil will be too large and your iris will catch some of the light, acting the same as if you'd put an aperture mask on the front of the telescope. For people with good eyes, the rule of thumb is:

Min Useful Magnification = 3.6x per Inch of Aperture = 0.142x per millimeter of Aperture

The number (of inches or mm) is equal to the reciprocal (1/x) of the diameter of the pupil in the observer's eye, which is nominally 7mm or 0.278".

Also useful is the focal ratio. Smaller values represent shorter telescopes for a given aperture and are called “Fast,” larger values represent longer telescopes for that same aperture and are called “Slow.”

Focal Ratio = Focal Length of Telescope / Aperture of Telescope.

We can rearrange these to get the following, which determines which eyepieces your telescope can use depending upon your telescope’s focal ratio:

Maximum Useful Eyepiece Focal Length (mm) = Telescope Focal Ratio / 0.142

Minimum Useful Eyepiece Focal Length (mm) = Telescope Focal Ratio / 2

But also keep in mind that unless you're going to get a 2" eyepiece, you can't get an eyepiece focal length more than about 32mm, since the apparent field of view would begin to go down.

Overpowering Your Telescope: 1000x Advertised on the Box!

Many telescopes, especially cheap department store telescopes, provide eyepieces of short focal lengths and low-quality high-power Barlow magnifiers to reach absurdly high focal lengths, so they can put a big number on the box. If a telescope is advertised using magnification instead of aperture, that’s a red flag to begin with. But this is especially true if the magnification advertised is more than 50x per inch of aperture or 2x per millimeter of aperture.

If you use more than the recommended magnification, the view you get will be dim, extra blurry, and may have noticeable and distracting diffraction effects.

Sometimes, sky conditions prevent viewing at high magnifications. Atmospheric turbulence can cause wobbly distortions known as “bad seeing”. On most nights, you can’t get above 200x. On excellent nights, you can go beyond 400x (with a large enough telescope) and still see clearly. On very poor nights, you might be limited to 100x. The altitude of an object above the horizon also matters--if it’s low in the sky, you’ll be looking through more air, and the image will be especially wobbly and roiling.

Underpowering & Exit Pupil

The rule given above for minimum magnification, 3.6x per inch of aperture, or 0.142x per millimeter of aperture, are based upon the assumption that your eye will open up to about 7mm. If your eye’s pupil is smaller than the exit pupil of the instrument, your iris will be acting as an aperture stop, blocking light from reaching your retina. For this reason, the minimum magnification of a telescope actually increases as the observer ages, as older observers tend to have more constricted pupils. 20 year olds tend to open their pupils to 8mm, 40-year-olds to 6mm, and 60-year-olds to 4mm. But to be sure you’ll have to measure the eye pupil yourself. (Dark room for adaptation, a mirror, a ruler, camera with a short flash--I’ve never done it myself, but you can probably figure it out.)

Exit Pupil of a telescope and eyepiece is just:

Exit Pupil (mm) = Eyepiece Focal Length (mm) / Telescope Focal Ratio

Exit Pupil (mm) = Aperture (mm) / Magnification

Or rearranged if you know the target exit pupil and want to know magnification:

Magnification = Aperture / Exit Pupil

You can use this to find the actual practical minimum magnification (and thus, maximum eyepiece focal length) that you can use with a given telescope.

Focal Ratio and Aberrations

The Focal Ratio, or the ratio between the focal length and the aperture of a given telescope, determines the range of possible eyepiece focal lengths your telescope can use, and can be derived from the formulas below. However, there’s another important thing to consider about telescopes and eyepieces.

For any kind of optical aberrations, be they chromatic aberration in an objective lens, spherical aberration in a cheap mirror; or false color, blur, and distortions originating in an eyepiece, a slower focal ratio will reduce those aberrations. This is why, before telescopes used achromatic doublets, they had to be extraordinarily long--otherwise there would be too much false color fringing. It also means that some eyepieces will perform well in a slow (long focal ratio) telescope, but show severe distortions in a fast (short focal ratio) telescope.

Note that this focal ratio is a descriptor of the telescope, not the eyepiece. A Barlow lens, in addition to magnifying the image, also increases the effective focal ratio of the entire telescope. Thus, an eyepiece with noticeable distortions when used on its own might perform better with the Barlow. Barlows will not fix aberrations originating in the objective lens or mirror, however.

Know Your Eyepieces

Different eyepieces can have wildly different designs, and even two eyepieces of the same focal length might have different use cases. They’re often named after the astronomer or optician who invented the arrangement, but I’ve listed a few more modern branded arrangements as well which come up often.

Eyepieces will have their focal length and usually the type of eyepiece (sometimes as a single letter, e.g. K or H, sometimes written in full, e.g. "Plossl" or "Ultra Wide Angle") printed somewhere on it.

The short version is that Huygens and Ramsden eyepieces are bad, Kellners are decent, Plossls are good, and there are other eyepiece designs which are expensive and excellent.

Most of the very cheapest beginner telescopes come with some very poor eyepieces indeed. The Huygens (H) type, which has just two elements, was designed in the 18th century for telescopes of extremely long focal lengths in a time when glass absorbed so much light that limiting the number of optical elements was necessary, and long focal lengths reduced aberrations. As a result, they are two-lens designs, with a lot of optical aberrations, and an uncomfortably small eye lens size, uncomfortably small field of view, and with short eye relief. The Ramsden (R) or Symmetrical Ramsden or Super Ramsden (SR) have a similar story. Two-element lenses with a lot of false color fringing. Though in my experience an SR eyepiece is superior to a Huygens, if only just.

The step up from the two-element eyepiece would be the three-element Kellner (K) eyepiece. This design is also sometimes known as a Modified Achromat (MA). The Kellner is effectively a modified Ramsden eyepiece, where the eye lens is an achromatic doublet. They can have fields of view around 40°. These are the bare minimum for useful observing, and you can do a decent job of telling apart Department Store Trash telescopes from Serious Instruments by whether they come with Huygens and Ramsdens vs Kellners or Plossls. I use a Kellner for most of my planetary observing. When used with a long focal length telescope, a Kellner can deliver very nice performance on planets and the Moon, as its small amount of lens elements delivers a clear, reflection-free image. They can also be found very cheap, and so they're a great upgrade path if you're on a budget.

The bread and butter of high quality beginner eyepieces is the Plössl (P) (sometimes called Symmetrical) eyepiece. These are four-element eyepieces, and it is the privilege of the modern day novice astronomer to be able to use them. Up until the last few decades, Plossls would use too much glass to be useful. But new anti-reflection coatings and high quality glass have become common in eyepieces, and so Plossls are shipped with practically every serious telescope, beginner or otherwise. Plossls have a generous field of view of 50° or more, and an eye relief of about 80% of their focal length. This is very comfortable for long-focal-length eyepieces of 20mm and up, but around 10mm they start to get seriously tiny, and nearly impossible to place your eye in the exit pupil when wearing eyeglasses. Avoid Plossls below a focal length of 9mm, but they’re excellent in longer focal lengths.

Towards the expensive end of non-specialized/proprietary eyepieces we have the Orthoscopic (Ortho) eyepieces. These have long eye reliefs at short focal lengths (about 80% the focal length), but have the narrowish fields of view of the Kellner (around 40°) and very small eye lenses. None of these come particularly cheap, and they’re not as common as Kellners and Plossls. Many planetary viewers use these because like the Kellner, they have high contrast by using relatively little glass, and they have a longer eye relief than the Kellner.

Another high end but sub-100-dollar eyepiece is the Rank Kellner Eyepiece, or Reverse Kellner Eyepiece (RKE). It has a 3-element design which is sort of like a Kellner backwards. The 28mm RKE, well known for shipping with the Edmund AstroScan, is a very popular eyepiece. It has a wide field of view, long eye relief, not much glass to produce reflections, and its users often describe a certain “floating stars” effect, causing the telescope to go away. I have a 27mm off-brand RKE myself, from my first telescope, the Bushnell Voyager 4.5x100. It’s an excellent eyepiece, but unfortunately mine has a barrel which is slightly larger than 1.25”, so I can’t use it in my other telescopes! They’re not cheap, but worth mentioning since they have shipped with a few beginner telescopes. Note that RKEs tend to have some somewhat severe distortions around the edge of the FOV when used with fast (f/5 or faster) scopes.

Erfle (EFL) eyepieces are an alternative to Plossls. They’re not usually found for very cheap, but I’ve seen a few with prices comparable to similar sized Plossls. They use 5 elements, and have 60° fields of view and good eye relief, but they’re unsuitable for high powers due to internal reflections and distortions.

Goldline Eyepieces or 66° Ultra Wide Angle are a generic term applied to a set of eyepieces sold by various manufacturers of a modified König optical arrangement, which has three lenses, true color performance, and a very wide field of view of 60-70°. They can have some distortions at the edge of the field of view. The 20mm and 15mm Goldlines are more or less standard Königs, but the 6mm and 9mm variants use an additional achromatic doublet lens to function as a Barlow Lens. They’re highly recommendable to the beginner because they can often be found for low costs and provide wide fields of view with a very long eye relief--usually longer than the focal length.

BST StarGuider Dual ED, also sold under several other brand names, is another affordable eyepiece design which is physically quite beefy and has a 60° field of view and plenty of eye relief. This seems to be another variant of the König but with different additional lens elements, including a Barlow lens in the short-focal-length versions. They’re a fair bit more expensive than the Goldlines, however. I have not used them myself, but they seem to be popular and well-liked by their users, and I intend to acquire one sooner rather than later.

HR Planetary Eyepieces and their identical clones sold under different brands, also called 58-degree Planetary Eyepieces, are another popular entry-level design which is similar to the StarGuider Dual ED designs, but optimized for a wider variety of short focal lengths, again using a Barlow in the light path. At this point they are my favorite affordable eyepiece for short focal lengths. Even in fairly fast telescopes they remain fairly sharp throughout most of the field of view, and in slow scopes they remain fast right up to the edge.

Avoid Kits/Sets

Eyepiece kits seem like attractive deals, but often you don’t need every single eyepiece in the kit, and buying one or two of the useful eyepieces in the kit is more cost effective than buying the whole thing. Many sets come with Plossls in short focal lengths which, as discussed, aren’t comfortable to use. Some eyepiece kits also include filters, which are another topic entirely--but suffice to say that these are best bought a la carte as well. If you have a kit already, don’t be afraid of looking into replacing some of the eyepieces, especially if they’re short focal length Plossls. Don’t even try to use an accessory kit full of Huygens.

Replacing Eyepieces

If you have Huygens or Ramsden eyepiece, you will need to replace them as soon as possible to get good optical performance and comfort out of your telescope. You should have at least two eyepieces, one for wide-field scanning and one for medium-high magnification. If your telescope came with an H20mm and an SR4mm eyepiece, you can replace them with a 25mm or 20mm Kellner and either a 10mm or 6mm Kellner for high power viewing. Odds are your telescope’s objective is actually pretty decent, but don’t spend too much money on very expensive eyepieces. For a somewhat nicer upgrade, go for a 25mm or 20mm Plossl for the low power eyepiece.

If your telescope came with Kellners or Plossls, you’ll get more mileage from expanding your collection rather than replacing it.

Expanding your Collection: Wide Field

You may be surprised to learn that there’s lots of good reasons to want a low magnification. First, they can provide much brighter images, making large dim objects pop out. Second, they provide wide true fields of view, allowing you to appreciate a large amount of starry sky, and fit in large Deep Sky Objects like the Pleiades, Andromeda Galaxy, and Beehive Cluster. If you have a long-focal-ratio telescope, minimum power/maximum true field of view might not be provided with an eyepiece using a 1.25” barrel, as the best you could do is a 32mm Plossl. (Plossls above 32mm focal length provide a smaller apparent field of view as well as smaller true field of view). On the other hand, a telescope with a focal ratio faster than f/4.5 will have a maximum eyepiece focal length of around 32mm anyway, so fast telescopes need to use shorter eyepieces.

Designs other than Plossls can reach very wide true fields of view at shorter focal lengths/higher magnifications. However the majority of these are expensive and/or proprietary designs. When choosing a very wide field 1.25” eyepiece, I recommend going with a 32mm Plossl for telescopes with focal ratios above f/4.5. For faster telescopes, get an eyepiece with the longest focal length useful in your telescope (maybe a little under that if you’re older) with the widest field of view you can get, or just that focal length in a Plossl.

If your telescope has a 2” focuser, I do seriously recommend getting a wide-field 2” eyepiece for your lowest power. In addition to lots of specialized/proprietary designs, you can find Plossls and Erfles in these focal lengths and barrel sizes as well. They’ll just be a lot more expensive.

However, keep in mind that the best magnification to view a deep sky object is not always the lowest. Though it can be best for large, low-surface brightness objects, and for finding objects in your field of view as a supplement to your finderscope, a medium power can often be better--a larger object with lower surface brightness might stand out against a dark background better than a small object with a higher surface brightness against a lighter background, and in that case a medium power is better. Some deep sky objects are so small that they can’t be resolved beyond fuzzy stars without a higher power, such as the Ring Nebula M57.

Expanding your Collection: High Power

For a long time I made do with a 9mm and 10mm Kellner eyepiece which came with my second telescope, the regrettable AstroMaster 114EQ. However, I eventually wanted a higher power eyepiece for observing Jupiter & Saturn with my 6” Dobsonian. The K10mm worked fine, but didn’t provide as much power as I wanted. Odds are your telescope either comes with a 10mm Kellner or Plossl, or if you have a higher power eyepiece, it’s probably a very low quality Ramsden or Huygens. If your telescope doesn’t come with a 9mm or 10mm eyepiece, consider getting one for medium-high power viewing.

My first high power upgrade was a 6mm Kellner, which in my particular telescope brought me to 200x. Despite the tiny eye lens and fairly short eye relief, it worked very well, providing a sharp, crisp view. But I wanted to get to the highest possible power, so I bought a 2.5x Barlow to use with my 10mm Kellner. It actually performed worse in some respects--the field of view was larger, but there was a bright lens ghost and severe internal reflections. Same story with the 15mm Goldline and the Barlow.

The 6mm Goldline is an excellent choice for a planetary eyepiece due to its low cost and wide field of view, but if your eye pupil is constricted by bright lights (for example, lunar observing), you will see kidneybean-shaped blackouts when your eye isn’t exactly in the exit pupil. However, it will have slightly lower contrast than the same focal length of a Kellner due to the extra lens elements.

When choosing your first high power eyepiece, I recommend reaching either close to the maximum magnification of your telescope, or about 200x, whichever comes first. You’ll rarely need magnifications above 200x, and that can come later.

If you need an eyepiece with a focal length of below 5mm, consider getting a larger eyepiece and a Barlow, as most very short focal length eyepieces are very expensive specialized arrangements, and a very short focal length Kellner, Plossl, or Orthoscopic will have an eye relief far too short to use comfortably.

Most specialized high-power eyepieces (including Goldlines and StarGuiders mentioned earlier) include a Barlow lens group in the optical layout for what would otherwise be a long focal length eyepiece, to produce especially high powers. Since the Barlow is more versatile, you might consider getting the Barlow as its own lens instead.

Expanding Your Collection’s Versatility: Barlow Lenses

A Barlow Lens is a negatively-curved lens placed before the focal point of the telescope, which multiplies the focal length of the telescope. (If you’re only interested in visual astronomy, this is equivalent to dividing the focal length of the eyepiece, but this is not technically true from the point of view of the light path.) A camera with no eyepiece lens at all will see a larger image when a Barlow is added in front as well.

A Barlow takes the form of an adapter unit which is placed into the telescope’s focuser (or star diagonal), and the eyepiece fits into the Barlow.

Barlows usually come in 2x magnification, but can be found in 2.5x, 3x, 1.5x, or even 5x.

A Barlow is not something to cheap out on. You need an achromatic (2-element lens) or an apochromatic (3 or more lens elements) Barlow, or else you’ll see lots of false color. Odds are that if your telescope came with a Barlow, it’s a cheap singlet plastic model included solely to reach whatever absurd magnification was advertised on the box, and it should be ignored or replaced.

A Barlow will effectively double the magnifications you have access to with a given telescope and set of eyepieces, assuming that:

• You don’t have any redundant magnifications (a 20mm eyepiece with a 2x Barlow makes a 10mm eyepiece redundant.)

• You don’t overpower the telescope (a 5mm eyepiece with a 2x Barlow will overpower all but the fastest telescopes)

Barlows can provide a lot of versatility, and when building an eyepiece collection they’re a good way to get more mileage out of your collection. However, there are a few drawbacks.

Because of the extra glass, they can introduce internal reflections and halos around bright objects, reducing contrast. Especially cheap ones can introduce false color fringing and severe distortions. With optics, you get what you pay for.

When buying a Barlow, look for the number of lens elements and anti-reflection coatings. I recommend a 3-element Apochromatic Barlow, but a 2-element Achromatic one can suffice.

Barlows will also increase the eye relief of eyepieces used with it, but they’ll do so proportionally to the original eye relief. So a short focal length, short eye relief eyepiece might not extend its eye relief much, but a large Plossl with a long eye relief will have a much longer eye relief when Barlowed, to the point that the eye relief might be too long. This is exacerbated by “Shorty” Barlows.

Expanding your Collection: Middle Powers

If you’ve decided not to use a Barlow lens, you may want to pick up at least one or two more focal lengths of eyepiece in between the 20mm-9mm range. If you already have a 9mm or 10mm eyepiece, you should get an eyepiece around 15mm. The 15mm Goldline is the worst of its set, but still provides a comfortable eye relief and wide field of view. There’s also the StarGuider, which is just fine at this power. Even a 15mm Kellner or Plossl is acceptable.

A 15mm eyepiece is a useful mid-range magnification for a lot of uses. In my 6” f/8 Dobsonian, I’ve found it is great for examining subtle detail in Globular Clusters, and taking a closer look at some planetary nebulae and galaxies. It doesn’t sacrifice image brightness too much, but it still provides a high enough magnification to observe otherwise elusive detail.

Just remember that you can get the equivalent magnification of a 16mm eyepiece if you use a 2x Barlow and a 32mm Plossl.

Consolidating your Collection: Zoom Eyepieces

A Zoom eyepiece sounds like the perfect solution: a wide range of focal lengths available in a single ocular, to be changed as simply as twisting the barrel. The truth is that zoom eyepieces are a compromise. Zooms have somewhat narrow fields of view at the long-focal-length end, they tend to be heavy, their many glass elements result in internal reflections and loss of contrast and brightness, and they’re more expensive than a single normal eyepiece. (Though they are undoubtedly cheaper than many normal eyepieces). One nice feature is, at the short-focal-length end, they actually tend to have longer eye relief and wider fields of view than Plossls, Kellners, and Orthoscopics.

With all optics, you get what you pay for. If you buy a very cheap zoom eyepiece, don’t expect it to perform anything like as well as a single eyepiece. Probably the best zoom eyepiece in price & performance is the Celestron 8-24mm eyepiece. It provides a wide range of magnifications and it performs reasonably well. This eyepiece is used in Library Loaner Telescopes because, after it is permanently installed by the astronomy club, it is very simple to operate and can't be lost.

If you can only afford one eyepiece, or you want an ultraportable option that doesn’t require an eyepiece case, this is something to consider. However most of the time you’ll get much better performance out of a set of multiple eyepieces.

Conclusion

Like all telescope optics, there are inevitable compromises one must make, and different telescopes with different apertures and focal ratios prevent me from laying any specific suggestions in stone. However, I hope this will give you the understanding you need to choose eyepieces on your own from an educated perspective.

If you’re going to go for specialized expensive eyepieces, be sure to do your own research. Read reviews which aren’t associated with the seller’s website. But consider that starting off with a cheaper eyepiece (Plossls or Kellners at the minimum) will work about as well optically and will leave more money left over for other accessories. You shouldn’t have to buy an eyepiece that is as expensive as the entire telescope to get some good viewing done!