So you are looking to buy a telescope or already have one but are wanting the best telescope eyepieces to get better detail in your views. Here is what to look out for and the sizes that matter when choosing an eyepiece collection for your telescope.
In a hurry? Check out the top brand, Tele Vue, at Amazon.
What are the best telescope eyepieces?
For the best telescope eyepieces, look for brands like Tele Vue, which may cost you that bit more, but are quality built types. Size-wise, consider a 10-14 mm for general use, a 7-10 mm as a basic workhorse, one smaller again for detailed views (e.g. of planets), and another larger for location objects in the sky.
A typical eyepiece set would cover low power, medium power, and high power. You’ll find a chart of eyepiece sizes to suit different purposes featured below in this telescope eyepieces guide.
Eyepieces are important components of your telescope. They contribute to half of the optics in a refractor telescope and about a third in a reflector. Thus, choosing the right eyepiece will improve your experience.
Why get extra eyepieces?
The eyepieces that come with a new telescope are often the 9 mm and 25 mm or just the basic working-horse type.
If included, the 2x Barlow lens will double the magnification of the eyepieces, meaning effectively you’ll also have 4.5 mm and 12.5 mm eyepieces with your 9 and 25 mm inclusions.
(Barlow lenses come in other sizes apart from 2x. I cover more on using a Barlow lens in this article about using a Barlow lens.)
With a Barlow lens, you might cover all the sizes needed for the variety of viewing options you are seeking, but the eyepieces may not have the right eye relief or the best quality opticals.
It’s about enhancing your experience. Most new telescopes come with eyepieces but these are not always the best quality or give you the best views possible with the telescope.
This telescope eyepiece guide covers some main features and metrics to know in buying telescope eyepieces.
What to consider about that best telescope eyepiece set
If you are thinking of buying a ready-compiled eyepiece kit, it’s best to check whether the kit includes the telescope eyepiece sizes that are best for you and whether some will be surplus.
A few good quality eyepieces may be a better investment than a set with extras you won’t use.
That aside, a set that includes filters and lens may be the best telescope eyepieces for beginners especially to get accustomed to their use and gain an understanding of the construction and mechanics.
The best eyepieces for your telescope
If you want better views of the details on planets, buy yourself additional eyepieces to those that come with the telescope.
Apart from better quality optics, you will get more out of your telescope with extra eyepieces that give you the right magnification and field of view for your viewing objective.
Viewing comfort & usability
You want comfort while observing. And, of course, you want eyepieces that you’re going to use. So their construction and mechanics matter. Eye relief is another.
Construction & mechanics
Features to look out for include the barrel size and the type of lens and its coating.
First up, take note of the barrel size on your telescope, i.e., the diameter of the eyepiece slot. Most are either 1.25″ or 2″. You will need to shop for eyepieces matching that diameter.
Or alternatively, you could search for an adapter if you really need to use a different diameter of the eyepiece to that of the barrel.
Type of optical lens
You can get optical lenses with as many as eight elements. These are the more sophisticated eyepiece designs for which you’ll pay extra.
One thing to know is that not all eyepieces suit every telescope. Plössl eyepieces, which have four lens elements, for example, are not recommended for fast telescopes (f/5 or lower) or Dobsonians.
Eye relief is the max distance where you can position your eye away from the top eyepiece lens and still see the full field of view.
Having your eye jammed up close say with a 5–8 mm eye relief can become uncomfortable.
Eye relief especially matters if you need to wear eyeglasses while observing. This may be the case if you have strong astigmatism. For this, look for long eye relief above 15, possibly 18–20 mm, to help.
Focal length – Size considerations
As always noted, the smaller the eyepiece focal length the greater the magnification.
Best eyepiece size for your viewing purpose
Telescope eyepiece field of view
The field of view, measured in degrees, is how much night sky you will see.
The size of the eyepiece field-stop determines the apparent (AFOV) or the width of the light circle seen when looking through the eyepiece.
The larger the AFOV, the more sky you’ll see at a certain magnification.
So you’ll need to know the AFOV of the eyepiece. A standard Plossl eyepiece, for example, has an AFOV of 50°.
The apparent angular sky width ranges from 40° to 100°.
The true field of view (TFOV) is the amount of sky you will really get to see.
What this means is that if you have two eyepieces with the same AFOV, say 100°, but with different FLs, e.g., 13 mm and 21 mm, each will show a different amount of the sky or TFOV. The 13 mm has nearly twice the magnification of the 21 mm and so you will see an object in the sky nearly twice as big. Because both give 100° circle, the 13 mm with have a smaller TFOV because you are only fitting in half of the image from the 21 mm.
TFOV = Eyepiece AFOV ÷ magnification.
There is a limit to magnification. A rule of thumb is to stay within twice the telescope’s aperture in millimeters.
Or, in inches, multiply the aperture by 60 (some say 50x is better given average atmospheric conditions) for the maximum usable magnification of your telescope under normal conditions.
By changing eyepieces you change the magnifying power of your telescope.
Eyepieces usually have their focal length marked on the piece.
Shorter focal lengths correlate with higher magnifications.
How to work out eyepiece focal length to achieve a certain magnification
Telescope Focal Length ÷ Magnification = Eyepiece Focal Length (FL)
Example: A telescope with a focal length of 800 mm and you are wanting magnification of 200x requires an eyepiece of 4 mm FL.
Tip: Always make sure you are using the same units, e.g., millimeters (mm).
You need to know the limits of your telescope’s magnification range (minimum and maximum usable power) when buying eyepieces, as the aperture size will restrict how far you can go.
Let’s take the Skywatcher 120 mm (4.7″), included in my article covering the best telescopes for planet viewing. It has f/7.9 and a focal length of 900 mm. It comes with eyepieces with focal lengths of 5 mm and 20 mm. The maximum useful magnification is 283x.
Magnification of the two included eyepieces are 45x (with the 20 mm) and 180x (using the 5 mm).
The 5 mm is not bad.
What’s the best telescope eyepiece for viewing planets?
For the best telescope eyepiece for viewing planets you will need pieces that will give you 150x up to say 240x depending on the limits of your telescope and the planet of interest.
If you want good views of Saturn’s rings, for instance, you’ll probably need something that will give you 200x or 240x magnification.
For seeing features of Jupiter, you might only need 100x or 150x magnification.
So in choosing the eyepiece, first decide the magnification you need. Note: you will be constrained by the maximum magnification limit of your telescope, so check this also.
Next, divide the focal length of your telescope by the magnification required and this will give you the eyepiece focal length (mm) that should give you the views you need.
How to calculate eyepiece size in terms of focal length (EFL):
Telescope FL ÷ magnification = Eyepiece FL
Example: To calculate the eyepiece FL, take the telescope FL (e.g. 900 mm) and divide it by the magnifications (150x, 200x, 240x). This gives you 6 mm for the 150x, 4.5 mm for the 200x, and 3.75 mm for the 240x. The smaller focal length supports the higher magnification.
Note: In practical terms, getting good views using magnifications over 200x will depend on the observational conditions as well as your telescope capabilities.
How to choose eyepieces for your telescope
If you’re looking for a useful set, the following may be useful.
Calculating eyepiece sizes for a useful set
The set will need to fall within the exit pupil limits.
Why is exit pupil important? It is the diameter of the light beam exiting the eyepiece and entering your eye. Note: the larger the exit pupil (ep), the brighter the image you are likely to see.
But it has limits: no more than 7 and no less than 0.5 mm (the average user’s pupil diameter under dark condition). Outside that range, the light is wasted.
How to calculate exit pupil of eyepiece
Exit Pupil (ep) = Eyepiece FL ÷ Telescope f/ (Focal Ratio)
We start with half the f/ of the telescope as the first estimate for an eyepiece FL. Using the Skywatcher (TFL 900 mm f/7.9) as an example, half the telescope’s focal ratio (f/) gives us the eyepiece focal length of 4 mm.
Dividing the eyepiece focal length into the telescope focal length we get the magnification.
900 mm ÷ 4 mm = 225x magnification
Using this 225x magnification as a starting point and adding magnification increments of 1.5x (with eyepiece increments usually 1.4x, 1.5x, or 1.6x), we calculate the exit pupil (ep) for increasing eyepiece sizes until reaching a sizes that yields an exit pupil outside a useful limit.
Eyepiece FL based on a telescope with 900 TFL and f/7.9:
As explained earlier, the useful exit pupil upper limit is 7, but this can vary from 5 to 7 maximum, depending on light conditions and the users’ age 1.
Anything that gives an exit pupil greater than 7 is usually wasted and for older users, 5 would be the maximum limit. The limit of 7 is the average pupil diameter of youthful dark-adapted eyes. The limit decreases with age 1.
So, in the above case, you can include eyepieces of focal lengths between 4 mm and 45 mm.
What are the best eyepieces for fast telescope
The general idea is that a fast telescope has a short focal length (f/7 or below), which produces a wider field of view of the sky than that of a long focal-length telescope.
What would be the best eyepiece for a fast telescope? Not all eyepieces work well in fast scopes. Coma image distortion worsens with fast telescopes, say f/6 or lower, and correction is needed to improve views. As a general rule, high-end types, like Tele Vue Plössl eyepieces, work best as they are well-corrected for coma effects seen with fast scopes.
Best brands of telescope eyepieces
Who makes the best telescope eyepieces? Many consider Tele Vue as the best telescope eyepiece brand, especially when it comes to the Nagler type, for the range of focal lengths. You’ll find these eyepieces at reasonable prices at Amazon – See details.
Some other good brands of eyepieces for general observing of the night sky include:
- Gosky Plössl eyepieces
- Celestron X-Cel LX
- Celestron 93220
- Celestron 93432 Luminos
- Baader Hyperion
- Orion Lanthanum
- Orion 8728 Sirius Plössl eyepieces
- Some users also swear by GSO.
- Jay C. Bradley, Karl C. Bentley, Aleem I. Mughal, Hari Bodhireddy, Sandra M. BrownJ “Dark-adapted pupil diameter as a function of age measured with the NeurOptics pupillometer”. Refract Surg. 2011 Mar; 27(3): 202–207. Published online 2010 May 17. doi: 10.3928/1081597X-20100511-01
- Featured image source: Nick Kinkaid, Attribution-NoDerivs 2.0 Generic (CC BY-ND 2.0)
- FOV, Randy Culp, https://www.rocketmime.com/astronomy/Telescope/Magnification.htm