So you are looking to buy a telescope or already have one but are wondering about 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.
Eyepieces are important components of your telescope. An eyepiece contributes 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.
Navigate This Guide
- Aim For Viewing Comfort & Useability
- Getting The Best Visual Impact
- Optimize Observational Strategies
- Compiling The Right Eyepiece Collection
A typical eyepiece set would cover low power, medium power, and high power.
The eyepieces that come with a new telescope are often the 9 mm and 25 mm, which are 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 investing in a good Barlow lens.
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.
Why Get Extra Eyepieces For Your Telescope?
Why do I need extra eyepieces? 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.
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.
Viewing Comfort & Usability
It’s about comfort while observing. And, of course, you want eyepieces that you’re going to use.
Construction And Mechanics
You want ones that will work with your telescope as well as quality pieces to give you the best views.
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.
Fully multi-coated (FMC) glass optics enhances the transmission of light rays. This provides for high achromatic photos of distant objects such as Venus and Mars.
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.
As always noted, the smaller the eyepiece focal length the greater the magnification.
|2-4.9 mm||Give very high magnification. Can be very difficult to use except under perfect observational conditions and viewing the very bright objects, like the moon.|
|5–6.9 mm||Good for bright objects such as the moon and bright planets. Very high power, so work best with steady observational conditions.|
|7–9.9 mm||Very comfortable high magnification eyepieces. Excellent for observations of bright objects. A basic working horse for any eyepiece collection.|
|10–13.9 mm||General use. Good for all objects including bright nebula and galaxies. Provide a useful mid/high range magnification.|
|14–17.9 mm||Yields effective mid-range magnification and will help resolve globular clusters, galaxy details, and planetary nebulae.|
|18–24.9 mm||Work well for wide-field and extended objects. These are mid-range magnification for viewing objects such as galaxy clusters and large open clusters.|
|25–30.9 mm||Works as a finder eyepiece. Use these to locate objects. They are extra wide field eyepieces, which are also useful for viewing large nebula and open clusters.|
|31-40 mm||Work even better as finder eyepieces. Get excellent extended views and large star fields.|
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.
Focal Length of Eyepieces
Eyepieces usually have their focal length marked on the piece.
By changing eyepieces you change the magnifying power of your telescope. Shorter focal lengths correlate with higher magnifications.
How To Work Out The 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.
Calculating Exit Pupil
What is the exit pupil? It is the diameter of the light beam exiting the eyepiece and entering your eye. By and large, the greater the magnification, the smaller the exit pupil.
Also, the larger the exit pupil (ep), the brighter the image you are likely to see. But the limit is no more than 7 mm and no less than 0.5 mm (the average user’s pupil diameter under dark condition). Otherwise, the light is wasted.
How To Calculate Exit Pupil For An Eyepiece
Exit Pupil (ep) = Eyepiece FL ÷ Telescope f/ (Focal Ratio)
The useful exit pupil can vary from 0.5 to 0.7 mm minimum and from 5 to 7 mm maximum, depending on light conditions and the users’ age 1.
Let’s take the Skywatcher 120 mm (4.7″), included in my article covering 4 best telescopes for planet viewing. It has f/7.9 and focal length 900mm. 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 for planetary viewing, but there’s some room for improvement if you want to get good views of Saturn’s rings for instance. Say, you are looking for 200x or 240x magnification. How do you get this?
To calculate the eyepiece FL, take the telescope FL (900) and divide it by the magnifications (200x, 240x).
EFL = TFL/magnification
This gives you 4.5 mm for the 200x and 3.75 mm for the 240x. As seen, a smaller focal length corresponds to a higher magnification.
Note: in practical terms, reaching magnifications over 200x could require clear atmospheric conditions, so your location will affect the outcome. (I wrote this article that covers dark sky places for stargazing.)
How To Choose Eyepieces For Your Telescope
Calculating Eyepieces For A Set
Using the Skywatcher (TFL 900 mm f/7.9) as an example, we start with half the telescope’s focal ratio (f/) as the eyepiece focal length, i.e., 4 mm.
This gives us a 225x magnification. Using magnification increments of 1.5x (with eyepiece increments usually 1.4x, 1.5x, or 1.6x), this will give us the exit pupil (ep) for each.
Eyepiece FL based on a telescope with 900 TFL and f/7.9:
|EFL (mm)||Exit Pupil||Magnification|
|4.00 mm||0.51 ep||225x|
|6.00 mm||0.76 ep||150x|
|9.00 mm||1.14 ep||100x|
|13.50 mm||1.71 ep||67x|
|20.25 mm||2.56 ep||44x|
|30.38 mm||3.84 ep||30x|
|45.56 mm||5.77 ep||20x|
So, you could look at eyepieces with focal lengths between 4 mm and 45 mm. Anything that gives an exit pupil greater than 7 mm is usually wasted. The limit of 7 mm comes from it being the average pupil diameter of youthful dark-adapted eyes. But it is worth noting that this decreases with age 1.
Best Eyepieces For Telescopes
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 are reasonable prices at Amazon – See details.
Some other good brands of eyepieces for general observing of the night sky include Gosky Plössl, Celestron X-Cel LX, Celestron 93220, Celestron 93432 Luminos, Baader Hyperion, Orion Lanthanum, and Orion 8728 Sirius Plössl. 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