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 are some points to consider and the metrics that matter when choosing between telescope eyepieces for sale.
What this Covers
- Why Get Extra Eyepieces For Your Telescope?
- Features and Metrics To Consider
- How to Calculate the 3 Main Metrics
- A Working Example
About Telescope Eyepieces
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.
A typical eyepiece set would cover low power, medium power, and high power.
The eyepieces that come with a new telescope are often the 9mm and 25mm, which are the basic working horse type.
If included, a Barlow lens will double the magnification of the eyepieces.
You may think about buying a ready-compiled eyepiece kit, but three or four good quality eyepieces might serve you better in the long run than buying an entire telescope eyepiece set with pieces you might never use or find problem with. Something to think about.
Why get extra eyepieces for your telescope?
Why do I need extra eyepieces? Apart from better quality optics, you 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.
Telescope Eyepiece Buying Guide
This telescope eyepiece guide covers some main features and metrics to know in buying telescope eyepieces.
Features and Metrics To Consider
First up, take note of the barrel size, which is 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 eyepiece to that of the barrel.
Type of Optical Lens
You can get types 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.
Focal Length Eyepieces
The eyepiece focal length is the focal length of the telescope divided by the eyepiece magnification.
How to Calculate Eyepiece Focal Length
Eyepiece Focal Length (FL) = Telescope FL ÷ Eyepiece Magnification
The magnification of the telescope relates to the focal length of both the telescope and the eyepiece.
How do you work out the telescope’s magnification? The ‘magnification of telescope’ formula is the telescope focal length (TFL) divided by the eyepiece focal length (EFL).
Note: There is a limit to magnification. A rule of thumb is to stay within twice the telescope’s aperture in millimeters. In addition, if you are using high magnification, collimation may affect your viewing.
How to Calculate Eyepiece magnification
Eyepiece Magnification = Telescope FL (focal length) ÷ Eyepiece FL
What this means: With a smaller eyepiece focal length, you’ll get higher magnification for an individual telescope.
Here’s an example: A telescope with a focal length of 800 mm with an eyepiece with FL of 10 mm means a telescope magnification of 80x. Whereas, an eyepiece with a focal length of 25 mm with the same telescope would yield a magnification of 32x (800÷25).
Tip: Always make sure you are using the same units, e.g., millimeters (mm).
The limits of your telescope’s magnification range (minimum and maximum usable power) need consideration when buying eyepieces, as the aperture size may restrict how far you can go.
Times your aperture (in inches) by 60 (some say 50x is better given average atmospheric conditions) and this will give you the maximum usable magnification of your telescope that you can expect under normal conditions – as a general rule of thumb. Above that, will have low contrast.
Field of View
This is how much night sky you will see. It is measured in degrees. There is both the apparent (AFOV) and the true field of view (TFOV).
The larger the AFOV, the more sky you’ll see at a certain magnification, being the apparent angular sky width, ranging from 40 to 100 degrees. The AFOV is governed by the size of the eyepiece field stop.
The TFOV (in degrees) is the amount of sky you will really get to see. You’ll need to know the AFOV of your eyepiece for this. Most Plössl eyepieces have an AFOV of 50 degrees.
TFOV = Eyepiece AFOV ÷ magnification.
Eye relief is the max distance where you can position your eye away from the top eyepiece lens and still see full field of view. It’s about comfort while observing. 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 eye glasses while observing. This may be the case if you have a strong astigmatism. For this, look for a long eye relief, say 18–20 mm, to help.
This is the diameter of the light beam exiting the eyepiece and entering your eye. The greater the magnification the smaller the exit pupil.
Generally the larger the exit pupil (ep), the brighter the image you will 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 The Eyepiece
Exit Pupil (ep) = Eyepiece FL ÷ Telescope f/ (Focal Ratio)
In this, you need to consider the user’s age. So, 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 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 5mm and 20mm. The maximum useful magnification is 283x.
Magnification of the two included eyepieces is 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 get the eyepiece FL, take the telescope FL (900) and divide it by the magnifications (200x, 240x). 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 where to find dark sky places for stargazing.)
Working It Out For A Set
Start with the pupil exit (ep). Using the above example, we start with half the telescope’s f/ (7.9) as the eyepiece focal length, i.e., 4 mm, giving us a 225x magnification and will use magnification increments of 1.5x. (with eyepiece increments usually 1.4x, 1.5x, or 1.6x). The TFL is 900 mm.
Eyepiece FL based on telescope with 900 TFL and f/7.9:
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.
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.
As noted above the smaller the eyepiece focal length the greater the magnification.
- 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, http://www.rocketmime.com/astronomy/Telescope/Magnification.htm