Wanting better planet or moon observations? There are many modern eyepiece designs to choose from and knowing what to buy is a tad daunting. This is the reason I put this guide together. It includes a selection of eyepieces and what to know to help you decide on what’s right for you. (updated January 2021)
For the best eyepieces, there are brands like Tele Vue offering Nagler, which will cost a bit but is a quality built design that will give you “space ship views”. Read on…
My experience with accessories that come with a new telescope is that the eyepieces don’t cover all expectations.
And…Eyepieces are important since they contribute to half of the optics in a refractor telescope and about a third in a reflector.
How many eyepieces do I need?
In terms of how many eyepieces to get, you need at least three to cover a useful range of magnifications: a low (40–20mm), a medium (19–13 mm), and a high 12–4 mm, along with a Barlow lens, for a set considered useful by those experienced in the field.1
Why get extra eyepieces?
As an accessory item, a new telescope often comes with 9 mm and 25 mm eyepieces, and sometimes a Barlow lens, which increases the range of magnifications.
While the 9 and 25 mm are basic working-horse types and useful, these inclusions with your purchase are usually of lesser quality than others on the market. And, the eye relief of these may not always suit the user.
If included, a 2x Barlow lens will double the magnifications, 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 a number of magnifications apart from 2x. If you want to know how to use them, see my article covering what you need to know about Barlow lenses.
Types of eyepieces
You can get optical lenses with as many as eight elements. These are the more sophisticated eyepiece designs for which you’ll pay extra. The following lists the types commonly used today in order of their number of elements and expected quality.
Once the best available, they are still preferred by many for medium to high magnifications. These types introduced to the user a four-element design, providing sharp images with a slightly wider FOV than earlier models. The construction saw the end of essentially all optical aberrations.
These can be relatively cheap in price and they are fine for planetary viewing.
Plössl lenses have four elements but are considered superior to the Orthos. They have a slightly wider FOV. They are useful for low, medium, and high power and great for observing the planets and the moon. They are also reasonably cheap in price.
This modern design has 6 or 7 elements. With this much glass, we’re moving up towards the higher end of the market price-wise. But going by what users say, there’s no disappointment in owning one of these.
An example of the wide-field optics is the Panoptic by Tele Vue, a premium buy.
The Nagler came on the market in 1981, introduced by Tele Vue. It’s regarded as the finest in the medium to high power category for giving sharp images. This design provides a wide FOV.
Here we’re at the upper end of the scale, price-wise. However, the user experience has been described as “looking at the universe through a space-ship window”.
The Ultra Wide-Angle is a similar design to the Nagler and was introduced by Meade soon after Tele Vue brought in the Nagler.
The best telescope eyepieces to get
Putting the types aside, you want comfort while observing and a price you can afford. And, of course, you want eyepieces that you’re going to use. So first up, take note of the barrel size on your telescope. This is the diameter of the eyepiece slot. Most are either 1.25″ or 2″. Make sure to shop for eyepieces matching that diameter or otherwise, you’ll need to buy an adapter so they fit.
Cost-wise, for your ocular collection, consider spending at least 1/3 of the cost of your telescope. Good quality eyepieces tend to hold their value and can be used on subsequent telescope purchases, so look at it as a long term investment.
Is a zoom eyepiece a good idea?
I’d probably steer away from zoom eyepieces. Experienced amateur astronomers like Terrence Dickinson rate these as undesirable for astronomy. Dickinson gives two reasons: a restricted field of view and usually an inferior optical performance.1
Eye relief and corrective glasses
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 to the lens with a short eye-relief, or worse none, when using high power can cause discomfort for most users.
But it particularly matters if you need to wear eyeglasses while observing. Not everyone who normally wears eyeglasses needs them with a telescope. I cover who is most likely to need them in my article on eyewear with telescopes or binoculars. In the case you do need to wear them, look for a lens with long eye relief, e.g. at least above 15 but possibly up to 18–20 mm, to help.
long eye relief range
Also known as high eye relief, the following are known brands with a series providing comfort for eyeglass users:
- Tele Vue – Delos
- Vixen – LV series
- Celestron – Xcel
- Pentax – XW Series
field of view – apparent and true
The field of view, measured in degrees, is how much night sky you will see.
The true field of view (TFOV) is what you actually see of the sky. The apparent field of view (AFOV) is the width of the light circle seen when looking through the eyepiece.
The TFOV is governed by the telescope’s magnification (with the said eyepiece) and the AFOV as follows:
TFOV = AFOV ÷ magnification.
This means the higher the power, the less of the sky (TFOV) you’ll see at a given AFOV.
Imagine you have two eyepieces with the same AFOV, say 100° for simplicity, but are different in FL, e.g., 13 mm and 21 mm. Each will show a different amount of the sky or TFOV.
With close to double the magnification of the 21 mm, the 13 mm will give a view of an object that’s nearly twice as big. But 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.
So it’s worth knowing the AFOV, which can range from 40° to 100°.
Exit pupil size
Why is exit pupil important? The exit pupil is a measure (ep) indicating the diameter of the light beam exiting the eyepiece and entering your eye. It is important because if it is too large, light is exiting outside of your eye and is being wasted.
The larger the exit pupil (ep), the brighter the image you’ll likely see, but the upper limit of it being useful is 7 for the average youthful dark-adapted eyes.
Individually, this figure can vary from 5 to 7, depending on light conditions and the users’ age.2 For older users, 5 can be the maximum limit. The limit decreases with age.2
To work out the range of useful eyepieces to get for your telescope and this factor: Exit Pupil (ep) = EFL ÷ Telescope Focal Ratio (f/)…
The following table shows the results of this for a telescope with 900 TFL.
It starts with a magnification of 225x, which is the TFL divided by an EFL of 4 mm (half the f/ of the telescope) i.e., 900 mm ÷ 4 mm. The proceeding rows displays the eyepiece sizes that correspond to exit pupils calculated with magnification increments of 1.5x (eyepiece increments are often 1.5x, though can be 1.4x and 1.6x) until an ep is reached outside the useful limit (7, being for the average youthful user).
This is a manual way of looking at the useful eyepieces, namely those between 4 mm and 45 mm, for the 900 mm telescope.
Range of magnifications
As always noted, the smaller the eyepiece FL the greater the magnification. A rule of thumb with magnification 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.
Eyepiece Focal length and magnification
The following is general guide only:
Magnifications for viewing planets
In choosing eyepieces, you might want to decide on the required magnifications. The following is a guide only for planet and moon observations.
For viewing Saturn’s rings see my article on seeing Saturn through a telescope.
For seeing features of Jupiter, 100x or 150x magnification, depending on the conditions.
Note: In practical terms, getting good views using magnifications over 200x will depend on the observational conditions as well as your telescope capabilities.
Calculating eyepiece based on magnification:
Divide the TFL by the magnification you are aiming for to get the EFL (mm).
TFL ÷ magnification = EFL
By changing eyepieces you change the magnifying power of your telescope.
Shorter focal lengths correlate with higher magnifications.
Eyepieces usually have their focal length marked on the piece.
Tip: Always make sure you are using the same units, e.g., millimeters (mm).
Telescope eyepiece Brands
Who makes the best telescope eyepieces? Many consider Tele Vue as the best telescope eyepiece brand, especially when it comes to the Nagler type, that suit low, medium, and high power applications. 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.
- Dickinson, T. 2019. NightWatch: A Practical Guide to Viewing the Universe. Firefly Ontario.
- 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