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 that offer Nagler, which will cost a bit more than others but are a quality built design to give you “space ship views”. Read on…
My experience with accessories sold 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.
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.
Al Nagler is the founder of Tele Vue Optics.
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.
How many eyepieces do I need?
Consider having at least three good telescope eyepieces to cover a useful range of magnifications: a low (40–20mm), a medium (19–13 mm), and a high 12–4 mm.
Why get extra eyepieces?
A new telescope might come with 9 mm and 25 mm eyepieces, and sometimes a Barlow lens, which increases the range of magnification.
While the 9 and 25 mm are basic working-horse types and useful, these inclusions 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, which you can use to 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.
The best 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.
List of variables to consider
- Barrel size matches that of the focuser on your telescope
- Price matches your budget but also gives adequate quality
- True field of view it provides
- High power view offered
- Coating that allows clear views
- Exit pupil size suited to you
- Eye relief for comfort
Tele Vue 9 mm Nagler 1.25″
Eye relief 12 mm
7 elements / 4 groups
What to look for in a telescope eyepiece
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.
There are some sized 0.965-inch. This smaller size come with the cheaper telescopes, which typically can only accomodate eyepieces with few elements (i.e. orthoscopic), small fields of view (about 30°), and poor eye relief.
Dobsonian reflectors and top-end apochromatic refractors generally use the larger 2-inch barrel size.
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.
Maximize the True field of view
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 your eyepiece.
There are two ways to calculate AFOV.
- TFOV = AFOV ÷ magnification
The TFOV is governed by the telescope’s magnification (with the said eyepiece) and the AFOV as follows:
- TFOV = eyepiece field stop diameter ÷ telescope’s focal length x 57.3
This means the higher the power, the less of the sky (TFOV) you’ll see at a given AFOV.
Here’s an example: You have two eyepieces with the same AFOV, say 100° for simplicity. But, they have different focal lengths, say 13 mm vs 21 mm. Each will show a different amount of the sky or TFOV.
Which one will give a smaller TFOV? Answer: The 13 mm. Here’s why…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 of the telescope eyepiece, which can range from 40° to 100°.
Maximize your high power view
In maximizing your high power view, be aware that you are limited by the maximum usable magnification of your telescope under normal conditions. I cover this in max magnifications.
- By changing eyepieces you change the magnifying power of your telescope.
Remember, the smaller the eyepiece focal length (EFL) the greater the magnification.
- Shorter focal lengths correspond with higher magnifications.
The following is general guide on EFL and particular use:
- Eyepieces usually have their focal length marked on the piece.
Choosing eyepiece based on magnification:
In choosing eyepieces, you might want to decide on what’s going to give you the best view for the target planet as well as the limitations of your telescope. The following is a guide.
For viewing Saturn’s rings I have an article on seeing these through a telescope.
For seeing features of Jupiter, 100x or 150x magnification, depending on the conditions.
In practical terms, getting good views using magnifications over 200x will depend on the observational conditions as well as your telescope capabilities…
Telescope capabilities — Max Magnifications
Something to be aware of when buying extra eyepieces is the limit of useful magnification.
A rule of thumb with magnification is to stay within…
- Twice the telescope’s aperture in millimeters, or
- Multiply the aperture in inches by 60 (some say 50x if less than optimum atmospheric conditions)
This gives you the maximum usable magnification of your telescope under normal conditions.
Telescope focal length (TFL) divided by magnification gives your the eyepiece focal length (EFL).
TFL ÷ magnification = EFL
Tip: Always make sure you are using the same units, e.g., millimeters (mm).
Eyepiece lens Coating
The options in eyepiece lens coating are untreated, multi-coated, or fully multi-coated.
Magnesium fluoride (MgF2) is a common material in coatings.
Here is the basic difference…
Untreated: No coatings on any of the lenses. 10% light passing through is reflected.
Multi-coated: one lens surface or more treated with multiple layers of anti-reflective coatings. More light passes through.
Fully multi-coated: all lens surfaces are treated with multiple layers of anti-reflective coatings. Much more light passes through.
Eyepieces with coatings will show blue or purple reflections when held up to the light.
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.
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.
This is especially important for wearers of corrective glasses.
It particularly matters if you need to wear eyeglasses but not everyone who normally wears eyeglasses needs them. I cover this in my article on eyewear with telescopes or binoculars.
If you 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.
Short eye reliefs are found in Plössls and orthoscopic types. They are generally around 12–15 mm.
The variety of telescope 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.
It can get confusing with the naming of eyepieces. How do they differ? Which is better? The following lists the commonly used descriptions of telescope eyepieces in order of their number of elements and expected quality.
- Modern wide-field
- Ultra wide-angle
|Type||Number of Elements||AFOV|
|Ultra wide-angle||multiple||up to 85º|
Once the best available, the orthoscopic variety 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.
Th wide-field 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. The user experience has been described as “looking at the universe through a space-ship window”.
The Ultra Wide-Angle is a similar design of eyepiece to the Nagler and was introduced by Meade soon after Tele Vue brought in the Nagler.
Is a zoom eyepiece a good idea?
I’d probably steer away from zoom eyepieces. Experienced amateur astronomers like Terrence Dickinson rate zoom eyepieces as undesirable for astronomy. Dickinson gives two reasons: a restricted field of view and usually an inferior optical performance.1
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
- Nightwatch, a Practical Guide to Viewing the Universe by Terence Dickinson (available at Amazon). This book contains sky charts and has a spring binding and so is practical for use on location.
- 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