Choosing The Best Telescope Eyepiece

With the right eyepieces you get better views. There are many modern eyepiece designs to choose from and knowing what to buy is a tad daunting. This is the reason I put together this guide. It includes a selection of eyepieces and what to know to help you decide on what’s right for you.

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. Eyepieces are sometimes called oculars.

Why get extra eyepieces?

A new telescope will come with eyepieces. The purchase might include a 9 mm and a 25 mm eyepiece, and sometimes a Barlow lens.

While the 9 and 25 mm are basic working-horse types and useful, these inclusions are often bottom of the range type in quality. And, if you need to wear glasses while looking through the telescope, they probably won’t be satisfactory for comfort.

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
• Celestron
• Baader
• Orion
• Meade

How many eyepieces do I need?

To cover a useful range of magnification, three eyepieces will do — one in each category of power: low, medium, and high. Or… opt for two eyepieces, a low- and a medium-power, plus a Barlow (per Terence Dickinson).

POWER CATEGORY	Focal length (mm)
Low	20–40
Medium	13–19
High	4–12

• Eyepieces usually have their focal length marked on the piece in mm along with the type of eyepiece.

The thing to note — the smaller the eyepiece focal length (EFL) the higher the power.

Barlow lenses

A 2x Barlow lens will double the power of the eyepieces.

Example:

• For ease of number use, imagine you have a 40 mm and a 18 mm, with a 2x Barlow lens. You’ll also get the equivalent of a 20 mm and a 9 mm eyepiece, respectively. This gives you a high-power option (9 mm) from a medium-power eyepiece (18 mm).

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.

Doing the rounds of forums, I discovered a range of recommendations:

• 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

Quality eyepieces can improve any telescope’s performance.

Terence Dickinson, Astrophotographer and Author, NightWatch: A Practical Guide To Viewing The Universe.

Terence Dickinson offers a list of his favorites in his NightWatch guide. These include:

• Panoptics by Tele Vue
• Series 5000 Plössls by Meade
• Series 5000 Ultra Wides (Nagler type) by Meade

According to Dickinson, any Plössl is good value. I have Plössl eyepieces and they work well.

What to look for in a telescope eyepiece

Here are variables to consider for a good eyepiece collection:

1. Barrel size matches that of the focuser on your telescope
2. Price matches your budget but also gives adequate quality
3. True field of view
4. Limitation of high-power use
5. Coating that allows clear views
6. Exit pupil upper and lower limits
7. Eye relief for comfort

Barrel size

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.

Price

Cost-wise, for your ocular collection, consider spending at least 1/3 of the cost of your telescope. That’s what Dickinson estimates.

He also reasons that 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.

field of view

The apparent field of view (AFOV) of the telescope eyepiece can range from 40° to 100°.

With a narrow FOV in a manual telescope, you spend more time fiddling to find and centre your target object.

Broadly speaking, the higher the power, the narrower the field of view.

A wider field corresponds to a wider measure of the sky and is thus suited to viewing the Milky Way star clouds, open star clusters such as the Seven Sisters, nebula, and large galaxies.

Limits of high power Use

In choosing a high-power eyepiece, keep in mind the maximum practical magnification of your telescope. Pushing beyond this will give you fuzzy images and a nightmare of trying to find and keep the object in the view.

Keep in mind the practicalities that can limit the use of high magnifications.

You can estimate the practical maximum magnification of your telescope, using one of two rule-of-thumb methods:

1. Twice the telescope’s aperture in millimeters (mm)
   So, if you have a 90mm aperture, then twice that, 180, is the max useful magnification for that telescope.
2. Multiply the aperture in inches by 50x
   For a 4″ aperture, multiply it by 50 and you have 200x as the magnification to stay within.

But here’s the thing…

In practice, it could be as much as half these limits, e.g. under poor seeing conditions.

Your useful magnification limit may fit somewhere in these ranges:

APERTURE SIZE	Multiply the aperture in inches by:
up to 5″	8–40
> 5″	6–30

Exit pupil – What is it?

The exit pupil is the light beam diameter (mm) exiting the telescope eyepiece and entering your eye. This is relevant for choosing telescope eyepieces (as well as binoculars).

Exit pupil (ep) is calculated by dividing the aperture (mm) by magnifying power:

APERTURE ÷ MAGNIFICATION = EXIT PUPIL

Using my Dobsonian as an example: a 10-mm eyepiece in a 150-mm telescope with focal length of 1200-mm:

150 mm ÷ (magnification: 1200÷10) = 150÷120 = 1.25 mm ep

Another way I can do this is to use the telescope’s focal ratio (f/):

EYEPIECE FOCAL LENGTH (MM) ÷ f/ = EXIT PUPIL

Example, using the same specs (all in millimeters – mm):

10 ÷ (f/: 1200÷150) = 10 ÷ 8 = 1.25 mm ep

Exit pupil – the ideal

The idea is to avoid an exit pupil that’s too narrow, since this means too little light passing to your eye. An exit pupil less than 0.5 mm would be practically useless. Some users prefer to stay above 1.0 mm exit pupil to get decent views.

My 10-mm eyepiece will give me decent views. But if I go shorter, say a 4-mm eyepiece, I’m looking at an ep of 0.5-mm, which is close to practically useless.

This means when choosing high-power eyepieces — consider the exit pupil and whether the eyepiece focal length is too short to give you the views you desire.

At the opposite end of the scale…The exit pupil should stay within your pupil size, or light is being wasted.

The larger the exit pupil, the brighter the image you’ll see but the limit is based on the size of your pupil under dark conditions. This is about the lowest magnifying power that will be useful in using the full aperture.

This dilated pupil size can vary between users and with users’ age.² The dark-adapted average size is 7 mm for youthful eyes. As people age, their pupil size tends to shrink. For older users, 5 mm is more likely.²

To find the longest eyepiece focal length, aka lowest magnifying power:

aperture (mm) ÷ your pupil size (mm) = magnification

Example: 150-mm aperture ÷ 5-mm eye pupil = 30. So, 30x is the lower limit of power.

To find the eyepiece focal length (mm) corresponding to the magnifying power:

telescope focal length ÷ aperture x pupil size = eyepiece FL

Example using the same telescope as above and a 5-mm pupil size:

1200 ÷ 150 aperture x 5-mm pupil size = 40 mm

In this case, a low power eyepiece within 40 mm in length is relevant. Considering an eyepiece longer than this would mean light wasted AKA less of the aperture is used.

To put it another way: if I used an eyepiece that gave 20x power, the exit pupil would be 7.5-mm (150÷20). My 5-mm pupil size is ⅔ this size, which means light entering eyes equates to ⅔ of the telescope’s aperture. It’s acting as a 100 mm (4″) rather than the full 150 mm (6″) telescope.

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 are the basic differences…

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.

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.

Eyepieces with coatings to reflect multiple bands of wavelengths will show blue or purple reflections when held up to the light.

Eye relief

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. 

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

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.

• Orthoscopic
• Plössl
• Modern wide-field
• Nagler
• Ultra wide-angle

Type	Number of Elements	AFOV
Orthoscopic	4	30–50º
Plössl	4	50°
Modern wide-field	6–7	55–65º
Nagler	7–8	82º
Ultra wide-angle	multiple	up to 85º

Orthoscopic

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

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.

Modern Wide-field

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.

Nagler eyepiece

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.

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”.

This design provides a wide field of view.

Tele Vue Nagler with 100º field of view
See it at Amazon

Ultra wide-angle

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 in the undesirable category.

Two reasons¹ to steer away from these:

• a restricted field of view
• usually an inferior optical performance

See also: How To Clean Telescope Eyepieces [+Dos & Don’ts]

Information sources

1. 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.

2. 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