How to Choose Eyepieces for Your Astronomy Telescope

By: Brian Ventrudo
November 19, 2018

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How to Choose Eyepieces for Your Astronomy Telescope
Figure 1 – Eyepieces for astronomical telescopes. From left to right, the Starguider 25mm eyepiece, Nikon 17.5mm 72-degree eyepiece, 9mm Nagler eyepiece, and the Explore Scientific 10mm 52-degree eyepiece. Image credit: Agena AstroProducts

1. Introduction

Most telescope makers supply one or two eyepieces with their new telescopes to help you get started observing right away. While that's a nice gesture, these eyepieces are usually of modest quality and capability, and they are rarely suited to the full range of observing you are likely to encounter in the night sky. After getting a new telescope, most amateur astronomers soon want to acquire more eyepieces, or better eyepieces, to enhance the visual observing experience. But eyepieces, especially premium eyepieces, are expensive, and many new astronomers wonder how to choose the best eyepiece for their budget and their telescope without wasting time and money.

One approach is to buy a ready-made set of eyepieces, typically 5 or 6 of the same design with a wide range of focal lengths, that are sold by telescope makers and eyepiece vendors. But some of these focal lengths are not necessarily useful for all telescopes, the quality of the glass and mechanics may not be what you're hoping for, and view through these eyepieces will almost certainly not measure up to what you see through higher-quality eyepieces.

Most experienced amateur astronomers suggest a different approach. They suggest you buy two or three eyepieces of good quality, with the possible addition of a Barlow lens, to achieve three or more levels of magnification. We believe this is good advice. Specifically, as your budget allows, we suggest you choose these eyepieces:

  • A low-power wide-field eyepiece for taking in large celestial objects or framing multiple objects in the same field. Also good for finding objects before switching to higher magnifications
  • A medium-power eyepiece for teasing out more detail in extended objects like the Orion Nebula, further magnifying smaller objects like galaxies and small star clusters. Higher magnification also darkens the background sky to help improve image contrast
  • A high-power eyepiece for seeing detail on planets and the Moon, for resolving double stars, and for observing small objects like planetary nebulae

This makes for a more manageable set of optics, and you can purchase each eyepiece separately as your budget allows. If you follow this advice, you will find that a few really good eyepieces give you far more enjoyment than a box full of middling eyepieces.

But exactly which two or three eyepieces should you choose?

The answer is different for each person, and it depends on budget, the focal length and focal ratio of your telescope, and your observing skill and interests. It's difficult to suggest a single solution for everyone. But this article will show you the 'thinking process' by which you can make the best eyepiece choices for your situation. Our approach is based on matching your eyepiece selection to the capabilities of the human eye, while also accounting for other key eyepiece specifications, personal preferences, and budget. As you read through this article, you will come to understand not only which types of eyepieces might work best for you, but also why they work best.

How to Choose Eyepieces for Your Astronomy Telescope
Figure 2 – The position of an eyepiece in the focal plane of a telescope's objective lens. Image courtesy of Al Nagler at Tele Vue Optics.

2. Essential Eyepiece Specifications

First, let's take a quick look at the key eyepiece specifications all amateur astronomers should understand before diving in to the following discussions.

2.1 Focal Length

The purpose of an eyepiece is to magnify the image projected by the objective lens or mirror of a telescope, and the magnifying power is directly related to the focal length. The magnification of an eyepiece is given by this well-known relation:

Magnification = Focal Length of Telescope / Focal Length of Eyepiece

This relation means an eyepiece with a smaller focal length gives a higher magnification for a particular telescope.For example, a telescope with a focal length of 1200mm and an eyepiece of 12mm has a magnification of 100x. With an eyepiece of 6mm focal length, it has a magnification of 200x.

Eyepieces come in a huge range of focal lengths, from less than 2mm to as long as 56mm (and longer). However, as you will see shortly, not all telescopes work well at the extreme ranges of eyepiece focal lengths. It is possible to have too much or too little magnification.

Table 1 summarizes the ranges of eyepiece focal lengths available and how each range of focal lengths applies to visual observation of the night sky.

Focal Length Range (mm)Best Uses
3-6 High magnification viewing of the Moon, planets, and double stars. Can provide TOO MUCH magnification with SCT and longer focal-length telescopes.
7-13 Medium-to-high magnification for viewing galaxies, globular clusters, and planetary nebulae and wider double stars. Also good for moderate magnification of lunar features. Good for planets on nights of moderate-to-poor seeing.
13-18 Low-to-medium magnification for viewing extended objects like galaxies and larger open star clusters; works well for globular clusters with longer focal length telescopes.
19-24 Lower magnification for faster reflectors and refractors. Excellent for extended nebulae and larger galaxies.
24+ Lowest magnification for sweeping large angular views of the Milky Way in dark sky. Also good for centering objects in the field of view before switching to higher magnification. Longer focal lengths in this range are often in eyepieces with 2" barrels.
Table 1 – A summary of eyepiece focal lengths and their most common applications.

2.2 Apparent and True Field of View

The apparent field of view (AFOV) of an eyepiece is the apparent angular width of sky presented to your eye. It usually ranges from about 40° to 100°. Eyepieces with a larger AFOV show you much more sky for a particular magnification. That's usually a good thing, especially if you want to take in extended celestial sights such as nebulae and large galaxies in a single view. A large AFOV is also a benefit if you have a Dobsonian telescope without a motor drive because an object stays in view longer before you have to nudge your telescope. But eyepieces with a larger AFOV are usually larger and more expensive. The AFOV is a design parameter of each eyepiece, and it's governed by the size of the field stop of the eyepiece (see Section 2.3).

The true field of view (TFOV) is the amount of sky you can really see with an eyepiece. It's calculated as:

True Field of View (TFOV) = Apparent Field of View (AFOV) / Magnification

For example, a 24mm eyepiece used with a telescope of 1000mm focal length yields a magnification of 42x. Figure 3 shows the approximate true field of view of three eyepieces when used with this telescope when observing the Pleiades star cluster. One eyepiece has an AFOV of 50°, another of 68°, and another of 82°. These eyepieces give an approximate TFOV of 1.2°, 1.6°, and 2.0°, respectively. The eyepiece with the widest field of view shows an area of sky 2.7x greater than the eyepiece with the narrowest field of view.

How to Choose Eyepieces for Your Astronomy Telescope
Figure 3 - The true field of view of the Pleiades seen with a telescope of 1000mm focal length and three eyepieces, each with 24mm focal length, but with an apparent field of view of 50°, 68°, and 82°. Each eyepiece gives the same magnification, but the eyepieces with the larger AFOV show a larger angular area of sky.

2.3 Barrel Size and Field Stop

Then, there is barrel size. This is the diameter of the metal barrel or tube that slides into the focuser of the telescope. Most astronomy eyepieces have a barrel that is 1.25" in diameter, and most focusers accept eyepieces of this size. Larger eyepieces with longer focal lengths come with 2" barrels which can only be used in focusers that accept this barrel size. Some older Japanese-made telescopes and many inexpensive and lower-quality “department store" telescopes use eyepieces with a barrel size of 0.965". These eyepieces are less common and generally of lower quality. They only work in focusers that accept this barrel size, or with standard 1.25" focusers with an adapter.

While far less common, there are also eyepieces that have a 3" barrel. These eyepieces are used in larger telescopes to get very wide fields of view.

Each eyepiece also has a field stop, which is usually a metal ring inside the eyepiece that limits the field of view and produces a sharp well-defined edge that's visible around the field. A larger field stop allows a wider cone of light to enter the eyepiece and produces a larger apparent field of view. The field stop is specified in millimeters, although most eyepiece manufacturers, with the probable exception of Baader and Tele Vue Optics, do not list the field stop in their specifications. If you do know the field stop of an eyepiece, you can calculate the true field of view as:

True Field of View (TFOV) = Field Stop Diameter *57.3 / Telescope Focal Length

The field stop of an eyepiece cannot exceed the inside diameter of the eyepiece barrel. For 1.25" eyepieces, the field stop cannot be larger than 27mm, for example. This limits the field of view of each 1.25" eyepiece design, and it limits the maximum focal length you can achieve in a 1.25" eyepiece in each type of optical design. For example:

  • For a 1.25" eyepiece with a 50° apparent field of view, the maximum focal length is 32mm
  • For a 1.25" eyepiece with a 68° apparent field of view, the maximum focal length is 24mm
  • For a 1.25" eyepiece with an 82° apparent field of view, the maximum focal length is about 18mm

In all three of these examples, the field stop is at the maximum of 27mm for the 1.25" barrel. To get a longer focal length (and therefore a lower magnification) with each of these designs, it's necessary to move to an eyepiece with a 2" barrel that accommodates a field stop of up to 46mm. That's why most 2" eyepieces have a long focal length and large apparent field of view.

In principle, all eyepieces, even shorter focal length eyepieces, can have a 2" barrel. It's possible to have, for example, an 8mm Plossl eyepiece with a 2" barrel. But such an eyepiece would have a field stop of only 6.5mm, so there is no point in placing such a small field stop in a larger, heavier, and more expensive 2" barrel diameter.

How to Choose Eyepieces for Your Astronomy Telescope
Figure 4 - Two 1.25" eyepieces, a Tele Vue 9mm Nagler eyepiece (left) and a Tele Vue 24mm Panoptic eyepiece (right). The field stop of the 24mm eyepiece is effectively the aperture of the eyepiece barrel, about 27mm, while the field stop of the 9mm Nagler eyepiece is much smaller (in this design, it lies within the eyepiece itself). Credit: AgenaAstroProducts.

2.4 Exit Pupil

Next up is the exit pupil of an eyepiece, which is a critical specification that we will use for explaining how to choose eyepieces.The exit pupil is the diameter of the beam of light that projects from the eyepiece into the entrance pupil of your eye. You find the exit pupil with this relation:

Exit Pupil = Focal Length of Eyepiece / Focal Ratio of Telescope

This means that the exit pupil of an eyepiece is not strictly a specification of an eyepiece; it also depends on the telescope. For example, an eyepiece with a focal length of 32mm, when used with a telescope of focal ratio f/8, produces an exit pupil of 4mm. The same eyepiece used with a telescope of focal ratio f/5 produces an exit pupil of 6.4mm.

Because of the physiology of the human eye, the exit pupil of an eyepiece (which depends on your choice of telescope) should be somewhere between a minimum of 0.5mm to 0.7mm and a maximum of 5 mm to 7mm (depending on your age and your eye). You'll learn more about this important concept in Section 3 where we use it to help understand how to choose the right eyepieces for your telescope.

2.5 Eye Relief

Eye relief is the maximum distance you can place your eye from the top lens of the eyepiece while seeing the full field of view. This specification is related to viewing comfort and head position. It's generally a less pleasant experience if you have to jam your eye right up to the eyepiece, and that's often the case with eyepieces with a short eye relief of around 5mm to 8mm. If you wear glasses while viewing, which is important if you have significant astigmatism in your observing eye, then a long eye relief of 18mm to 20mm is essential. Some eyepieces, especially longer focal length eyepieces, have a very long eye relief that makes it tricky to position your eye to get the entire field of view.

3. Key Considerations for Choosing an Eyepiece

Now that you have some knowledge of eyepiece specifications, it's time to keep an important truth in mind: not every eyepiece will work well with every telescope in all situations for all observers. There are, however, a few subjective considerations to keep in mind that aren't easy to quantify. Let's have a look at the most important factors in matching an eyepiece to a telescope.

3.1 Maximum, Minimum, and "Optimum" Magnifications

Perhaps the most critical consideration is the maximum magnification that is possible, or at least advisable, with your telescope. The maximum advisable magnification is governed by the quality of the optics and alignment of your telescope and by the seeing conditions and steadiness of the atmosphere.

The most common and time-tested rule of thumb suggests not exceeding a magnification of about 50x per inch of aperture of your scope. For a 4" telescope, for example, you should stick to less than 200x with your shortest focal length eyepiece. Any higher and the view will become unacceptably dim and less sharp. On nights of average seeing conditions, especially with larger telescopes, you might only get 30x to 40x per inch of telescope aperture. In fact, when spending money on an eyepiece that gives you high magnification, it makes sense to plan for many nights of average seeing rather than just a few nights of superb seeing each year.

Maximum magnification is also limited by your eye and the exit pupil of an eyepiece and your telescope. As mentioned above, most visual observers should make sure that an eyepiece yields an exit pupil greater than 0.5mm to 0.7mm. Any smaller and your visual acuity will suffer because so little or your retina is illuminated. A small exit pupil is also a problem if you suffer from distracting "floaters" in your observing eye. As an example, if you have a telescope with a focal ratio of f/10, your shortest focal length eyepiece (which produces greatest magnification) should have a focal length of 5mm to 7mm, but no shorter.

Exit pupil is also a consideration in setting the minimum magnification you can (or should) try to get with an eyepiece and your telescope. The pupil of human eye under conditions of maximum dark adaptation, only opens to a diameter of 7mm. For observers older than 40 or 50 years of age, that drops to about 5mm to 6mm. That places a limit on the maximum exit pupil of an eyepiece; if it's any greater, the light does not enter the eye and is essentially wasted. For example, with a Dobsonian reflector with a focal ratio of f/5, your eyepiece with the maximum focal length (which produces lowest magnification) should have a focal length of 25mm to 30mm, roughly, and certainly no larger than 35mm.

A large exit pupil is also a concern with reflecting telescopes because of the central obstruction of the secondary mirror. The obstruction can take up 25%-30% (or more) of the diameter of the exit pupil, so the obstruction becomes more noticeable in the field of view when using eyepieces with larger exit pupils, or if the entrance pupil of the observing eye is reduced by a flash of stray light during an observing session. Refracting telescopes, of course, do not have a central obstruction.

Visually, in telescopes of a given aperture, eyepieces that yield the same exit pupil result in the same image brightness. For example, a 10mm eyepiece with an f/4 telescope results in an exit pupil of 2.5mm. A telescope of the same aperture with a focal ratio of f/8 will produce a 2.5mm exit pupil with a 20mm eyepiece. In both cases, the image brightness is identical.

Is there an optimum magnification for a particular telescope? Not exactly. But there is a guideline used by experienced observers, especially for deep sky objects, that suggests using an eyepiece that results in an exit pupil near 2mm, give or take a few tenths of a millimeter, provides the best visual observing experience with many deep sky objects. That's because an exit pupil of about 2mm confines light from your telescope to the most sensitive part of your retina. The best exit pupil varies slightly from person to person. And it even varies among the many types of celestial objects you can observe. But a 2mm exit pupil, or even a little less, is a good starting point in selecting eyepieces of moderate magnification, and we'll use this guideline in Section 4.

It is also important to understand that the image of extended objects like galaxies and nebulae will get considerably dimmer as you increase magnification: double the magnification and your image will become four times fainter! Since the telescope collects the same amount of light regardless of the eyepiece used, the same amount of light is spread over a larger image in short focal length eyepieces. This is a basic law of physics that beginners don't always grasp, leading to disappointment with very high-power eyepieces. Stars, which are point sources of light, do not get dimmer with increased magnification. In fact, larger magnification helps improve the contrast of stars against the background sky. In practice, images of stars at high magnification can be degraded by the effects of unsteady atmosphere.

 

3.2 Telescope Focal Ratio

Now let's look at a specification of a telescope that is important to consider when choosing an eyepiece: the focal ratio. The focal ratio of a telescope is:

Focal ratio (f/#) = Focal Length of Objective / Diameter of Objective

For example, a telescope with a mirror of 8" diameter (200mm) that has a focal length of 2000mm has a focal ratio of 10, written asf/10. A telescope with a smaller focal ratio is often called a "faster" telescope because it produces a brighter image of extended objects, which means you can take a photograph in less time than in a "slower" telescope.

Telescopes with a focal ratio of less than f/5, more or less, make extra demands on an eyepiece. That's because these telescopes have a pronounced field curvature, which means when you adjust the focuser of the telescope to bring stars to focus at the center of the field of view, the stars at the edge of the field of view are slightly out of focus, and vice versa. The focal plane of eyepieces themselves also have a slight field curvature, along with other optical imperfections. It turns out that more expensive, premium eyepieces are more rigorously designed to minimize the effect of the field curvature of both eyepiece and telescope, especially when used with telescopes with a focal ratio less than f/5, and especially with eyepieces that have a large apparent field of view of greater than 68°, give or take. In less expensive eyepieces, objects at the center of the field generally look fine, but there is some degradation of the image at the edge of the field. If you want very sharp images across the entire field of view, then premium eyepieces from manufacturers like Tele Vue, Pentax, Baader, Nikon, and Explore Scientific tend to work better than less expensive eyepieces with 'fast' telescopes.

No eyepieces can correct for aberrations inherent to the telescope itself such as coma, which is pronounced in reflecting telescopes faster than about f/5. Coma results in comet-shaped stars at the edge of the field of view of an eyepieces, especially with longer focal length eyepieces that have a large AFOV. Coma can be reduced by using a separate optical device called a coma corrector that inserts into the focuser between the telescope and the eyepiece. These devices can be somewhat expensive, so some observers minimize the effects of coma by simply looking towards the center of the field of view and ignoring the stars at the edge, or by buying eyepieces with a smaller field of view.

Some premium fast refractors incorporate a Petzval design which uses additional lens elements between the objective and the eyepiece to eliminate field curvature. 

'Slower' telescopes, those with a focal ratio greater than f/8, tend to be more forgiving of less expensive eyepieces when it comes to getting good focus across the entire field of view.

3.3 Observing Interests and Personal Preference

When it comes to choosing an eyepiece, observing preferences are important to consider. For example, regardless of the focal length of an eyepiece, some amateur astronomers very much like the expansive view with somewhat costly wide-field eyepieces that have an AFOV of 82° or even 100°. Some very much do NOT enjoy such a view and prefer to stay with eyepieces with an AFOV of 70° or less; these eyepieces are less expensive and are more forgiving of optical aberrations in a telescope.

Eye relief is also a matter of personal preference. Some, especially eyeglass wearers, prefer eyepieces with a long eye relief. Some don't like long eye relief because it makes it hard to hold your head in the right position to see the full exit pupil of the eyepiece. There are some longer eye-relief eyepieces that include an adjustable eyeguard that makes it easier to achieve a comfortable viewing position.

In terms of cost, or having the 'best that money can buy', some visual observers want to have the best possible specifications and performance from an eyepiece. Some are willing to enjoy 80%-90% of the best possible performance at half the price (or less) of the finest eyepieces on the market.

Observing interests are also worth considering. If you are primarily interested in observing small objects such as planets, double stars, planetary nebulae and so forth, you can use eyepieces with a smaller AFOV and fewer lens elements, which may improve image contrast and color fidelity. If you prefer to sweep wide fields of the Milky Way with a small refractor looking for nebulosity and rich star fields, an eyepiece with a wide AFOV and long focal length, and possibly even a 2" barrel, might be the way to go.

If you are not sure what kind of eyepiece you prefer, or what kind of observing interests you, then consider attending a star party where you can look though several types of eyepieces and telescopes and discuss the pros and cons of various designs with other stargazers.

3.4 Price

Here's some good news: thanks to modern design and manufacturing methods and off-shore suppliers, you can get a good eyepiece today for $100 or less, especially if you have a telescope that is f/8 or slower. Or you can get a premium eyepiece for $250-$500 depending on the focal length and a few other factors. The difference between these price points comes down to the quality of the glass and anti reflection coatings, the mechanics and the fit and finish, and the optical performance, especially in faster telescopes (see Section 3.2). The choice of how much to spend is yours. But also keep in mind that many brands of premium eyepieces such as Tele Vue, Nikon,and Pentax hold their value well over time.

4. Choosing Eyepieces for Your Telescope

Now let's examine three examples to help you through the thought process of selecting quality eyepieces for your telescope and personal situation. In this section, we'll discuss the concepts explained in the previous sections by using 'case studies' for three common and popular types of telescopes: a 4" (102mm) f/7 ED apochromatic refractor, an 8" f/10 Schmidt-Cassegrain compound reflector, and a 10" f/4.7 Dobsonian reflector.

The discussion here will aim to choose three eyepieces for each telescope:

  • A longer focal length eyepiece that gives low magnification, and which has an exit pupil of about 3-5mm
  • A medium focal length eyepiece that gives an exit pupil of about 1.7mm to 1.8mm
  • A short focal length eyepiece that gives a high magnification (about 30x to 40x per inch of aperture for each telescope) and an exit pupil of about 0.7mm

This range of focal lengths will also be chosen to allow you to extend your range of magnification by adding a 2x Barlow lens to your kit without significant overlap in the magnification. With judiciously chosen eyepieces and a good Barlow lens, you will get a set of optics with four to six magnifications that will serve you well for many years.

As for the choice of AFOV, this is a matter of budget and personal preference. For most observers, an apparent field of 60° to 70° is a good choice for an eyepiece. If you have more to spend, you can consider an AFOV of 82° eyepiece, but these eyepieces tend to be expensive and bulky at the longer focal lengths, and so they tend to offer the best value when used for medium and high magnification. Another option is to choose an eyepiece with a larger AFOV (>70°) for your lowest-power eyepiece, then choose less expensive eyepieces with a smaller AFOV for the medium and higher magnifications.

Also, for simplicity in these discussions, we'll restrict our choices to eyepieces with a 1.25" barrel, even at the longer focal lengths. These eyepieces are lighter in weight and less expensive, and they work with commonly available 1.25" Barlow lenses.If you want to acquire a 2" eyepiece for getting very wide fields of view at lower magnifications, the discussions below are equally valid. But you will need to make sure your focuser can handle the size and weight of a 2" eyepiece. If you wish to use a 2" eyepiece in a Barlow lens, then you need to get a Barlow that fits 2" eyepieces. Not all do.

4.1 Case Study 1 – Choosing Eyepieces for a 4" ED/Apochromatic f/7 Refractor

A good ED or apo refractor is a lightweight and versatile instrument for sweeping the night sky along the Milky Way, for observing the Moon and brighter deep-sky objects, and getting a respectable look at Jupiter, Saturn, and Mars when they are near opposition.In this eyepiece 'case study', we choose an ED refractor with an objective lens of 4" (102mm) aperture, a focal ratio of f/7, and therefore a focal length of 714mm.

How to Choose Eyepieces for Your Astronomy Telescope
Figure 5 – A 100mm ED (extra-low dispersion) refractor. Image courtesy of Sky-Watcher USA.

For the highest-power eyepiece, we want something that gives an exit pupil of about 0.7mm. For this focal ratio, that works out to an eyepiece focal length of about 5mm which will give a magnification of 143x. That's about 36x for each inch of objective diameter, which works on nights of average seeing or better. There are dozens of eyepieces with a 5mm focal length, with prices of less than $40 to more than $300. Get one with an AFOV of at least 60°, and more if your budget allows.

For the lowest-power eyepiece, you want something that gives an exit pupil of about 5mm. For this focal ratio, that works about to an eyepiece focal length of about 35mm. In this focal length range, there are several 32mm Plossl eyepieces (with a 45°-50° AFOV) that will fit the bill such as those from Baader, GSO, and Tele Vue. These eyepieces will yield a magnification of 22x and a TFOV of 2.2°.

For those with a higher budget for their lowest-power eyepiece, an Explore Scientific 68-degree eyepiece or a Tele Vue Panoptic with a 24mm focal length gives a magnification of 30x, but still yields a nice big TFOV of 2.2°. The exit pupil for these eyepieces is 3.4mm, much smaller than a fully dilated entrance pupil, but it's still in the sweet spot of retinal sensitivity. These more expensive eyepieces will also do a better job correcting for the field curvature of the telescope (Section 3.2).

How about a medium power eyepiece? If we want a 2mm exit pupil, that means we need an eyepiece with 14mm focal length that gives 50x. If we go with a slightly smaller exit pupil of 1.7mm, that means a focal length of 12mm and a magnification of 60x. Such an eyepiece with a 60° AFOV will show a full degree of sky.

With a 5mm, 12mm, and 32mm eyepiece, you get magnifications of 22x, 60x, and 143x, which is a good range of magnifications for this instrument.If you add in a good 2x Barlow with these eyepieces, you get 22x, 44x, 60x, 120x, 143x, and a big 286x (which is only useable with superb optics under a rock-steady sky, at best).

Focal Length (mm)AFOV (°)Mag. (x)Exit Pupil (mm)TFOV (°)Comments
5 60 143 0.7 0.4  
12 60 60 1.7 1.0  
32 50 22 4.6 2.2 Could also choose a 24mm eyepiece with a 68-degree AFOV Table 2– A summary of possible eyepiece choices (focal length and AFOV) for a 102mm f/7 ED refractor. A 2x Barlow lens will double the number of available magnifications.
Table 2-- A summary of possible eyepiece choices (focal length and AFOV) for a 102mm f/7 ED refractor. A 2x Barlow lens will double the number of available magnifications.

These eyepiece selections are just an example of what could work with this telescope, and you may wish to adjust your choices to match your own needs. There are many variations that will work. For example, instead of choosing three eyepieces, you might choose a 24mm eyepiece with a 68° which gives 30x and an8mm eyepiece that gives 90x. Add a 2X Barlow and you get magnifications of 30x, 60x, 90x, and 180x, all with the exit pupils that match most eyes very well.

4.2 Case Study 2 – Choosing Eyepieces for an 8" f/10 Schmidt-Cassegrain Telescope

A good Schmidt-Cassegrain has been a versatile workhorse scope for amateur astronomers for nearly 50 years. In this case study, let's work with a telescope with an aperture of about 8" (200mm), a focal ratio of f/10, and therefore a focal length of 2000mm.

How to Choose Eyepieces for Your Astronomy Telescope
Figure 6 – An 8" f/10 Schmidt-Cassegrain telescope. Image credit: Celestron

For the highest-power eyepiece, we want something that gives an exit pupil of about 0.7mm. For this focal ratio, that means an eyepiece with a focal length of about 7mm. It will give a magnification of 285x. That's about 36x for each inch of objective diameter, which works well on nights of average seeing or better. Again, there are many of eyepieces with a 7mm focal length, with a wide range of prices. Get one with an AFOV of at least 60°, and more if your budget allows. If you live in a region where good atmospheric seeing is uncommon, a magnification of 285x might be too high. In such a case, you might consider an eyepiece that produces a lower magnification, such as one with a focal length of 9mm or 10mm to produce a magnification of 222x or 200x, respectively. 

For the lowest-power eyepiece, you want something that gives an exit pupil of about 5mm. For this focal ratio, that works about to an eyepiece focal length of about 50mm. Such eyepieces tend to be quite bulky and they do not come with a 1.25" barrel size, so for this telescope let's go with a 32mm Plossl with a 50° AFOV which gives 63x and a TFOV of 0.8°. Or choose a 24mm 68° that gives 83x and also a TFOV of 0.8°. The exit pupils of these eyepieces are 3.2mm and 2.4mm, still a good range for visual observing. For this case study, let's pick the 24mm eyepiece.

How about a medium power eyepiece? If we want a 2mm exit pupil, that means we need an eyepiece with 20mm focal length that gives 100x. Let's get a slightly smaller exit pupil of 1.8mm, which means an 18mm eyepiece that gives 111x. Such an eyepiece with a 60° AFOV will show 0.54°, about the size of the full Moon.

With a 7mm, 18mm, and 24mm eyepiece, you get magnifications of 83x, 111x, and 285x, a good range of magnifications for this instrument. If you add in a good 2x Barlow with these eyepieces you get 83x, 111x, 166x, 222x, and 285x. The Barlowed 7mm eyepiece gives a very high 570x, likely only useable with superb optics and collimation and very steady sky.

Focal Length (mm)AFOV (°)Mag. (x)Exit Pupil (mm)TFOV (°)Comments
7 60 285 0.7 0.2  
18 60 111 1.8 0.54  
24 68 83 2.4 0.8 Could also choose a 32mm eyepiece with a 50-degree AFOV.
Table 3 - A summary of possible eyepiece choices (focal length and AFOV) for a 200mm f/10Schmidt-Cassegrain telescope. A 2x Barlow lens will double the number of available magnifications.

Again, these eyepiece selections are just an example of what could work with this telescope, and you may wish to adjust your choices to match your own needs.

4.3 Case Study 3 – Choosing Eyepieces for a 10" f/4.7 Dobsonian Telescope

Now let's outfit a Dobsonian with some good eyepieces. We'll pick a 10" f/4.7 Dob, a popular telescope with experienced stargazers because it combines significant light collecting ability with a reasonable degree of portability. Focal length: 1200mm.

How to Choose Eyepieces for Your Astronomy Telescope
Figure 7 – A 10" Dobsonian reflector. Image courtesy of Sky-Watcher USA.

For the highest-power eyepiece, to get an exit pupil of about 0.7mm, we need an eyepiece with a focal length of 3mm. That gives a magnification of 400x, about 40x per inch of aperture. That's a bit high for most circumstances, so let's go with a 4mm eyepiece for 300x and an exit pupil of 0.9mm. Get one with as large an AFOV as you can afford. Even a magnification of 300x is a stretch on many nights, so a 5mm eyepiece that gives 240x would work as well.

For the lowest-power eyepiece, to get an exit pupil of about 5mm, we need an eyepiece with a focal length of about 24mm. That gives a magnification of 50x. If possible, get such an eyepiece with a 68° AFOV for a true field of view of 1.4°. As mentioned in Section 3.2, premium eyepieces tend to offer a greater degree of correction at the edge of the field of view for such a fast telescope. So if you can afford it, go with an eyepiece from Tele Vue Optics, Explore Scientific, Baader, Pentax, and others.

For a medium power eyepiece, if we want a 2mm exit pupil, that means we need an eyepiece with a about a 10mm focal length that gives 120x. Such an eyepiece with a 60° AFOV will show 0.5°, the size of the full Moon.

With a 4mm, 10mm, and 24mm eyepiece, you get magnifications of 50x, 120x, and 300x, a good range of magnifications for this instrument. If you add in a good 2x Barlow with these eyepieces you get 50x, 100x, 120x, 240x, and 300x. The Barlowed 4mm eyepiece gives a very high 600x, likely only useable with superb optics and collimation and very steady sky.

Focal Length (mm)AFOV (°)Mag. (x)Exit Pupil (mm)TFOV (°)Comments
4 60 300 0.9 0.2  
10 60 120 2.1 0.5  
24 68 50 5.1 1.4 Could also choose a 32mm eyepiece with a 50-degree AFOV.
Table 4 -A summary of possible eyepiece choices (focal length and AFOV) for a 250mm f/4.7Dobsonian reflector. A 2x Barlow lens will double the number of available magnifications.

Again, these eyepiece selections are just an example of what could work with this telescope, and you may wish to adjust your choices to match your own needs. With this telescope, for example, you can also consider a just two eyepieces, a 10mm and 24 mm eyepiece, along with a 2x Barlow. That gives you 50x, 100x, 120x, and 240x.

While we're sticking with 1.25" eyepieces in this case study, you might consider a good 2" eyepiece with a Dobsonian of focal ratio f/4 to f/5 to get the widest possible true field of view. For example, a 35mm eyepiece with a 68-degree AFOV would give a magnfication of 34x with a 10" f/4.7 Dobsonian and a big AFOV of 2.0 degrees. A 31mm eyepiece with an 82-degree AFOV would give a similar AFOV at a magnfication of 39x. Of course, a 2" eyepiece requires a 2" focuser and a mount sturdy enough to hold the added weight of the eyepiece.

5. Summary

This article has walked through the thinking processes and important specifications to consider when choosing eyepieces for an astronomy telescope. We used an approach based on matching your eyepiece selection to the capabilities of the human eye, while also accounting for other key eyepiece specifications, personal preferences, and budget. Using the principles in this article, you can understand not only which types of eyepieces work best for your interest and your telescope, but why they work best.

NOTE: The author would like to thank Al Nagler of Tele Vue Optics for many insights and suggestions that have been incorporated into this guide.

6. References

For further reading:

Brian Ventrudo
About the Author

Brian Ventrudo is a writer, scientist, and astronomy educator. He received his first telescope at the age of 5 and completed his first university course in astronomy at the age of 12, eventually receiving a master's degree in the subject. He also holds a Ph.D. in engineering physics from McMaster University. During a twenty-year scientific career, he developed laser systems to detect molecules found in interstellar space and planetary atmospheres, and leveraged his expertise to create laser technology for optical communications networks. Since 2008, Brian has taught astronomy to tens of thousands of stargazers through his websites OneMinuteAstronomer.com and CosmicPursuits.com.