Choosing Eyepieces for Your Telescope

Opticians have been designing eyepieces for more than 300 years. Many classic designs (Huygens, Ramsden, Kellner, Orthoscopic, Plossl) are more than a century old. There are also some new, proprietary eyepiece designs that offer wider fields, sharper images, and/or longer eye relief.

Optical Correction

The main goal of any eyepiece design is to get all the light rays to form a sharp image. The difficulty of pulling it off depends on the f-ratio of the telescope. Telescopes with low f-ratios require more highly corrected eyepieces because the cone of light entering the eyepiece is converging more sharply. With an f/10 telescope, any well-made eyepiece will give a sharp image. With an f/4 telescope, only the best modern eyepieces will yield completely sharp images from out to the edge of the field of view. Eye Relief
The optical design also determines the eye relief (distance from your eye to the eyepiece lens when the image is in focus). If you wear glasses, you probably need at least 15mm and preferably 20mm of eye relief to see the entire field of view. With insufficient eye relief the outer portion of the viewing field will be cut off, resulting in a “keyhole effect”. In traditional eyepiece designs, eye relief is proportional to focal length: the shorter the focal length, the shorter the eye relief. However, some of the newer eyepiece designs provide luxuriously long eye relief regardless of focal length — a real boon to eyeglass wearers.

Apparent Field of View

Finally, the optical design determines the size of the field of view you see. An eyepiece's apparent field of view is the angular diameter, expressed in degrees (°), of the circle of light that the eye sees. It is analogous to the screen of a television (not the actual picture seen on it). Most eyepieces have an apparent field of about 40° to 50°. The true field (or real field) of view is the area of sky seen through the eyepiece when it's attached to the telescope. The true field can be approximated using the formula:

True Field = Apparent Field
_____________

Magnification

For example, suppose you have an 8" Schmidt-Cassegrain telescope with a 2000mm focal length, and a 20mm eyepiece with a 50° apparent field. The magnification would be 100x (2000mm ÷ 20mm). The true field would be 50 ÷ 100, or 0.5° — about the same apparent diameter as the full Moon.

Some older designs (e.g., Ramsden, Huygens) and microscope eyepieces cover only 30° of apparent field. Newer designs span 60° or more. If you switch from a 30° eyepiece to a 60° eyepiece at the same magnification, you’ll see twice as large a field. You can spend a lot of money on high-performance eyepieces that cover super-wide fields, but many observers feel that 50° is enough. Others enjoy the “spaceship porthole” effect of using as wide a field as possible.  

Eyepiece Types

Huygenian: The two-element Huygenian eyepiece was invented by Christiaan Huygens (pronounced “HOY-kens”) in the 1600s. This design is inferior to more recent designs, so it is now obsolete, except that some Huygenian (“H”) eyepieces are still supplied with cheap imported telescopes. Eye relief is extremely short and the apparent field is small. The 18th-century Ramsden design is much better, but still not up to today’s standards (though it is used on some microscopes that have very high f-ratios).

Kellner: The three-element Kellner, together with its close relatives the Achromatic Ramsden (“AR”) and Modified Achromatic (“MA”), is the least expensive eyepiece suitable for serious astronomy. It gives sharp, bright images at low to medium powers. Best used on small to medium-size telescopes, Kellners have apparent fields around 40° and reasonable eye relief, though short at higher powers. They’re good, low-cost performers, far superior to simpler Ramsden and Huygenian designs. A 40mm Kellner is an inexpensive way to get very low power on most telescopes.

Orthoscopic: The four-element “ortho” was once considered the best all-around eyepiece, but has lost some of its luster because of its narrow field compared to newer designs. Orthos have excellent sharpness, color correction, and contrast, and longer eye relief than Kellners. They are especially good for planetary and lunar observing.

Plossl: Today’s most popular design, the 4-element Plossl provides excellent image quality, good eye relief, and an apparent field of view around 50°. High-quality Plossls exhibit high contrast and good sharpness out to the edge. Ideal for all observing targets. Twenty years ago, these were considered “luxury” eyepieces for the well-heeled; today they are normal general-purpose eyepieces. Eyeglass wearers can generally use Orthoscopics and Plossls with focal lengths of 17mm or greater.

Erfle: The 5- or 6-element Erfle is optimized for a wide apparent field of 60° to 70°. At low powers, its big “picture window” viewing area provides impressive deep-sky views. At high powers, image sharpness suffers at the edges.

Ultrawides: Various improved designs incorporating 6 to 8 lens elements boast apparent fields up to 85° — so wide you have to move your eye around to take in the whole panorama (which some people like and others don’t). Light transmission is slightly diminished because of the additional lens elements, but otherwise the image quality in these eyepieces is very high. So, too, can be their price.

Choosing the right eyepiece design depends on what you plan to view, how finicky you are about image quality and field of view, and how much you are willing to spend.

Barrel Size
Eyepieces come in several different barrel diameters, .965", 1.25", and 2". The smallest size is found mostly on low-end “department store” telescopes and should be avoided, if possible. Most amateur telescopes are designed to accommodate the 1.25" eyepiece size. The large, 2" models are used mostly with higher-end telescopes, and offer increased field of view and brighter images.

Illuminated-Reticle Eyepieces
These eyepieces have etched crosshairs or other reticle grid patterns at the focal plane that can be illuminated so they’re visible in the dark. An external illuminator arm incorporating a small red LED light, a watch battery or two, and a potentiometer for varying the brightness is screwed into the specially made eyepiece.

An illuminated reticle eyepiece is needed for guiding exposures in astrophotography, and is useful for aligning a finderscope with the main telescope. It also comes in handy when drift-aligning an equatorial mount. Just like regular eyepieces, illuminated models come in different designs, such as Kellner, Orthoscopic, and Plossl.

Part 2: How Many Millimeters?

Choosing Magnification and Focal Length
If you’ve ever used the same telescope at different powers, you know that you have a choice of a small, sharp, bright image, or a big, blurred, dim image. The reason is twofold. First, the telescope gathers a fixed amount of light, and at higher magnifications, or powers, you’re spreading the same light over a larger area, so the image is dimmer. Second, because light consists of waves, even a perfect telescope picks up only a limited amount of fine detail in the image. Magnifying the image beyond a certain point does not reveal more; it just makes the image look blurry. This is called empty magnification.

So the first step in choosing eyepieces is to decide what magnifications you want to use and what eyepiece focal lengths will give them. (Eyepiece focal lengths are expressed in millimeters.) The formula is:

Magnification (or power) = Telescope focal length (mm)
__________________________

Eyepiece focal length (mm)

Or, put another way,

Eyepiece focal length (mm) = Telescope focal length (mm)
__________________________

Magnification


For example, a telescope with a 2000mm focal length used with a 20mm eyepiece will give 100 power (2000/20 = 100).

How Exit Pupil Relates to Power
The powers at which a telescope will work well depend on its aperture. A larger telescope gathers more light and captures a broader wavefront, giving sharper images. One handy way to classify powers is in terms of "power per inch" of aperture. For example, 80x on an 8"-aperture telescope is 10 power per inch. Another way is to go by the size of the exit pupil (the bundle of light rays coming out of the eyepiece). Exit pupil size in inches is the reciprocal of power per inch. More commonly, exit pupil size is calculated in millimeters using these formulas:


Exit pupil size (mm) = Telescope aperture in mm
__________________________

Telescope magnification


Exit pupil size (mm) = Eyepiece focal length in mm
__________________________

Telescope f-ratio


The exit pupil must be smaller than the pupil of your eye, or else some of the light rays will not make it into the pupil (light will be wasted). A young person’s fully dark-adapted eyes may have 7mm-wide pupils. As you age, maximum pupil diameter decreases. For middle-aged adults, the practical maximum is more like 5mm.

At the other end of the scale, at magnifications that yield an exit pupil in the range of 0.5mm to 1.0mm, empty magnification begins to set in, depending on the quality of your telescope and your eyes. In other words, this much magnification really starts to degrade the image you see. Here’s a table of how various powers stack up:

Power
Range
Exit Pupil Size Power Per Inch Power
(3" Telescope)
Power
(8" Telescope)
What It’s Used For
VERY LOW 4.0 - 7.0mm 3 - 6x 10 - 18x 28 - 50x Lowest usable power. Wide-field views of deep-sky objects under dark skies.
LOW 2.0 - 4.0mm 6 - 12x 18 - 36x 48 - 100x General viewing; finding objects; most deep-sky observing.
MEDIUM 1.0 - 2.0mm 12 - 25x 36 - 75x 100 - 200x Moon, planets, more compact deep-sky objects, wide double stars.
HIGH 0.7 - 1.0mm 25 - 35x 75 - 100x 200 - 280x Moon and planets (in steady air), double stars, compact clusters.
VERY HIGH 0.5 - 0.7mm 35 - 50x 100 - 150x 280 - 400x Planets and close double stars in very steady air.

Practical Focal Lengths for Eyepieces


To determine what eyepieces you need to get powers in a particular range with your telescope, you can use the formulas above, or you can take a shortcut by using the following table:

Power
Range
Eyepiece
(f/4 Telescope)
Eyepiece
(f/8 Telescope)
Eyepiece
(f/10 Telescope)
Eyepiece
(f/15 Telescope)
VERY LOW 16 - 28mm 32 - 56mm 40 - 70mm* 60 - 105mm*
LOW 8 - 16mm 16 - 32mm 20 - 40mm 30 - 60mm
MEDIUM 4 - 8 mm 8 - 16mm 10 - 20mm 15 - 30mm
HIGH 2.8 - 4mm* 6 - 8mm 7 - 10mm 10 - 15mm
VERY HIGH 2.0 - 2.8mm* 4 - 6mm 5 - 7mm 7 - 10mm

*Eyepieces in these ranges are not normally practical. See below.

Commonly available eyepieces have focal lengths between 6mm and 40mm. The eyepiece barrel diameter limits how long a focal length can be practically employed. A 32mm Plössl or a 40mm Kellner uses the entire diameter of a standard 1.25" eyepiece barrel; a longer focal length will not cover a wider field. On the other hand, in most eyepiece designs, very short focal lengths (less than 6mm) suffer from impracticably small lenses, which require you to position your eye impossibly close. Some newer eyepiece lines, such as Orion’s Lanthanum series, do provide eyepieces as short as 2.3mm with long eye relief.

If you need focal lengths longer than 40mm, some telescopes allow use of a 2" eyepiece barrel and eyepieces up to 60mm focal length or so. That’s enough to get you into the very-low-power range even with an f/15 refractor.

How Many Eyepieces Do I Need?
A few. You can observe for a long time with one low-power and one high-power eyepiece, although eventually you will want a few more focal lengths for more magnification options. Avoid the temptation to go all the way to the limits (very low and very high) until after you’ve filled in the middle range. For example, for an f/10 telescope, a 25mm and a 9mm eyepiece make a good starter set; you can add something around 15mm and perhaps 6mm next.

With a several different eyepieces, you have a better chance of hitting the optimal power for the particular object you are observing, given the sky conditions at the time. Usually, you’ll want to start out with low power (i.e., long eyepiece focal length, such as 25mm or 30mm) to get the object in the field of view of the telescope. Then you might try a slightly higher-power (shorter focal length, maybe 18mm or 15mm) eyepiece and see if the image looks any better. If it does, swap in an even higher-power eyepiece, etc., until you hit that "sweet spot" where image brightness, image scale, and the amount of visible detail combine to form the most pleasing view.

You can also use a 2x barlow lens to boost the power (or reduce the effective focal length) of any eyepiece by a factor of two. Thus, instead of a 3mm eyepiece, you can use a 6mm eyepiece with a 2x barlow and get the same magnification. By using a barlow lens you can get away with having fewer eyepieces in your collection. To gain the maximum benefit from the barlow, choose eyepiece focal lengths that are not multiples of each other. In other words, if you have eyepieces of 25mm, 12.5mm, and 6mm — multiples of 2 — then a 2x barlow won’t provide much in the way of additional magnifications. But if your eyepieces are 25mm, 15mm, and 10mm, then use of the 2x barlow with each, respectively, will provide 12.5mm, 7.5mm, and 5mm effective focal lengths — just like having three additional (and different!) eyepieces.

What Does Parfocal Mean?
Eyepieces that are parfocal can be interchanged without the need for refocusing. This is desirable (but not necessary) when switching eyepieces while looking at the same object. Often, eyepieces of the same design, from a given manufacturer, will be parfocal. But the same eyepiece design from different manufacturers will likely not be parfocal.

This information is from Orion Telescopes website.

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