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Choosing Eyepieces
Choosing Eyepieces

A telescope simply will not work without an eyepiece. The optical elements of an eyepiece allow you to focus light collected by a telescope, so you can observe a sharp view of the object or area where the telescope is pointing. With an eyepiece installed in the telescope focuser, you can bring extremely distant objects into focus for magnified study.   

While most new telescopes include one or two eyepieces to get started with, purchasing additional eyepieces can significantly increase the functionality of any telescope, new or old.

Let's take a few moments to learn a bit more about the most common terms and specifications used to describe features of telescope eyepieces. With this knowledge, you'll be well-prepared to select ideal eyepieces for your own telescopes.

Focal Length and Magnification
Perhaps the most important specification of an eyepiece is its focal length. The focal length of an eyepiece, along with the focal length of the telescope the eyepiece is used with, determines the magnification the combination provides.

So the first step in choosing eyepieces is to decide what magnifications, or powers, you want to use and what eyepiece focal lengths will give them. Since both eyepiece and telescope focal lengths are expressed in millimeters, the formula used to determine magnification is:

Magnification = 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).

The above formula dictates that a telescope eyepiece with a shorter focal length yields a higher magnification than an eyepiece with a longer focal length. For example, a 10mm eyepiece will always provide a higher magnification than a 25mm eyepiece. This relationship is important to remember while choosing eyepieces: the lower the eyepiece focal length, the higher the relative magnification will be. The actual magnification will depend on the focal length of the telescope the eyepiece is used with.

If you've ever used a single telescope at different powers, you know that you have a choice of a small, sharp, bright image at lower magnification; or a big, blurred, dim image at higher power. The reason is twofold. First, the telescope gathers a fixed amount of light, and at higher magnifications, or powers, you're spreading the same amount of light over a larger area, so the image will always be dimmer. Second, because light consists of waves, even an optically 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" and can change depending on the object or area viewed.

Field of View: Apparent and True

A telescope eyepiece's apparent field of view is the angular diameter, expressed in degrees (), of the circle of light that the eye sees. It is similar to the screen of a television (not the actual picture seen on it). Most eyepieces have an apparent field of about 40 to 50. Specialized wide-field telescope eyepieces can boast apparent fields ranging from 60 to 100 or more. Such wide-field and Ultra-Wide eyepieces are preferred by amateur astronomers who enjoy the "spaceship porthole" effect of using as wide a field as possible.
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" Cassegrain telescope with a 2000mm focal length, and a 20mm eyepiece with a 50 apparent field. The magnification would be 2000mm 20mm = 100x. The true field would be 50 100, or 0.5 - about the same apparent diameter as the full Moon.

Eye Relief and Corrective Lenses

The optical design of an eyepiece determines the eye relief, which is the distance from your eye to the eyepiece lens when the image is in focus. If you wear corrective lens eyeglasses while using a telescope, we recommend looking for telescope eyepieces with at least 15mm, and more preferably 20mm, of eye relief to see the entire field of view comfortably. With insufficient eye relief the outer portion of the viewing field will be cut off, resulting in a "keyhole effect" which can be frustrating.  In more traditional telescope eyepiece designs, eye relief is proportional to focal length: the shorter the focal length, the shorter the eye relief. However, some of the more modern eyepiece designs provide luxuriously long eye relief regardless of focal length a real boon to eyeglass wearers. If you like to keep your eyeglasses on while using a telescope, the eye relief of an eyepiece is a very important specification to consider.

Barrel Size
Most quality telescope eyepieces come in two different barrel diameters, 1.25", and 2". A smaller, 0.965" barrel 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 and relatively large-aperture telescopes, and offer increased field of view and brighter images since they transmit more light than smaller sizes.

Optical Correction

The main goal of any telescope eyepiece design is to get all the light rays collected by the telescope to form a sharp image. Depending on the f-ratio of the telescope, this can be a difficult task. Telescopes with low f-ratios require more highly corrected eyepieces because the cone of light entering the eyepiece is converging more sharply. With a relatively low f-ratio telescope, such as an f/4 optical tube, only the best modern eyepieces will yield completely sharp images all the way out to the edge of the field of view. Some older designs may result in blurred views around the edge of the field of view, but the center will remain sharp. In telescopes with a relatively high f-ratio, such as an f/10 telescope, any well-made eyepiece will give a sharp image.

How Exit Pupil Relates to Power

The powers or magnifications at which a telescope will work well depend on the aperture of the instrument. In general, 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 term "exit pupil" describes the size of 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, meaning the light will essentially 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 closer to 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.

 

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. Some parfocal eyepieces may require a nominal amount of focus adjustment.

Illuminated-Reticle Eyepieces
These telescope eyepieces have etched crosshairs or other reticle grid patterns at the focal plane that can be illuminated so they're easily visible in the dark. An external illuminator arm incorporating a small red LED light, a button-cell 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 manual guiding exposures in astrophotography, and is useful for aligning a finder scope with the main telescope. It also comes in handy when drift-aligning an equatorial mount, or performing an alignment procedure for a computerized GoTo or IntelliScope system. Due to the presence of crosshairs in the field of view, illuminated reticle eyepieces aren't recommended for normal viewing through a telescope, although they can prove very useful for specific applications. 

So, How Many Eyepieces Do I Really Need?
The short answer is a few. The long answer depends on your own goals. You can observe for a long time with one low-power and one high-power telescope 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, and so on.

With a several different telescope 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 view 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. Remember: higher power doesn't necessarily equal a better view.

To Zoom or not to Zoom?

Some telescope eyepieces provide a range of different focal length settings. Such "zoom" eyepieces can be very convenient if you don't like the idea of storing and carrying a number of separate eyepieces in order to use different magnifications. Many amateur astronomers enjoy using zoom eyepieces since they make it possible to increase or decrease power without swapping out eyepieces from the telescope focuser. In general, zoom eyepieces do not perform quite as admirably as single focal length eyepieces, due to the fact they are typically made with more optical elements, which can reduce overall image brightness and clarity. However, many amateur astronomers consider the increased versatility and convenience of a zoom eyepiece to be preferable to an assortment of single focal length eyepieces. Many high-end premium zoom eyepieces are designed to optimize performance, but they can be a bit pricier than other zoom eyepieces.   

Using a Barlow Lens with Eyepieces
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 lens and get the same magnification. Using a Barlow is easy: just insert an eyepiece into the Barlow lens, then insert the Barlow/eyepiece combination into the telescope focuser and adjust until the image is sharp. By using a Barlow lens you can get away with having fewer eyepieces in your collection, while still having a variety of magnifications at your disposal. To gain the maximum benefit from the Barlow lens, 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. Since Barlow lenses add glass elements into the light path, you may notice a slight dimming of the image when alternating from an eyepiece to a Barlow and eyepiece combination. Barlow lenses are also available in 3x and 5x models, for those looking to significantly increase power. 

Closing Thoughts

Using different eyepieces can profoundly increase the versatility and functionality of any telescope. While shopping for eyepieces, remember these basic tips:

  • Consider the focal length of your telescope, or telescopes, to make sure the eyepiece will provide an appropriate magnification to suit your needs.
  • If you wear eyeglasses while using a telescope, pay attention to the eye relief specification of different eyepieces, as ample eye relief can improve comfort and ease-of-use while wearing corrective lenses.
  • Depending on your observing goals, consider the apparent field of view of your eyepiece choices.
  • If versatility is paramount, consider a zoom eyepiece or Barlow lens to increase the amount of possible magnifications to use.

Keep in mind the specifications described above and you're sure to choose an ocular that will provide you with night after night of enjoyment with your telescope(s). And remember, Orion is here to help with any questions you have along the way. Just send us an email at sales@telescope.com and we'll help you find the right telescope eyepieces.

Clear skies!

 

Details
Date Taken: 03/15/2011
Author: Orion Staff
Category: Accessory Guides

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