Eyepiece lens

In a telescope, the objective lens or mirror collects light and brings it to focus creating an image. The eyepiece is placed at the focal point of the objective to magnify this image. The amount of magnification depends on the focal length of the eyepiece. An eyepiece consists of several lens elements in a housing with a barrel on the bottom designed to fit a the opening on a telescope.

Modern research-grade telescopes do not use eyepieces. Instead, they have high-quality digital cameras mounted at the focal point, and the images are viewed on a computer screen. Some amateur astronomers use their telescopes the same way, but most still look through eyepieces.

Eyepiece Specifics

The focal length of telescope eyepieces is measured in millimeters, and typically ranges from 3 mm to 50 mm. The magnification delivered at these focal lengths depends on the telescope. Magnification is calculated by dividing the focal length of the telescope objective (the main lens or mirror) by the focal length of the eyepiece. So, the shorter the focal length, the higher the magnification. For example, a 25 mm eyepiece in a telescope with a 1200 mm focal length would magnify objects 48 times. A 4 mm eyepiece in the same scope would magnify 300 times.

Elements are the individual lenses. The first eyepieces had only one element which delivered highly distorted images. Two and three-element designs were invented soon after, and quickly became standard due to the improved image quality. Today, engineers assisted by computer-aided drafting software have designed eyepieces with seven or eight elements that deliver exceptionally large, sharp views.

Field of view can refer to two different measurements. The actual field of view is the angular size of the amount of sky that can be seen with a given eyepiece and telescope combination. It is typically one tenth of a degree to two degrees. (For reference, the full moon spans one half degree.) The apparent field of view is how large the image is compared ones own field of view. This measurement ranges from 35 to over 80 degrees. Field of view is usually abbreviated FOV. The context should make clear which of the two meanings is intended.

There are three standard barrel sizes. The smallest is 24.5 mm (0.965 inches). However, the only telescopes that come with this size anymore are cheap ones found in toy stores and malls. Many of these eyepieces are plastic, and some even have plastic lenses. High-quality eyepieces with this barrel size are hard to find. 1¼ inches (31.75 mm) is the most common, and two-inch (50.8 mm) barrels are also available. (Since the amateur telescope market got its start in the United States, the two main barrel sizes are measured in inches.) The practical upper limit on focal lengths for eyepieces with a 1¼ inch (31.75 mm) barrel is about 32 mm. With longer focal lengths, the edges of the barrel itself intrude into the view limiting its size. With focal lengths longer than 32 mm, the FOV is cut to below 50 degrees, which most amateurs consider to be too small. The larger two-inch (50.8 mm) size helps alleviate the problem listed above. The upper limit on these eyepieces is about 50 mm. The trade-off is that these eyepieces are usually more expensive, and they won't fit in some telescopes.

To see images properly through an eyepiece, the eye needs to be held at the correct distance. This distance is called the eye relief. Eye relief ranges from about 2 mm to 20 mm. Long focal-length eyepieces usually have ample eye relief, but short focal-length eyepieces are more problematic. All short-focal length eyepieces used to have short eye relief, and that is still common. However, modern designs with many lens elements can corrected this making viewing at high power more comfortable—especially for eyeglass wearers. People who wear eyeglasses need more room, so they need eye relief of 15 mm to 20 mm.