Eyepiece
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An eyepiece is a type of lens that is attached to a variety of optical devices such as telescopes and microscopes. It is called an eyepiece because it is usually the lens that is closest to the eye when someone looks through the device. 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. The barrel is shaped to fit in the opening on a telescope. The image can be focussed by moving the eyepiece nearer and further from the objective, and most instruments have a focusing mechanism to allow this without requiring that the eyepiece be manipulated directly.
The eyepieces of binoculars are usually fixed in place, causing binoculars to have a pre-determined magnification and field of view. With telescopes and microscopes, however, eyepieces are usually interchangeable. By switching the eyepiece, the user can adjust what is viewed. For instance, eyepieces will often be interchanged to increase or decrease the magnification of a telescope. Eyepieces also offer varying fields of view, and differing degrees of eye relief for the person who looks through them.
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.
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Eyepiece designs
Technology has developed over time and there are a variety of eyepiece designs for use with optical telescopes. They vary in their internal lens configuration, and different designs are sometimes more appropriate both for different types of viewing, and for different types of telescope. Some eyepiece designs are:
- Huygens
- Ramsden
- Orthoscopic
- Kellner
- Erfle
- Plossl
- Nagler
Eyepiece properties
There are several properties of an eyepiece that the user of a telescope is likely to be interested in when comparing eyepieces and deciding what to use.
Focal length
Eyepieces have a focal length, which is usually expressed either in millimetres, or by indirectly stating the magnification that the eyepiece produces when combined with a particular telescope or microscope.
Telescope eyepieces
The focal length is probably the most frequent of properties referred to by amateur astronomers when describing telescope eyepieces. In astronomy, the focal length is usually expressed using millimetres, and typically ranges from 3 mm to 50 mm. The actual 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. Magnification increases, therefore, when the focal length of the eyepiece is shorter or the focal length of the telescope is longer. 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 telescope would magnify 300 times.
Some astronomers refer to the resulting magnification power, rather than the focal length, when describing the eyepiece used for observations. It is often more convenient to express magnification in observation reports, as it gives a more immediate impression of what view the observer actually saw. Due to its dependence on properties of the particular telescope in use, however, magnification power alone is meaningless for describing a telescope eyepiece.
Microscope eyepieces
Microscope eyepieces are usually listed by power instead of focal length. Common eyepiece powers are 10X, 15X, and 20X. The total magnification of a microscope image is calculated by multiplying the eyepiece power by the objective power. For example, a 10X eyepiece with a 40X objective will magnify the image 400 times. The powers specified assume the accepted visual distance of closest focus of 250 mm, so the focal length can be calculated by dividing the eyepiece power into 250. So a 10X microscope eyepiece has a focal length of 250 mm/10 or 25 mm. Formerly microscope tubes were made to the length of 250mm, but in more recent microscopes this has been reduced to 160mm to make the instrument a more convenient size (modern instruments alternatively use objectives designed for an infinite tube length).
Elements
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
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 with 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.
Calculating the actual field of view
To calculate the actual field of view that results when a particular eyepiece is in a telescope of a specific focal length, the formula is (57.3) x (diameter of eyepiece field stop in mm) / (focal length of telescope objective in mm). The result is expressed in degrees. The field stop is the narrowest aperture light entering the eyepiece must pass through to reach the field lens of the eyepiece. The focal length of the telescope objective is the diameter of the objective times the focal ratio. For example, a 6" reflecting telescope has a focal ratio of f/8. The focal length of the telescope is 48 inches, or 1219.2 mm.
Barrel diameter
There are three standard barrel sizes for telescopes. The two main barrel sizes are usually expressed using inches, because the amateur telescope began in the United States.
The smallest standard barrel size is 24.5 mm (0.965 inches). The only telescopes still manufactured with this size, however, are low-quality telescopes usually 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.
Eyepieces with barrel sizes of 1¼ inches (31.75 mm) are the most commonly used. Eyepieces with 2 inch (50.8 mm) barrels are also available. The practical upper limit on focal lengths for eyepieces with 1¼ inch (31.75 mm) barrels 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 available field of view is cut to below 50 degrees, which most amateurs consider to be too small. Using larger 2 inch (50.8 mm) size helps alleviate this problem. The upper limit of focal length with 2 inch eyepieces is about 50 mm. The trade-off is that these eyepieces are usually more expensive, and won't fit in some telescopes.
Microscopes have standard barrel sizes measured in millimeters: 23.5 mm and 30 mm, slightly smaller than telescope barrels.
Eye relief
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 spectacle wearers. People who wear spectacles need more room, so they need eye relief of 15 mm to 20 mm.ja:アイピース pl:Okular