Even when you know all the terminology, sifting through the countless telescope eyepieces can be a daunting task. If you are in the market for an eyepiece chances are you have a solid background in astronomy vocabulary but you may not have thought of the words as they apply to telescope eyepieces. We can offer a brief refresher course.
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Actual Field of View – This is a measurement of the amount of sky that can be seen with a specific pairing of eyepiece and telescope. It usually ranges between 1/10 of a degree and 2 degrees.
Apparent Field of View – This is a measurement of the amount of sky that can be seen through an eyepiece alone. It measures how large an image can appear to the viewer. Eyepieces now have apparent fields of view of up to 100 degrees, which can offer the feeling of being immersed in space.
Barlow Lens – The English engineer Peter Barlow invented this optical element that can be placed immediately adjacent to an eyepiece to increase magnification. Many modern eyepieces incorporate an interior lens for magnification but adding a Barlow to the eyepiece can make a nice view enhancement in many situations.
Doublet – When two lenses are attached together they form a doublet, a new type of lens. The lenses are usually made of different types of glass, engineered for specific purposes that work together.
Eye Relief – The distance away from the eyepiece your eye can be and still see the whole field of view. Eye relief is considered ideal when the eye can be positioned at a comfortable distance behind the rear lens and still see the whole field of view.
Ramsden – Two plano convex lenses with the same focal length combine to create this type of eyepiece, invented in 1782 by Jesse Ramsden.
Huygenian – C. Huygens created the first of these in about 1662. This eyepiece design consists of two planoconvex lenses with their convex surfaces facing the objective lens. This design allows some aberrations in certain focal lengths.
Kellner – Carl Kellner made a variation of the Ramsden eyepiece in 1849. The Kellner eyepiece uses a 3-lens design. An achromatic doublet is used in place of the eye lens in the Ramsden to correct chromatic aberration.
Nagler – Invented in 1979, the Nagler eyepiece is designed to give an ultra-wide field of view and good correction for aberrations. The design features up to eight optical elements that are grouped in different patterns to achieve various specified results. With all those features Naglers can get a little heavy and expensive but they offer a wide field of view.
Orthoscopic – A four-element eyepiece came along in 1880 due to the efforts of Ernst Abbe. A convex-convex triplet lens and a convex singlet eye lens combine to offer good eye relief and a quality image. These eyepieces have lost a bit of favor in recent history due to the rise of the Plossl, but they still have a great reputation for use in viewing planets and the Moon.
Plossl – Arguably the most often used eyepiece design of all, the Plossl, was originally designed by Georg Simon Plossl in 1860. The design uses at least four optical elements, usually two doublets, and is known for offering good views of both deep space objects and planets.
Erfle – This design has its origins in the military world. Heinrich Erfle patented it in August of 1921. It is a wide-field eyepiece consisting of three lenses. Two or three of them are doublets. This design offers a wide field of view and good eye relief.
Field Stop – This term refers to a ring inside the eyepiece that forms an edge around the field of view. Its size is directly related to the field of view – the larger the ring the larger the field of view.
Focal Length – The distance from the primary eyepiece lens to where the object viewed comes into focus.
Parfocal – This term refers to an eyepiece’s ability to be swapped with eyepieces of different magnification without causing you to have to refocus your telescope. There is always a little refocusing but parfocal eyepieces are designed to minimize the time spent doing it.