All of this comes at a double price. The first is their weight. The larger focal lengths when used on smaller telescopes will generally need rebalancing of the main telescope. The other one is the price itself. The cost of these eyepieces is understandably high and will generally cost more than the typical beginner's telescope. But those who purchase them are generally very satisified with their performance so feel it was money well spent.

Arm microscopefunction

To calculate the magnification of any given eyepiece, you simply take the focal length of the telescope and divide it by the focal length of the eyepiece. So for a telescope with a 1200mm focal length, that 15mm eyepiece will give a magnification of 80X.

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Here we list many of the eyepiece types, but please take the following notes. First of all, you can reasonably ignore a lot of the simpler ones because you'd have difficultly buying them because they're not manufactured. Even some of the more complex ones are now considered practically obsolete, much of it due to modern telescopes coming (in general) much shorter focal lengths which makes them much smaller and lighter. On the other end of the spectrum are the really exotic models with 6+ lens segments. They are both heavy and expensive. This type has proliferated because of the shorter focal length telescopes and the fact that new eyepieces can be designed via computer software where they can not only be designed, they can be virtually tested.

Eyepiece functionin microscope

Kellner. A Kellner eyepiece is really the Ramsden design with the inclusion of corrective achromatic doublet. It was developed by Carl Kellner back in 1849 and was the first achromatic eyepiece. These are still readily available and popular particularly because of their relative low cost, but they also perform fairly well in low to medium power with telescopes of f/6 and longer. They are superior to either the Huygens or Ramsden designs. They typically have an apparent FOV of 40-50°.

Why are there so many different eyepieces? Basically it's function and of course price always plays a part. As we've already shown the single most important specification of an eyepiece is its focal length because that will determine the magnification of your telescope view. To the beginner that's all they really need to know, but there is so much more.

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What iseyepiecein microscope

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Huygens. This was the first multiple lens eyepiece design. It consists of two plano-convex lenses with the plane sides toward the eye. Developed about 1660 by Christiaan Huygens who discovered by using two air spaced convex lens with the proper spacing there was no transverse chromatic aberration. The leading lens is called the field lens and the other the eye-lens. Though the design works okay with relatively long focal length objectives (f/10 and over), with shorter focal lengths it doesn't work well. And the apparent FOV is only 40°.

Orthoscopic. This was the first eyepiece design with a triplet component. Developed by Ernst Abbe in 1880, it was considered the high-end of consumer eyepieces. It has almost perfect image quality and good eye relief. However, it has a fairly narrow apparent FOV at 40-45° so they are mainly good for planetary viewing. The popularity of this eyepiece has waned since anti-reflective codings have become the norm and the Plössl eyepiece is now preferred.

Convex Lens. This is one of two designs that are the simplest eyepieces as it is a single lens. It was probably the very first eyepiece lens used in the early 1600's when the telescope was invented in the Netherlands. The huge problem with this design is chromatic aberration, also known as color fringing. They only really work with extremely long focal length telescopes which are simply impractical because the tube would be unacceptable long. Consequently you're not going to find commercially made eyepieces with this design.

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The left and center images above are magnified exactly the same. The difference is the left one has a much larger apparent FOV. The right image shows the same apparent FOV as the left, but the magnification is much more. Of course this immediately begs the question: why would you ever buy an eyepiece with a smaller field of view? The simple answer is price because wide field eyepieces have more glass elements and are therefore more difficult to manufacture. You will see this in detail when we discuss the various eyepiece types.

König. The König was designed in 1915 by Albert König, a German optician. It is essentially a simplified Orthoscopic. The design allows for high power and very good eye relief. The apparent FOV is 55°. It has been largely replaced by the Plössl so it is now a hard eyepiece to find.

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A word of caution, especially because these eyepieces are so popular. They are not easy to make plus the quality of the glass needed. Consequently, the quality of these eyepieces can vary considerably from one manufacturer to the next. Sometimes within the same manufacturer.

Nosepiece microscopefunction

RKE. The RKE was designed by David Rank for Edmund Scientific who marketed this eyepiece. It is essentially a reverse adaptation of the Kellner eyepiece, but provides a slightly larger apparent FOV. The RKE stands for either Rank Kellner Eyepiece or Rank-Kaspereit-Erfle.

To really understand why there are so many different eyepieces available, even with the same focal length, you need to learn how they work.

Also, not surprising is these eyepiece designs have been copied so you have other choices in the so-called exotic eyepieces. But even within Tele Vue Optics, new designs are still being developed. The original Nagler design went on to Nagler Type-2, 3, 4, 5, and 6. They also have DeLite (62° with long ER), Panoptic (68°), Delos (72° with long ER), and finally, Ethos which have an apparent FOV of 100°.

Eyepieces for your telescope basically come in just two sizes, but several other standard sizes do exist. All eyepieces are threaded at the bottom in order to accommodate filters.

One unique aspect of this eyepiece design is that the focal plane lies between the two lenses. Consequently this eyepiece cannot be used as a magnifier. Another almost unique feature is it can be used for solar projection for extended periods of time because the two lenses are not cemented together as they are in other designs. Heat will generally melt the cement rendering an eyepiece nearly useless.

Objective lens microscopefunction

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After an eyepiece's focal length, the most important feature is the apparent field of view (FOV). All FOVs are not the same. In the cheapest eyepieces apparent FOV can be as small as 30°. The widest ones are over 90°. In fact those extremely wide ones are so large you have to actually look around to take in the entire field.

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One shortcoming of this design (and all doublets and triplets) is internal reflections. This has been overcome with the use of anti-reflection coatings. We believe all modern eyepieces automatically come with anti-reflective codings.

Functionof microscope

Monocentric. We add this eyepiece design almost as a footnote as it has become very rare. It is a achromatic triplet lens with two pieces of crown glass cemented on either side of a flint glass element. It was invented by Hugo Adolf Steinheil and is given the name Monocentric because it has a common center. The design is free from ghost reflections and shows a bright contrasty image which was a very desirable feature before anti-reflective coatings became available. But it has a narrow field of view (about 25°) so only really used by planetary observers.

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Ramsden. This was the second multiple lens eyepiece. Designed by Jesse Ramsden in 1782, it is very similar in design to the Huygens as it also has two air spaced lenses. The spacing is less than one eye-lens focal length apart, typically 7-8/10ths of the focal length of the eye-lens. A separation of 1 focal length would never be used because any dust on the lens will be in focus. The apparent FOV is 40°. But the Ramsden is only slightly better than a Huygens and not widely manufactured anymore for telescopes.

Microscope parts and functions

Interchangeable eyepieces are one of the two ways that make telescopes different from binoculars. The other being binoculars are simply a pair of identical small telescopes. Here we go in depth of what you really need to know plus what's in the category of "nice to know."

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Erfle. The Erfle is a 5-element eyepiece developed during World War I for military purposes. It was patented in 1921 by Heinrich Erfle. They were designed to have a wide field of view (60°) but only work well at low to medium powers. At high power they suffer from astigmatism and ghost images. They offer a large eye lens and good eye relief. But they are now difficult to find because newer designs offer wider fields of view.

Diopter adjustment microscopefunction

Nagler. This was the first of the new exotic eyepieces. They took having "multiple lens" eyepiece design to a whole new level. The name comes from its inventor Albert Nagler, the founder of Tele Vue Optics. His first designs gave an apparent FOV of 82°! And did this with good correction for astigmatism and other aberrations. These eyepieces have another huge advantage in that they perform especially well with short focal length telescopes. If you look at the diagrams you'll notice that it's a design that has a built-in barlow (i.e., negative) lens. So the positive lens series is not dealing with a really short cone of light. One consequence of this should not be an issue: the Nagler design cannot be used as a standalone magnifier.

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Plössl or Symmetrical. The Plössl has become one of the most popular eyepieces for amateurs. It was developed in 1860 by Georg Simon Plössl. It is sometimes referred to as Symmetrical because the two doublets can be identical. The eyepiece overtook the Orthoscopic design when anti-reflective codings became available as the Plössl has a good apparent FOV of 50° or more. Thus, it is good for both deep space and planetary viewing. The big shortcoming of this design is short eye relief (limited to 70-80% of the eyepieces focal length). This becomes very problematic when the focal length of the eyepiece is less than 10mm making the eye relief just 7-8mm. This makes viewing with eye glasses difficult.

Negative / Galilean Lens. This is the other simple eyepiece design as it is also a single lens. It was also one of the earliest designs and gets the Galilean Lens label because it was used in Galileo Galilei's 1609 telescope design. It only produces a very narrow field of view, but it does produce an upright image. Consequently it was used in early binoculars and opera glasses.

The eyepiece will determine the magnification. Simple enough. The eyepiece magnifies the image that is formed from the objective lens of a telescope. How much will any given eyepiece magnify? On every commercial eyepiece a number will be imprinted like the example on the left followed my the letters "mm" which stand for millimeters. This is the focal length of that eyepiece. Often, but not always the type of eyepiece will be shown. In the example it is Plössl. We will discuss the various eyepiece types further down.