As the name suggests, the field of view describes how big the circle of night sky is that you will see through your eyepiece. This size is measured in degrees. To give a frame of reference, the full moon occupies a circle of about 0. The apparent field of view is the limit of what the lens would show you if there were no magnification involved I know that sounds odd, but it will make sense in a moment.
True FoV is calculated by dividing the apparent FoV this is where it makes sense by the magnification which, as we saw earlier, is worked out with the focal length of your particular telescope. To simplify, we worked out above that a telescope with a mm focal length using a telescop eyepiece with a focal length of 25mm gives a magnification of 28x. The diagram below which is from this rather technical site illustrates really well the impact of magnification on the apparent field of view.
There are many different types of eyepieces, and this helpful page covers them in more useful detail than I can do here. Cheap to make because they only have two lenses in them so often packaged with low-end telescopes they both suffer from aberration defects see above.
Moving up the scale, these eyepieces have additional lenses and so, cost to reduce the achromatic aberration effects. A Kellner lens steps up from the Ramsden design because it includes an achromatic lens discussed above. Cheaper versions still tend to suffer from ghosting though, which is where fainter versions of the main image can be seen alongside it.
Manufacturers reduce or practically eliminate them using special coatings on the glass lenses themselves. Move along the eyepiece quality scale another notch and we come to the Orthoscopic eyepiece. Generally speaking, higher magnification works for the moon and planets too, because they are brighter objects to train your telescope on.
The smaller mm lenses are relatively cheap to buy, but the recommended 15mm to 30mm start to become more pricey. You will not believe how much this Nagler costs!
This article could still go on for a long time, as the amount of detail on eyepieces is staggering see these telescope eyepiece specification tables as an indication of the complexity you can take on but that feels like enough detail for this post. Works nicely on long focal length telescopes to show wide field and extended objects. Shorter focal length telescopes will enjoy great mid-range magnification of galaxy clusters and large open clusters. Longer focal lengths are good for large nebula and open clusters.
Shorter focal lengths are great for large objects such as the Orion nebula, views of the full lunar disc, large open clusters, and more. It also makes for good "locator" eyepieces in all focal lengths. These are well suited for shorter focal length telescopes for extended views and large, starry fields. These are exclusively the domain of shorter focal length telescopes. This magnification range is superb for showing large, starry vistas as well as extended nebulae with star fields, etc.
Exit pupil refers to the size of the bundle of light rays coming out of the eyepiece. Exit pupil size in inches can be calculated by:. In order for all the light rays to enter your pupil, the exit pupil must be smaller than the pupil of your eye. A young person's fully dark-adapted eyes may have 7 mm-wide pupils. As you age, the maximum pupil diameter decreases. For middle-aged adults, the practical maximum is closer to 5 mm.
At the other end of the scale, magnifications that yield an exit pupil in the range of 0. In other words, this much magnification starts to degrade the image you see. Although there is no specific number of eyepieces you should own, with a few 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 with low power i. Optical aberrations deviations from the "ideal" form are better corrected in some designs than others, but like everything in life, the higher the price, the better the quality. Even so, eyepieces with desirable characteristics still can be obtained at reasonable cost once you know what to look for.
At the budget end of the market, three-element meaning they have three lenses inside eyepieces labeled Kellner or "MA" modified achromat can turn in a good performance with scopes of long focal ratio, such as Schmidt-Cassegrains and traditional refractors.
They do not work well with telescopes of short focal ratio. The latter is a good all-rounder, particularly when antireflection multicoated. It is worth pointing out that observers who wear glasses to correct for simple long- or short-sightedness no astigmatism don't need to use them at the telescope; a twist of the focuser will remedy that.
High-power lunar and planetary viewing entails the use of short-focal-length eyepieces, which can be a problem owing to the small eye relief of conventional designs. This has spawned the development of six- to eight-element designs that combine comfortable eye relief and wide apparent fields across the focal-length range. They may feature exotic rare-earth glass elements to reduce optical aberrations still further. Many regard these Lanthanum and Ultrawide designs as the pinnacle of eyepiece evolution and rightly so , but for many people the price will be a barrier — and their physical size and weight may be an issue for delicately balanced small scopes.
While the Barlow is not strictly an eyepiece, our discussion would not be complete without mentioning this enormously valuable accessory. A Barlow is inserted into the telescope's focuser before the eyepiece and instantly doubles or triples, in some instances the magnification.
At the expense of a small loss of light, this very useful device can double your eyepiece investment by making each perform at two powers. Sharpness — Keeping every object-on an image in focus without any visible distortion is what we call sharpness.
The chances of an image being out of focus increases as we move away from the center of the lens. You can check for Sharpness by optically inspecting the image of a star and looking for any loss in image quality as it moves from the center to the edge. Brightness — When the light goes through a lens some of it reflects back while others pass through, which results in a dimmer image.
Most of the high-end optical lens comes with a special coating which reduces reflection and makes the image brighter. Well, brightness may not affect the already bright objects like the moon but it has a noticeable effect on dim stars. Contrast — Contrast is the difference in luminance between the brightest and darkest areas of an image. A good eyepiece which does not scatter light has good contrast. The light can scatter when the edges of an eyepiece are not properly sealed off.
The shiny internal casing can also help to amplify the scattered light by reflecting it. That is why most high-end eyepieces come with matte black internal casing. Using a low-quality glass and bad arrangement and calibration can also result in scattering and dispersion of light. Comfort — Properties like a longer eye relief distance from the lens within which the full viewing angle can be obtained along with soft eye rest which makes the viewing experience more comfortable.
A wider apparent field of view is also desirable. A good eyepiece has a number of design features that help with producing a clearer image. Some of these features are lacking or totally absent from cheaper eyepieces and that is what makes all the difference. Very cheap telescopes can sometimes come with a plastic lens. The plastic lens is very cost-effective and easier to make but has many disadvantages and should always be avoided.
Nowadays most of the lens are made of glass but as you go from the cheap eyepiece to expensive the quality of glass also increases. A good eyepiece has a really high-quality lens with no structural deformity and perfect shape. These lenses do not suffer from optical aberrations. Scattering can lower the quality of the final image and is also responsible for strange artifacts which appear when looking at a bright star called ghosting.
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