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Different Eyepiece Designs Huygens. This eyepiece design was invented by Christiaan Huygens in the 17th century. This two-element design is now considered outdated, but sometimes such eyepieces (marked with capital ‘H’) are supplied together with cheap telescope models. Eye relief and field of view are quite small. Ramsden eyepiece design, which is a modified version of the Huygens eyepiece design, is much more efficient but is also outmoded (although still used in some microscopes). Kellner. Three-element Kellner (and its close modifications – Achromatic Ramsden “AR” and Modified Achromatic “MA”) are considered to be the least expensive eyepiece for serious astronomy. Such eyepieces provide bright and clear images at small and medium magnifications. Kellner eyepieces work perfectly with small and medium-sized telescopes. These eyepieces have about 40° apparent fields of view and reasonable eye relief, though quite short for high magnifications. Orthoscopic. The four-element orthoscopic eyepieces were once considered the best eyepieces for universal use. But now such optical design clearly loses the competition to more recent designs because of its quite narrow field of view. Orthos provide excellent clarity, color correction, contrast, and larger eye relief than Kellner eyepieces. They are especially good for lunar and planetary observations. Plössl. The four-element Plössl design is the most popular eyepiece optical design that gives you excellent image quality, good eye relief, and a 50° apparent field of view. High-quality Plössl eyepieces provide high contrast and sharpness across the entire field of view. They are suited for any king of observation. Erfle. The 5- or 6-element Erfle eyepieces are optimized for a wide apparent field of 60° to 70°. At low magnification these eyepieces produce spectacular views of stellar fields. At high magnifications the edge-to-edge clarity suffers a little. Ultra Wide Angle Eyepieces. This group includes improved optical designs consisting of 6-8 elements and featuring wide fields of view up to 85°. It is so wide, you have to rotate your eye to see the whole panorama (by the way, not everyone loves it). Additional elements slightly increase light loss inside the eyepiece, but in general the resulting image quality is very high. So is the price.
Each telescope model has limited useful magnification power. When this limit is exceeded the picture starts to darken and blur. Keep that in mind when choosing an eyepiece. Magnification is calculated by dividing the focal length of the telescope by the focal length of the eyepiece. Accordingly, the focal length of the eyepiece equals the objective focal length divided by the magnification. For example, a telescope with 2000mm focal length and 20mm eyepiece will give you 100x increase. Highest practical power of a telescope directly depends on its aperture. Large telescopes are able to collect more light, capture a broader wavefront and, therefore, produce sharper views. The magnification also determines the size of the exit pupil. The exit pupil diameter can be obtained by dividing the telescope’s aperture by its magnifications. You can also use another formula: just divide the eyepiece focal length by the telescope’s focal ratio. The exit pupil must be smaller than the diameter of the pupil of the observer’s eye; otherwise some of the light rays will not make it into the pupil. Young people have great night vision; their eyes are fully adapted to the darkness, and therefore, they have pupils about 7mm in diameter. Maximum pupil diameter decreases with age. So an average middle-aged adult has 5mm-wide pupils. On the other hand, with exit pupil diameters less than 1 millimeter so-called ‘empty magnification’ appears, meaning that the picture quality decreases very rapidly. Magnification power range Exit pupil diameter, mm Magnification power per inch of aperture, x Magnification power (75-mm telescope), x Magnification power (200-mm telescope), x Application very low 4.0 – 7.0 3 – 6 10 – 18 28 – 50 Lowest usable power. Wide-field observing of deep-sky objects in dark sky. low 2.0 – 4.0 6 – 12 18 – 36 48 – 100 General observations, locating objects, observing most deep-sky objects. medium 1.0 – 2.0 12 – 25 36 – 75 100 – 200 Moon, planets, compact deep-sky objects, wide double stars. high 0.7 – 1.0 25 – 35 75 – 100 200 – 280 Moon and planets (steady air), double stars, compact clusters. very high 0.5 – 0.7 35 – 50 100 – 150 280 – 400 Planets and close double stars (very steady air). So how many eyepieces do you need? Just a few. You can use one low-power and one high-power eyepiece for a long time, but […]
Category: Numbers You might have noticed that the names of binoculars always include numbers, such as Levenhuk Bino Basic 10×40. If you are a beginner, you might not know what those number mean. The first number, 10 in our example, is the magnification. In other words, a magnification of 10 will make the object appear 10 times closer to you or 10 times larger than you would see it with the naked eye. Note that a magnification of 10 is probably all you would need for regular purposes. As the number increases, the image becomes more blurred and shaky and less bright. This happens because, along with the object, everything else is magnified, including the tremble in your hands. If magnification is higher than 10, you should use support for the binoculars, such as a tripod. The second number, 40 in our example, indicates the diameter of the front lens. The higher the number, the better it is for the image. So, 10×40 is much more comfortable than 10×25, for example. The only disadvantage of the higher number is that it increases the weight of the binoculars. Category: Field of view Field of view is another thing to keep in mind when you are deciding which binoculars to buy. In short, field of view refers to the amount of space you see through the binoculars. Suppose you have a wall 1000 yards ahead of you. With the field of view 250 you will see 250 feet of that wall without having to turn your head. Logically, as the magnification increases, you will see less of that wall, but in greater detail. Therefore, the higher the magnification, the smaller the field of view. If you wear glasses, you have to consider the eye relief of the binoculars. The eye relief is the maximum distance from the eye to the eyepiece at which the entire field of view is still observable. This distance shouldn’t be smaller than the thickness of your glasses. Category: Exit pupil Think of the exit pupil as the pupil of your eye on your binocular lens. It is calculated in millimeters by dividing the second number by the first, like this: 40/10=4. The exit pupil roughly tells you what the brightness of the image will be. For basic purposes 2.5-3 is just fine. For stargazing, 5-7 is better. Anything higher is not necessary, except for situations in which […]