11/11/2023 0 Comments Meade telescope eyepieceIf you wear corrective lens eyeglasses while using a telescope, we recommend looking for telescope eyepieces with at least 15mm, and more preferably 20mm, of eye relief to see the entire field of view comfortably. The optical design of an eyepiece determines the eye relief, which is the distance from your eye to the eyepiece lens when the image is in focus. The true field would be 50 ÷ 100, or 0.5° - about the same apparent diameter as the full Moon. The magnification would be 2000mm ÷ 20mm = 100x. True Field = Apparent Field ÷ Magnificationįor example, suppose you have an 8" Cassegrain telescope with a 2000mm focal length, and a 20mm eyepiece with a 50° apparent field. The true field can be approximated using the formula: The true field (or real field) of view is the area of sky seen through the eyepiece when it's attached to the telescope. Such wide-field and Ultra-Wide eyepieces are preferred by amateur astronomers who enjoy the "spaceship porthole" effect of using as wide a field as possible. Specialized wide-field telescope eyepieces can boast apparent fields ranging from 60° to 100° or more. Most eyepieces have an apparent field of about 40° to 50°. It is similar to the screen of a television (not the actual picture seen on it). This is called "empty magnification" and can change depending on the object or area viewed.Ī telescope eyepiece's apparent field of view is the angular diameter, expressed in degrees (°), of the circle of light that the eye sees. Magnifying the image beyond a certain point does not reveal more it just makes the image look blurry. Second, because light consists of waves, even an optically perfect telescope picks up only a limited amount of fine detail in the image. First, the telescope gathers a fixed amount of light, and at higher magnifications, or powers, you're spreading the same amount of light over a larger area, so the image will always be dimmer. If you've ever used a single telescope at different powers, you know that you have a choice of a small, sharp, bright image at lower magnification or a big, blurred, dim image at higher power. The actual magnification will depend on the focal length of the telescope the eyepiece is used with. This relationship is important to remember while choosing eyepieces: the lower the eyepiece focal length, the higher the relative magnification will be. For example, a 10mm eyepiece will always provide a higher magnification than a 25mm eyepiece. The above formula dictates that a telescope eyepiece with a shorter focal length yields a higher magnification than an eyepiece with a longer focal length. Magnification = Telescope focal length (mm) ÷ Eyepiece focal length (mm)Įyepiece focal length (mm) = Telescope focal length (mm) ÷ Magnificationįor example, a telescope with a 2000mm focal length used with a 20mm eyepiece will give 100 power (2000 ÷ 20 = 100). Since both eyepiece and telescope focal lengths are expressed in millimeters, the formula used to determine magnification is: So the first step in choosing eyepieces is to decide what magnifications, or powers, you want to use and what eyepiece focal lengths will give them. The focal length of an eyepiece, along with the focal length of the telescope the eyepiece is used with, determines the magnification the combination provides. Perhaps the most important specification of an eyepiece is its focal length. With this knowledge, you'll be well-prepared to select ideal eyepieces for your own telescopes. Let's take a few moments to learn a bit more about the most common terms and specifications used to describe features of telescope eyepieces. While most new telescopes include one or two eyepieces to get started with, purchasing additional eyepieces can significantly increase the functionality of any telescope, new or old. With an eyepiece installed in the telescope focuser, you can bring extremely distant objects into focus for magnified study. The optical elements of an eyepiece allow you to focus light collected by a telescope, so you can observe a sharp view of the object or area where the telescope is pointing. A telescope simply will not work without an eyepiece.
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