4/6/2023 0 Comments Orion skyseekerHowever, some of the more modern eyepiece designs provide long-eye relief regardless of focal length, which is especially beneficial to those who wear glasses. Eye relief is traditionally in proportion with focal length: The shorter the focal length, the shorter the eye relief. The true field would be 50/100, or 0.5° - about the same apparent diameter as the full moon.Įye Relief refers to the distance between your eye and the eyepiece lens when the image is in focus. The magnification would be 2000 mm / 20 mm = 100x. True Field = Apparent Field / Magnificationįor example, suppose you have an 8-inch Schmidt-Cassegrain telescope with a 2000 mm focal length and a 20 mm eyepiece with a 50° apparent field. The true field can be calculated using the following formula: AFOV's range from narrow (25° - 30°) to an extra-wide angle (80° or more).Īn eyepiece's true field of view is the angle of sky seen through the eyepiece when it's attached to the telescope. It is how much of the sky is seen edge-to-edge through the eyepiece alone. Other Telescope Eyepiece Specifications to Consider Field of View: Apparent and TrueĪn eyepiece's Apparent Field of View (AFOV) is expressed in degrees (°). It is very rare that the atmospheric seeing conditions will allow you to use these eyepieces on a longer focal length telescope since they're very high power. Ultra-short focal length (2-4mm) eyepieces provide very high power magnifications and are best for observing the planets and the moon on shorter focal length telescopes. If you're looking for a quality set of eyepieces for an affordable price, check out the TPO eyepieces here.Ĭomparing Eyepiece Magnification and Focal Lengths 2 - 4mm Eyepieces If you don't have the budget for three separate eyepieces, a zoom eyepiece can be a good choice in a pinch, but won't provide as much sharpness and clarity as individual eyepieces can. At a minimum, we recommend having at least three eyepieces: one low power, one medium power, and one high power eyepiece so you can observe anything you would like to. Take a look at the different eyepiece ranges below with examples of what magnification they provide. Instead, it's best to have a range of eyepieces from low, medium, and high power magnification eyepieces for your telescope. To get the best views no matter what the sky conditions are like, one single eyepiece won't do the job. This is why it's so important to have an arsenal of eyepieces to use so you can get the best views based on the ever-changing conditions of the night sky! For that reason, it is usually best to start observing with your longest focal length (lowest power) eyepiece first and work your way up in magnification until the view starts to degrade, and then find the eyepiece that hits the sweet spot. Most of us are not privileged to live under such perfect sky conditions, and more often than not, it is far from perfect. When atmospheric seeing is poor, the turbulent air will blur the view and cause the moon and planets to look wobbly: Atmospheric seeing is how still or unsettled the atmosphere around you is. This number is the Maximum Useful Magnification, assuming your sky conditions (atmospheric seeing) are perfect. Therefore, if your telescope has a focal length of 2000mm, you can use up to a 5mm focal length eyepiece. So, for example, if you have a telescope with an aperture of 200mm (8"), your Maximum Useful Magnification is 400x. You can also take the aperture in inches and multiply it by 50 to get the same result. Maximum Useful Magnification = Telescope Aperture (mm) x 2 There is a simple formula to determine the Maximum Useful Magnification of your telescope: This is known as Maximum Useful Magnification, and you may see this as a specification on telescopes. Your telescope's aperture is the main factor in what decides how much magnification you can use. This is limited by two factors: 1) your telescope's aperture, and 2) the conditions of the sky when you're observing. But why is that? Telescopes have a limit of how much magnification they can provide when paired with an eyepiece. You might have already concluded that a smaller telescope won't give as magnified of a view as a larger one. How Your Telescope and Sky Conditions Affect Magnification Note: When using your telescope at different powers, you generally have a choice of a small, sharp, and bright image at lower magnification or a larger, yet blurred and dim image at higher magnification. The reason being is that the telescope gathers a fixed amount of light, and at higher magnifications, the same amount of light is being spread over a larger area, resulting in a dimmer image. A 20mm eyepiece on a 2000mm telescope (2000 / 20) gives you 100 power (100x), this makes objects appear 100 times closer to you through the telescope than they appear to your unaided eye.Magnification = Telescope Focal Length (mm) / Eyepiece Focal Length (mm)
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