So a 17mm eyepiece has a focal length of 17mm. Notes: 1: Atmospheric seeing conditions (the sky) often limits the maximum usable magnification to 250-350x. Beyond this magnification, objects will continue to look bigger, but they won’t look better. So the telescope and eyepiece affect the magnification in different ways: Finding the focal length of an eyepiece is easy since it’s common to refer to eyepieces by their focal length. Magnification = Telescope focal length ÷ Eyepiece focal length For example, if you use a telescope of 1000mm focal length with a 25mm eyepiece, the magnification would be 40x (1000mm ÷ 25 = 40). So if your telescope has a huge 20-inch aperture, would you be able to comfortably view the skies at 1,000x magnification? Theoretical Maximum Magnification Notes: 1: Atmospheric seeing conditions (the sky) often limits the maximum usable magnification to 250-350x. As a rule of thumb, a telescope’s maximum useful magnification is 50 times its aperture in inches (or twice its aperture in millimeters). Sky & Telescope, Night Sky, and skyandtelescope.org are registered trademarks of AAS Sky Publishing LLC. When we view an object that is closer, our eyes act as a zoom and change the focal length to about 22mm. When you're out observing, you might reach out for that high-power lens to clarify features inaccessible from low-power eyepieces. It’s equal to the telescope’s focal length divided by the eyepiece’s focal length. How Much Power is Enough? In fact, as we’ll see in a moment, your telescope is actually capable of many different magnifications, by combining it with different eyepieces. To get started, we just need two numbers: 1. If you’re shopping for a telescope online, the focal length should be listed somewhere in the specs (if not the description). Most eyepieces are clearly marked with their focal length. Every telescope has a maximum usable magnification. a refracting telescope has an objective of focal length 70cm and eyepiece 5cm. Doubling the power gives you one-fourth the image brightness and reduces the sharpness by one half. We’ll talk more about the maximum usable magnification later in this article. Compute for the exit pupil diameter of the scope. Magnification (power): The amount that a telescope enlarges its subject. For example, if your telescope has an 8-inch aperture, the maximum usable magnification will be 400x. For example, a 2x Barlow doubles the magnification, and a 3x Barlow triples it. Depending on the quality and aperture of your telescope, as well as atmospheric conditions, higher magnifications may not look as good. If you want to calculate the magnification of your telescope, you’re in luck: It’s a surprisingly simple formula. More aperture makes for a brighter image. The angular magnification \(M\) of a reflecting telescope is also given by Equation \ref{eq2.36}. When you want to observe a star you will often switch eyepieces. A good backyard telescope for us amateur stargazers has an aperture of 80 mm to 300 mm (3.15” to 12”) or more. Although it’s useful to calculate magnifications, keep in mind that higher magnifications aren’t inherently better or more powerful. For a spherical mirror, the focal length is half the radius of curvature, so making a large objective mirror not only helps the telescope collect more light, but also increases the magnification of the image. Find out using this telescope calculator. If we look at something on the horizon a long way off, we see lots of things around that object i.e. I’ve created the tool below to make it super easy to calculate telescope magnifications. low magnification; suitable for viewing the moon, open clusters, and large Also remember that there are other factors that can affect views. Failing that, you can always visit the website of your telescope or eyepiece manufacturer. Then the image resolution at this magnification = 120/30 = 4 arcseconds. It will also give you an idea of what types of objects you could view with each magnification. By: Adrian R. Ashford and David Tytell So if you see an 8-inch Dobsonian or a 70mm refractor, that is referring to the aperture, not the focal length. The power of the telescope is the ability to magnify an object. We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites. The eyepiece has a field of view of 52°, so the field of view for the telescope at this magnification will be 52 ÷ 30 = 1.7°. the stars), the focal length, or the distance from the cornea to the retina, of a normal relaxed eye is about 1.7 cm (17 mm). Sky & Telescope is part of AAS Sky Publishing, LLC, a wholly owned subsidiary of the American Astronomical Society. When we look at an object which is an infinite distance away (e.g. CAUTION - telescope manufacturers will often advertise the magnification of the scope, and give really big, impressive numbers. The magnification of a telescope is a combined function of the scope and the eyepiece that is used, so the user can set the magnification to almost any arbitrary value by selecting a suitable eyepiece.

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