Magnification is the ability of a telescope to make a small or distant object look larger. Magnification is a relationship between two optical systems: the telescope and the eyepiece.
To calculate magnification, divide the focal length of the telescope by the focal length of the eyepiece. For example: The focal length of an 8" Schmidt-Cassegrain scope is usually around 2000mm so a 10mm eyepiece would produce about 200 power and a 20mm eyepiece would produce about 100 power.
A telescope can be used at a variety of powers for different applications with different eyepieces and one eyepiece would produce different magnifications with telescopes of different focal lengths.
The practical upper limit of magnification for a given scope is 60 power per inch of aperture under ideal conditions (e.g. stillness of air). For example, the maximum practical magnification for a 6" telescope would be 360 power.
High magnifications, between 100x and 200x, are often used in lunar and planetary viewing. Deep-sky targets like nebulae and galaxies benefit from lower powers which produce brighter, crisper images. Typical magnifications for deep-sky objects would be between 6 and 13 times the aperture in inches of a scope. For example, using an 8 inch scope you would typically use between 48 and 104 power.
You will not always want to make a small celestial object large and you sometimes want to make a faint object bright. A low-power, wide-field-of-view eyepiece can be as important to an astronomer as a high power view.
Field-of-View
Often an eyepiece manufacturer will state the "Apparent Field-of-View" for an eyepiece. To calculate the "Actual Field-of-View" a given eyepiece would have with a given telescope, you would divide the "Apparent Field-of-View" by the magnification.
For example, if the apparent field-of-view is 50 degrees and the magnification is 100 power then the actual field-of-view is ?of a degree.
Eye Relief
Eye relief is the distance from the final lens surface of an eyepiece to the pupil of the eye to see the whole field of view. Some high-power Plossl eyepieces have eye relief so short that you can feel your eyelashes on the glass lens. More comfortable eyepieces have longer eye relief.
If you must wear eyeglasses due to astigmatism, you will need an eyepiece with long eye relief, so that you can wear you glasses and still see the full field of view.
Calculators
Given your Telescope's focal length and the focal length and apparent field of view of a eyepiece calculate the power of the eyepiece with your telescope and the actual field of view with Scope City's .
Given your Telescope's focal length and aperture calculate the suggest powers of the eyepieces suited for your telescope with Scope City's .
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