One of the most misunderstood concepts in astronomy for a person who has never owned a telescope is how the magnification is calculated.

Calculating the magnification is dependent on the focal length of the telescope itself and the focal length of the eyepiece.

The two main types of optical telescopes are reflectors and refractors. A reflector uses a large curved mirror which focuses the incoming light, which eventually passes through an eyepiece, which diverges the light, so that it can form an image on your retina. A refractor has a lens which focuses the light instead of a mirror, and the light then passes through the eyepiece to diverge the light just as in with a reflector.

The focal length is the distance from the mirror or lens to the point where the light focuses.

For a particular telescope, the focal length of the telescope remains the same. Different magnifications are found by using eyepieces with different focal lengths.

The formula for working out magnification is:
Magnification = Telescope Focal Length / Eyepiece Focal Length

So, for example, if you have a telescope with a focal length of 900mm, and use an eyepiece with a focal length of 9mm, you will see objects magnified by 100x.

The inverse calculations are also easy, so you can find which eyepiece you need to use to magnify by, say, 50x very easily.

		public static double CalcTeleFocalLenMag(double pdEPFocalLen, double pdMag)
		{
			return pdEPFocalLen * pdMag;
		}

		public static double CalcEPFocalLenMag(double pdTeleFocalLen, double pdMag)
		{
			return pdTeleFocalLen / pdMag;
		}

		public static double CalcMagnification(double pdTeleFocalLen, double pdEPFocalLen)
		{
			return pdTeleFocalLen / pdEPFocalLen;
		}
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