How much magnification can I use with my CPC scope, and how much is too much?

A telescope's magnification is actually a relationship between two independent optical systems–the telescope itself and the eyepiece you are using. To determine power, divide the focal length of the telescope (in mm) by the focal length of the eyepiece (in mm). By exchanging an eyepiece of one focal length for another, you can increase or decrease the power of the telescope. For example, a 25 mm eyepiece used on a 2800 mm focal-length telescope would yield a power of 112x (2800/25 = 112) and a 10 mm eyepiece used on the same instrument would yield a power of 280x (2800/10 = 280). Since eyepieces are interchangeable, a telescope can be used at a variety of powers.

There are practical limits of magnification for telescopes. These are determined by the laws of optics and the nature of the human eye. As a rule of thumb, the maximum usable power is equal to 50-60 times the aperture of the telescope (in inches) under ideal conditions. Powers higher than this usually give you a dim, lower contrast image. For example, the maximum power on a 279 mm telescope (11” aperture) is in the range 550x-660x. As power increases, the sharpness and detail seen will be diminished. The higher powers are mainly used for lunar, planetary, and binary star observations. 

Most of your observing will be done with lower powers (6 to 25 times the aperture of the telescope in inches). With these lower powers, the images will be much brighter and crisper, providing more enjoyment and satisfaction with the wider fields of view.

Even with a lower-power eyepiece, a view can be blurry because of the Earth’s atmosphere. Heat waves and high-altitude winds move air around and cause differing temperatures of air to mix. This makes the air act like a weak lens that interferes with the light from a planet or a star by defocusing it. On nights that are like this, the situation gets worse when you magnify it using an even higher power eyepiece. 

A good way to increase magnification is to use a Barlow lens. A Barlow lens rated at 2x can be used with your existing eyepiece and will double the magnification of any existing eyepiece. 


Updated 12/24/13