Magnifying Power

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The ability of a telescope to enlarge images is the best-known feature of a telescope. Though it is so well-known, the magnifying power is the least important power of a telescope because it enlarges any distortions due to the telescope and atmosphere. A small, fuzzy faint blob becomes only a big, fuzzy blob. Also, the light becomes more spread out under higher magnification so the image appears fainter! The magnifying power = (focal length of objective) / (focal length of eyepiece); both focal lengths must be in the same length units. A rough rule for the maximum magnification to use on your telescope is 20 × D to 24 × D, where the objective diameter D is measured in centimeters. So an observer with a 15-centimeter telescope should not use magnification higher than about 24 × 15 = 360-power.

magnification enlarges fuzzy blob

The set of four figures below shows the effect of a larger objective size. They have the same magnification. These are ideal images of two stars separated by 0.5 arc seconds which would be the angular separation for stars at the Sun and Jupiter's positions if the system was 33 light years from us. The frames are 1.5 arc seconds square and are at the observation wavelength of 500 nanometers. The resolving power is given by thetaR and they all have the same brightness---the light in the bottom images from the large telescopes is just much more concentrated than for the small telescopes. The image from the 0.1524-meter telescope (image A) would take 30 minutes to make, but the image from the 5.08-meter telescope (image D) would take only 1.6 seconds! The exposure times for the other telescopes are given.

The pictures clearly show the increase in sharpness as the objective size is increased. The size of each of the blobs is the size of the smallest detail that can be seen with that telescope under ideal conditions. Atmospheric distortion effects (smearing of the binary star images to a blob the size of the entire frame) and obscuration and diffraction by the secondary and its supports are NOT shown here.

resolution of 0.15-m objective resolution of 0.5-m objective

resolution of 2.4-m objective resolution of 5-m objective

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last updated: 20 May 2001

Is this page a copy of Strobel's Astronomy Notes?

Author of original content: Nick Strobel