Example 3.1 The distance between the components of the binary star Z Herculis is 1.38". What should the diameter of a telescope be to resolve the binary? If the focal length of the objective is 80 cm, what should the focal length of the eyepiece be to resolve the components, when the resolution of the eye is 2'?

In the optical region, we can use the wavelength value of X « 550 nm. The diameter of the objective is obtained from the equation for the resolution (3.4),

The magnification is given by f o> f , and, thus, the focal length of the eyepiece should be f 80 cm f = - =-= 0.9 cm .

b) The magnification is m = ff f' = 1200mmf80mm = 15.

c) Assuming the angular diameter of the Moon is a = 31' = 0.52°, its diameter through the telescope is ma = 7.8°.

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