A telescope in normal adjustment is shown above. The objective and eyepiece lenses are separated by a distance equal to the sum of the focal lengths.
An object at infinity – a star or planet forms an image at the focal point of the objective lens. The image so formed becomes the object for the eyepiece lens, which then forms an image at infinity. The angular magnification is given by
The image so formed is inverted, virtual and larger than the original object.
The
number of a telescope is given by the ratio
where D is the diameter of the objective lens. We can increase the magnification by increasing the ratio
We can either increase the focal length of the objective, which also implies increasing the diameter
of the objective so that the f number is not too large. A largenumber means a dim image. The magnification is usually increased by changing the eyepiece for one of a smaller focal length and adjusting the separation between the lenses.