A Level Physics Notes: Optics – The Telescope
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.