The Simple Astronomical Telescope

Angular Magnification by Astronomical Telescope The angular magnification, M, (also sometimes called magnifying power) produced by an optical instrument is defined as

Light rays emerge parallel from th eyepiece lens Focal plane of the lens Focal length of eyepiece lens Focal length of objective lens

angle α is the angle subtended by the object with the unaided eye o’ represents rays from the top of the distant object o’ angle β is the angle subtended at the eye using the telescope o’ β o’ Rays from the top of the object (o’) appear at infinity. This is a virtual image

Light from object A (blue lines) meets at the principal focus of the objective lens.  It then spreads out until it meets the eyepiece.  The eyepiece is set at the focal length away from its principal focus.  Parallel rays emerge from the eyepiece.   At the same time parallel rays from object B arrives at the objective at a small angle a to the axis.  The light is focused onto the focal plane.  It then passes through the eyepiece to emerge as parallel rays.  The angle of these parallel rays is b to the parallel rays from A.

At the same time parallel rays from object B arrives at the objective at a small angle a to the axis.  The light is focused onto the focal plane.  It then passes through the eyepiece to emerge as parallel rays.  The angle of these parallel rays is b to the parallel rays from A.  

A simple expression for the magnification of the telescope = β/α fo α fe β α β h Using the small angle approximation

Concave mirriors

Parallel rays are brought to a principle focus by a concave mirror

The Cassegrain arrangement of mirrors to eyepiece

The Newtonian arrangement