Unlike popular belief, the main aim of an astronomical telescope is to gather a large amount of light, not magnification. When viewing very faint and distant objects like stars and galaxies, our main concern is the quantity of light reaching our eyes, then comes the size of the object. The telescope has a large aperture that collects light and focuses it at a single spot to give a bright image of faint objects. This image can then be magnified using a suitable eyepiece.
With an aperture of 120 mm, my telescope can capture about 1000 times more light than the average human eye.
Light rays from distant celestial objects are nearly parallel to each other. These parallel rays (shown in red and blue) enter the telescope from the left end and are incident on a concave or parabolic primary mirror. These rays after reflection begin to converge, moving towards the focus. They are reflected again off a plane (flat) mirror kept inclined 45 degrees to the axis. The rays finally converge at a point near the eyepiece focuser to form a relatively bright image of the faint celestial object. This image is magnified by the eyepiece to give a bright and enlarged final image.
The magnification of the telescope is given by the formula
M=Focal length of the primary mirror Focal length of the eyepiece