Formation of Image
In optical astronomy the object with which we work is the image of the light source formed by the principal image-forming part of the telescope, which is called an objective. The objective of the optical telescope is either a lens or a mirror. Light rays from the light source are refracted in passing through a lens and are reflected from a mirror. The image is produced where the light rays converge to a position known as the focus. The focal length of the objective is the distance behind the lens to the focus or the distance in front of the mirror to the focus. The image of a star is just a point of light, while that of an extended object, such as the moon, is inverted. In telescopes using either mirrors or lenses an eyepiece magnifies the image much as a reading glass magnifies small print. Or a photographic plate may be inserted into the focal plane of the objective instead of the eyepiece, transforming the telescope into a giant camera. In this case the objective lens or mirror serves as the camera lens. The advantage of photography over observing with the eye is that the photograph is available for later study and time exposures can record fainter sources than those the eye sees.
