Refracting telescopes use a system of lenses to gather and focus the incoming light for magnification by the eyepiece.
The objective lens is usually composed of two lenses made of glasses of different density . These combined lenses are termed achromatic and are designed to cancel out a phenomenon called chromatic aberration, an effect which is inherent in all lens systems which causes haloes to appear around images in the field of view. It is produced by a single lens acting as a prism and bending different wavelenghts of light (which we perceive as colours) by different amounts. Some, more expensive, astronomical telescopes use an apochromatic objective which combines three lenses to reduce this effect even further, although some manufacturers are now using this term to describe their high perfomance two lens objectives as well.
The clear diameter of the objective lens of a telescope determines its aperture and therefore its light-gathering power. Theoretically, this power continues to increase in direct proportion to the square of the aperture, so that a 100 millimeter ( 4" ) telescope is twice as powerful as one of 75 millimeters ( 3" ) aperture. In practice however, with increasing diameter the light-gathering power of large refracting telescopes can fall off rapidly due to increasing light absorption by the increasingly thicker lenses required to be employed. While this is not really an issue with the moderate sized refractors used by amateur astronomers, it is one of the reasons why large research instruments in use today are reflecting telescopes.
Well designed refracting telescopes incorporate a series of annular rings called light baffles inside the tube between the objective lens and the eyepiece. Light baffles are designed to increase contrast in the field of view by blocking unwanted stray light entering the objective.