In Figure 2 is depicted a lens and a mirror each of which is receiving the light from a distant star.  Note the lines with directional arrows in them, these represent light rays from the star.  Both surfaces of the lens are convex; that is, curved outward on each side.  The surface of the reflecting mirror is concave; in other words, curved inward.  You can see how starlight entering the lens is bent and concentrated by the lens into the dot image of the star.  Also, observe how the shiny surface of the mirror reflects starlight into a similar dot image.


 Stars are so far away that as far as your telescope optics are concerned, they are at an infinite distance.   Only an object which is effectively at an infinite distance from the telescope will be focused to a point at a distance from the objective called the focal length.  The point where this focusing of an infinitely distant object occurs is referred to as the focal point.  Objects which are very near to the telescope will be focused at a farther distance from the objective than the focal length.  The closer the object is to the telescope, the longer the distance to where the image is focused.


Yet another important term for you to know is f-ratio.  This value is equal to the focal length divided by the objective’s diameter.  Those readers who are 35 mm camera buffs may have run into this term before.


In the section of this text covering eyepieces and accessories,  you will see just how important it is to know the focal length and f-ratio of a lens or mirror.


Figure 3 illustrates the most common type of telescope in the refractor and reflector classes, respectively.


The lines with directional arrows in Figure 3 represent light rays leaving each tree.  You can see how light entering each telescope is concentrated by the objective and directed into the eyepiece.



copyright 2004 Singularity Scientific