The first refractors ever made were not achromatic

The first refractors ever made were not achromatic. They had only the convex element in their objective. Even so Galileo was able to discover craters on the moon and the satellites of Jupiter with such a crude telescope, though he saw some wild colors! The five dollar toy telescope of today is of this simplest variety which is referred to as a Galilean refractor.

To their credit, all commercial 2.4 inch refractors are achromatic. But did you notice that I said that the second element removes most of the false color? These telescopes, though much better than Galileans, will still show a false tinge of red and blue around any object observed. That little bit of chromatic aberration can lessen contrast in an image and prevent the viewing of very fine detail. There are more sophisticated apochromatic lenses which essentially remove all of a refractor’s false color. A third element added to the objective is usually what does the trick, though some modern hi-tech apochomats manage to do the job with just two elements.

Apochromatic refractors have the best image quality of all telescopes. This image quality is due to more than just the apochromat’s color-free image, as I shall explain later. Unfortunately, apochromats are very expensive. They are especially suited for observers of the planets who will spare no cost to have absolutely the best possible view. Luckily, there is another much cheaper way to have a telescope with NO chromatic aberration. It is the mirror based reflector telescope.

Examine the second telescope diagramed in Figure 3. Light from the tree falls on the concave objective mirror at the back of the telescope. The objective reflects this light and its curved surface causes the light to be concentrated onto a flat secondary mirror. Because the secondary mirror is tilted at an angle of 45 degrees, the light is reflected into the eyepiece at the side of the tube in order for the image to be seen.

The reflector telescope shown in Figure 3 with its flat and tilted secondary mirror is of the Newtonian type. So named because it was invented by Sir Isaac Newton, the same man who first described the Law of Gravity. There are other types of reflectors which use a different kind of secondary mirror, but we won’t discuss them for now.

Since the light entering a reflector telescope is not bent by passing through a lens, the image projected by the objective mirror has absolutely no false color! An objective mirror is not only much cheaper to produce than the same size apochromatic lens, it is cheaper than an achromat! A three inch apochromatic refractor might cost thousands of dollars. A three inch achromatic refractor may cost five hundred to seven hundred dollars. A three inch reflector might cost two hundred dollars or less. It is this economy of construction that has led to the largest telescopes in the world being reflectors.