Reflector or Refractor ?

The reflectivity of aluminum, however, is more-or-less constant, and from a standpoint of image brightness, it placed the reflector on a more equal footing with the refractor. In fact, until the quite recent development of anti-reflection lens
coatings, an aluminized mirror has had the same efficiency, in light-transmitting qualities, as an air-spaced achromatic objective lens9 of equal aperture.

Coming down to figures — due to reflection there occurs in an untreated lens a light loss of slightly more than four per cent at each of its surfaces. With these reflection losses to be accounted for, plus an absorption loss in the substance of the glass (amounting to about two per cent for lenses of moderate size), it is evident

that about 82 per cent of the original light is transmitted. In the reflector, after first deducting that area of the mirror's surface obscured by the diagonal, an equal percentage of the original light is found to be transmitted. Of course, this transmitted light is subject to another reflection by the diagonal, but the refractor will probably employ a star diagonal, the function of which is similar to that of the diagonal or prism of the Newtonian, so an equivalent loss may occur there. Therefore, with either instrument, the same amount of light reaches the eyepiece.


Reflector or Refractor continued