Make Telescopes: Understand Amateur Telescopes
Friday, July 31st, 2009 Subscribe To Our Feed
In the past there has been considerable discussion by those who make telescopes on the relative merits of reflector and refractor. From the standpoint of professional astronomers, there is no serious competition between them, as each type supplements the other in a well-rounded observing program. Let us first look at some of the optical characteristics of reflectors and refractors,
Very early in the 19th century, when advocates of the speculum mirror began to feel the challenge of the refractor, Dr. Nevil Maskelyne, English Astronomer Royal, ventured the opinion “that the aperture of a common reflecting telescope, in order to show objects as bright as the achromat, must be to that of an achromatic telescope as 8 to 5.”
The relative inefficiency of the reflector of that day was due to the fact that, even under most favorable circumstances, barely 40 per cent of the original light escaped absorption by the metal mirrors, the greatest losses occurring in the short and medium wave lengths. Even silver-on-glass mirrors are subject to considerable deterioration, especially under certain conditions of the atmosphere.
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 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, for those engaged in amateur telescope making, with either instrument, the same amount of light reaches the eyepiece.
It was discovered, however, in the latter part of the last century by those involved in who make telescopes that some lenses which had been tarnished by the elements transmitted more light than ones that were newly polished; it was found that this resulted from lessened reflections at the tarnished surfaces. Various processes of producing an artificial tarnish were attempted. At present, in the most satisfactory method, metallic salts (such as magnesium fluoride) are evaporated in a high vacuum onto the glass. Ideally, the refractive index of an anti-reflection fluoride coating should vary from that of glass at the glass-fluoride surface to that of air at the fluoride-air surface, in which case no reflection would occur.
Practically, the index of the coating should be equal to the square root of the index of the glass, and its thickness equal to a quarter of a wave length of yellow-green light. Then, if a refractor is contemplated, additional experience can be gained by making the optical flat that is so essential in the testing and figuring of the objective lens.
If you decide to begin to make a telescope, the experience of thousands of amateurs has shown that one’s teeth should first be cut on at least one good mirror. Then, if a refractor is contemplated, additional experience can be gained by making the optical flat that is so essential in the testing and figuring of the objective lens.
Technorati Tags: No Tags
Related Tags: No Tags
Possible Related Posts
