In past times there has been significant discussion by those engaged in amateur telescope making on the relative worth of reflector and refractor. From the standpoint of professional astronomers, there is no serious competition between them, as each kind supplements the other in a well-rounded observing program. An amateur who programs to build his own instrument and put it to use for general observing has other factors to consider. Allow us first look at some of the optic characteristics of reflectors and refractors. https://www.youtube.com/watch?v=8etze8eF2KU
Very early in the 19th century, when advocates of the speculum mirror started out to appreciate the challenge of the refractor, Dr. Nevil Maskelyne, English Astronomer Royal, embarked the view “that the aperture of a common reflecting telescope, in order to show objects as bright as the achromat, must be to that of your achromatic telescope as 8 to 5. inch
The relative inefficiency of the reflector of that day was because, even under most favorable circumstances, barely 40 per dollar of the original light escaped absorption by the metal mirrors, the very best losses occurring in the short and medium trend lengths. Even silver-on-glass wall mirrors are subject to extensive deterioration, especially under certain conditions of the atmosphere.
The reflectivity of light weight aluminum, however, is more-or-less continuous, and from a perspective of image brightness, it put the reflector over a more equal footing with the refractor. In reality, before the quite recent development of anti-reflection contact lens coatings, an aluminized hand mirror has had the same efficiency, in light-transmitting features, as an air-spaced achromatic objective lens of identical aperture.
Coming down to figures – due to reflection there occurs in an untreated lens a light loss of more than four per dollar each and every of its surfaces.
With expression losses to be paid for, plus an compression loss in the element of the glass (amounting to about two percent for lenses of modest size), it is obvious that about 82 % of the original light is transmitted. In the reflector, after first deducting that part of the mirror’s surface obscured by the diagonal, an similar percentage of the original light is deemed sent.
Of course, this sent light is subject to another reflection by the diagonal, but the refractor will probably use a star diagonal, the function of which is similar to that of the diagonal or prism of the Newtonian, so an equal loss may happen there. Therefore, for those involved in amateur telescope making, with either tool, the same amount of sunshine reaches the eyepiece.
It had been discovered, however, in these part of the last century, that some lenses which have been ruined by the elements sent more light than ones that were newly refined; it was found this resulted from lessened glare at the tarnished floors. 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 higher vacuum upon 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, whereby no expression would occur.
Practically, the index of the layer should equate to the square reason for the index of the glass, and its thickness corresponding to a quarter of an influx duration of yellow-green light. Simply the light at reverse ends of the obvious spectrum can now be reflected, amounting generally speaking to less than one per cent of this of the whole, and is detected by the purplish color given to the reflection.
In the perspective of an introduction to the optician’s trade, the experience of 1000s of hobbyists has shown that a person’s teeth should first be cut on at least one good mirror. After that, when a refractor is considered, additional experience can be gained by making the optical flat that is so essential in the testing and figuring of the objective lens.