Telescopes are usually designed to perform particular kinds of work.
Some are meant to be used chiefly for photography.
In general, for visual work, low-ratio telescopes with their wide fields are useful for comet seeking, variable star work, and the like.
The higher ratios are used in planetary study, double star observations, and in other fields where high powers and fine definition are required.
Some of these instruments are portable, and others must be mounted on a solid pier.
The amateur, however, usually will have formulated no particular plan of observation, except a desire to explore the heavens, and to see with his own eyes some of its wonders.
From the experience gained by amateur telescope makers, it has been found that the most practical and popular instrument for amateur use is the 6-inch f/8 Newtonian reflector.
Its concave mirror is 6" in diameter and its focal length 48".
The delicate task of parabolizing the mirror, while not easy, is not beyond the ability of a careful worker.
The 4-foot focal length makes for comfortable observing, and with a low-power eyepiece, the field of view is a trifle over one degree in diameter - more than twice that of the full moon.
The magnifications that may be employed permit of a modest size of mounting, which can be made portable.
Such a telescope should reveal stars of magnitude 12.
8, as compared with the 6th-magnitude limit of the unaided eye, and the 9th-magnitude limit of the average small binocular.
Theoretically, the mirror is capable of resolving double stars having a separation of % of a second of arc, but as magnifications exceeding about 30 per inch of aperture can seldom be used, it may not be expected to perform up to this limit.
This telescope will show the divisions in Saturn's rings; surface markings on the moon little more than a mile across should also be visible.
The purchase price of such an instrument of professional make is necessarily high, and many an amateur feels compelled to do without it.
But if he is possessed of some ingenuity and craftsmanship, and is willing to devote a few hours a week to the task, he can in a relatively short time build the telescope in its entirety, for a small fraction of that price.
Of course, many engaged in amateur telescope making feel that their mirrors are inferior to the professionals', but this is not necessarily true.
It has been frequently demonstrated that mirrors of professional make will seldom stand up to a test, because it is impossible for the professional optician to spend sufficient time on the mirror without losing money, whereas the amateur can, if he will, devote all the time and care necessary to produce a mirror of admirable figure.
Upkeep is slight for any telescope.
The reflective aluminum coating of the mirrors of a reflector is subject to deterioration from dust and the elements admitted by the open tube, but given the same protection when not in use that is accorded a refractor, at least two years of service should be realized before the aluminizing job need be repeated.
For those involved in amateur telescope making, the task is time consuming but extremely rewarding.