The history of the telescope makes for a compelling read. During the addition of the telescope, practical experiments with reflectors had already begun in 1639, but it was not until 1663 that they reached any distinction.
The Gregorian Telescope
In that year a Scottish mathematician, James Gregory, at the age of 24, published a treatise entitled Optica Promota. In this, he gave a classification of a compound exhibiting telescope employing two concave specula (metal mirrors). The more comprehensive one was to be perforated, and to have a paraboloidal exterior; the smaller was to be ellipsoidal. High magnification could be had with this device, the second reflection amplifying the focal length of the primary in the ratio of fs to Fs.
Assembly of the telescope was tried, but whatever chance it may have had of achieving creditably was lost by polishing the speculum on a cloth lap – putty (tin oxide) being used as the polishing agent. The unyielding lap was an impossible barrier to parabolizing, concern declined, and about 60 years were to elapse before a workable model was finally produced.
The Cassegrainian Telescope
Sieur Cassegrain, a Frenchman, in 1672 designed a second compound reflector, differing from Gregory’s in that it employed a convex secondary, to be of the hyperboloidal figure, placed inside of the center of the paraboloidal primary.
While the Gregorian is seen to be competent of higher magnification, all that is necessary can be had from the Cassegrainian, and it has the advantage of being a much more compact device. Although little was heard of this telescope for the next two centuries, it is worth observing that it survived the Gregorian, and is still widely used in observatories.
The Newtonian Telescope
The history of the telescope takes an interesting turn at this point. In the same year, Newton designed and constructed two small reflectors, of the type so popular with amateur astronomers today and which still bears his name. They were not large, as we know telescopes today, the effective apertures of the concave specula being about 1 1/3″. Their focal length was 6″, making the focal ratio f/4.5.6.
Newton, according to his Opticks (1704), polished his specula on the pitch, using putty as the polishing agent. His plans were natively calculated to yield a spherical surface, and it is quite probable that a close approach to that figure was attained. But the execution of even a spherical mirror of the proportions of Newton’s could hardly be satisfied because of the great amount of spherical aberration present.
Although Newton thought that his mirror might fail of good definition, he “despaired of doing the work” (parabolizing the speculum), yet he “doubted not but that the thing might in some measure be accomplished by mechanical devices.”
It might be concluded that if the center of the mirror were properly deepened, that is, given a shorter radius, or if the radii of the outer zones were progressively lengthened, or if a little of each were done, all the reflected rays could be brought to a common focus. That is a practical solution, and the resulting surface in each instance is a paraboloid.
The standard practice is to deepen the spherical mirror so that, for a 6-inch f/8 mirror, the glass removed in the operation is but half a wavelength of light in thickness at the center. Incredible though it seems, this represents the difference between poor and good definition.
The single-lens eyepiece of Kepler’s had already been improved, with the addition of another element, by Christian Huygens, a Dutch astronomer, and mathematician, about the year 1650. The field lens, like Galileo’s concave lens, is placed before the focal plane of the objective. As it is convex, however, it further converges the rays to form’ a slightly smaller image in a new focal plane, which is then magnified by the eye lens. Thus, a much wider field of view is encompassed by the eyepiece.
These were important improvements in the history of the telescope on the way to our present-day powerful telescopes