The Newtonian Telescope
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
Fig. 8. Model of the first reflecting telescope, made by Newton and presented by him to the Royal Society.
6", making the focal ratio f/4.5.6 To bring the light rays to a convenient place for observation, a plane speculum was used for the secondary reflection. This was placed at a 45° angle a short dis-
6The (f/ number of a telescope mirror or lens is the ratio of its focal length to its diameter. In the above case, the focal length of the mirror is 4.5 times its diameter. A mirror of similar diameter but greater focal length would be said to have a higher focal ratio
tance inside the focus of the primary, where it could deflect the rays out through an opening in the side of the tube. There the inverted image was magnified with a plano-convex eyepiece of about 1/6" focal length, giving a total magnification of about 36 diameters.
The plan of a modern Newtonian is shown in Fig. 9, and the reflection of rays in Fig. 47. As with the compound telescopes, the primary mirror must be a paraboloid, since a spherical surface cannot reflect parallel rays, such as those from a star, to a single
Fig. 10. Reflection of
parallel light rays by:
a, spherical mirror; b,
paraboloidal mirror. The
center of curvature of
the spherical mirror is
at C.
focus. This is shown in Fig. 10a, where rays striking the edge zones of a spherical mirror are brought to focus inside the focal point of the central-zone rays.7 The paraboloid (Fig. 10b) is the only surface that can bring parallel rays to a single focus. Newton, according to his Opticks (1704), polished his specula on pitch, using putty as the polishing agent. His methods were
ingeniously calculated to yield a spherical surface, and it is quite probable that a close approach to that figure was attained. But the performance of even a spherical mirror of the proportions of New-
7A zone is defined as a ring on the surface of the mirror, all points of which
are equidistant from the center. It is often convenient, however, to regard a
zone as having a finite width, notably the central zone which, on a 6-inch
f/8, is substantially equal in width to half the radius of the mirror.
ton's could hardly be satisfactory 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."
Referring back to Fig. 10a, it might be concluded from a study of the diagram 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 making of which is described in this book, the glass removed in the operation is but half a wave length 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. His
compound eyepiece is shown in Fig. 68. 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.
John Hadley
