Star Ware: The Amateur Astronomer's Guide to Choosing, Buying ...

18 Star Ware
Sir William Herschel, a musician who became interested in astronomy
when he was given a telescope in 1722, ground some of the finest mirrors of his
day. As his interest in telescopes grew, Herschel continued to refine the reflector
by devising his own system. The Herschelian design called for the primary
mirror to be tilted slightly, thereby casting the reflection toward the front rim
of the oversized tube, where the eyepiece would be mounted. The biggest
advantage to this arrangement is that with no secondary mirror to block the
incoming light, the telescope’s aperture is unobstructed by a second mirror;
disadvantages included image distortion due to the tilted optics and heat from
the observer’s head. Herschel’s largest telescope was completed in 1789. The
metal speculum around which it was based measured 48 inches across and
had a focal length of 40 feet. Records indicate that it weighed something in
excess of one ton.
Even this great instrument was to be eclipsed in 1845, when Lord Rosse
completed the largest speculum ever made. It measured 72 inches in diameter
and weighed in at an incredible 8,380 pounds. This telescope (Figure 2.7),
mounted in Parsonstown, Ireland, is famous in the annals of astronomical history
as the first to reveal spiral structure in what were then thought to be nebulae
and are now known to be spiral galaxies.
The poor reflective qualities of speculum metal, coupled with its rapid tarnishing,
made it imperative to develop a new mirror-making process. That evolutionary
step was taken in the following decade. The first reflector to use a
glass mirror instead of a metal speculum was constructed in 1856 by Dr. Karl
Steinheil of Germany. The mirror, which measured 4 inches across, was coated
with a very thin layer of silver; the procedure for chemically bonding silver to
glass had been developed by Justus von Liebig about 1840. Although it apparently
produced a very good image, Steinheil’s attempt received very little attention
from the scientific community. The following year, Jean Foucault (creator
of the Foucault pendulum and the Foucault mirror test procedure, among others)
independently developed a silvered mirror for his astronomical telescope.
He brought his instrument before the French Academy of Sciences, which
immediately made his findings known to all. Foucault’s methods of working
glass and testing the results elevated the reflector to new heights of excellence
and availability.
Although silver-on-glass specula proved far superior to the earlier metal
versions, this new development was still not without flaws. For one thing, silver
tarnished quite rapidly, although not as fast as speculum metal. The twentieth
century dawned with experiments aimed to remedy the situation, which
ultimately led to the process used today of evaporating a thin film of aluminum
onto glass in a vacuum chamber. Even though aluminum is not quite as highly
reflective as silver, its longer useful life span more than makes up for that slight
difference.
Although reflectors do not suffer from the refractor’s chromatic aberration,
they are anything but flawless. We have already seen how spherical aberration
can destroy image integrity, but other problems must be dealt with as
well. These include coma, which describes objects away from the center of
view appearing like tiny comets, with their tails aimed outward from the cen-