The Caldwell Objects

tember work commenced. The king also granted
a yearly salary of £50 to Caroline Herschel.
However, frequent flooding at Datchet and
consequent ill health slowed William's progress.
Thus he and his sister moved in 1786 to a house
at Slough, also in the neighborhood of Windsor
Castle. There William's health improved.
The telescope, its massive mount, and its
mirror all were designed exclusively by William
Herschel. The first of its two functioning mirrors
was cast in London; it spanned 49½ inches in
diameter and was 2½ inches thick. Ten men
polished it, giving the speculum "[a] very white
surface." The mirror was very brittle, however,
and came out of the mold thinner than intended.
The resulting weakness would never permit it to
give a good image, and the
458
mirror eventually was discarded.
However, first light for the 40foot
telescope was achieved with
this mirror on February 19,1787,
when Herschel crawled inside the
tube and hand-held the eyepiece
to find the focal point. The view
of the "Lucid Spot" in Orion was
far from perfect, but it was better
than expected and extremely
bright. The Herschels
immediately began trying to
refine the mirror.
In August 1787, King George
granted Herschel yet another
£2,000. A second mirror cracked
during the casting process, but in
February 1788 a third mirror was
cast successfully. This mirror
weighed 2,118 pounds, had a
diameter of 48 inches, and was
3½ inches thick throughout (its
back was not flat). An iron ring
surrounded the mirror,
and an iron cross supported it
from behind. The mirror was successfully polished
by machine.
The telescope's optical tube was 39 feet, 4
inches long, with a diameter of 4 feet, 10 inches.
Herschel did not deem the Great Telescope
completed until August 28, 1789 — the same
night he discovered the sixth satellite of Saturn
(Enceladus) with it. (This actually proved to be a
rediscovery, as Herschel had observed the
satellite two years earlier with the 20-foot
telescope but did not recognize it as such at the
time.) Herschel observed with the 40-foot at the
"front view," from an enclosed, adjustable seat
that was mounted on the lower side of the tube's
front end. No secondary mirror was used. The
primary mirror's focal point was placed 4 inches
above the lower side of the
Deep-Sky Companions: The Caldwell Objects