32 ASTRONOMY • MARCH 2020
hyperbolic secondary mirror at the Cassegrain focus
were sharper than those taken with the same mirror
at either the Newtonian or prime focus. The longer
focal ratio of the scope’s Cassegrain focus reduced
comatic aberration, or coma. (Focal ratio is a tele-
scope’s focal length divided by the size of its aperture;
smaller focal ratios are considered faster.)
Coma affects all parabolic mirrors by stretching a
star’s image as the object moves off-axis. The image
may be perfect at the center of the field, but closer to
the edge, it becomes distorted. This necessarily limits
the size of high-quality pictures taken using para-
bolic systems.
Ritchey wondered whether a more refined hyper-
boloid secondary mirror might reduce — or perhaps
even eliminate — coma altogether. He wrote to
Chrétien in Paris to see if he could come up with a
workable solution mathematically. Chrétien deter-
mined that a highly specialized hyperboloid primary
and secondary mirror, one concave (curved inward)
and the other convex (curved outward), could reduce
coma to zero.
When he received the news, Ritchey was delighted
at the possibilities their “new curves” represented and
began to conceive of the 100-inch telescope as the
world’s first “Ritchey-Chrétien” telescope. Aninstrument of this type would create perfect images
everywhere without the need for a collimating lens,
which then-current ref lectors required to “f latten” the
view they produced for better image quality. The tele-
scope’s Cassegrain setup would make possible a pri-
mary mirror with a fast focal ratio, reducing the
length of the telescope’s tube. This would give the
telescope a more compact profile, enabling a smaller
dome and a compact, lighter frame and mounting
system, thus lowering overall building costs. The idea
was as ingenious as it was prescient.Old vs. new
Ritchey immediately set about trying to convince Hale
and Mount Wilson assistant director Walter Adams
that the 100-inch should use the new curves — a move
that would ultimately prove to be Ritchey’s undoing.
By 1910, the 100-inch project was woefully behind
schedule and over budget. With funds from the trust
created by hardware magnate John D. Hooker, for
whom the telescope was named, running out, and
Hooker’s increased anxiety and frustration at the
delays, Hale went into emergency mode.
Meanwhile, Ritchey continued to assert that the
telescope ought to be a Ritchey-Chrétien system,
rather than the standard parabolic Cassegrain system.George Ritchey
(second from the
right) sits with
fellow astronomers,
including George
Ellery Hale (fifth
from the right,
seated on the high
wall behind the two
men on the lower
steps), in front of
Yerkes Observatory
in Williams Bay,
Wisconsin, in
August 1898. YERKES
OBSERVATORY, UNIVERSITY OF
CHICAGO, COURTESY OF AIP
EMILIO SEGRÈ VISUAL
ARCHIVES AND SPECIAL
COLLECTIONS RESEARCH
CENTER, UNIVERSITY OF
CHICAGO LIBRARY