Astronomy - USA (2020-03)

Secondary mirror

(hyperbolic)

Eyepiece

Primary mirror

(hyperbolic)

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George Ritchey had great vision and skill. Henri Chrétien

was a brilliant astronomer and optical engineer. Together,

they created one of today’s best telescope designs.

BY RON VOLLER

ien telescope was born

worked on direct photography with Mount Wilson’s

newly completed 60-inch telescope. Chrétien was a

brilliant optician in his own right and was impressed

by the incredible performance and optics of the tele-

scope, which was Ritchey’s latest and greatest achieve-

ment. Ritchey was equally impressed with Chrétien’s

knowledge, skill, and creativity as an engineer. This

mutual respect and admiration led them to correspond

regularly after Chrétien’s return to Paris a year later.

New curves

At about that time, Ritchey was designing the 100-inch

Hooker Telescope at Mount Wilson, which he and the

observatory’s director, George Ellery Hale, assumed

would employ a paraboloidal primary mirror with

an array of f lat, parabolic, and hyperbolic secondary

mirrors.

But while comparing photos taken with the 60-inch

ref lector, which had a parabolic primary mirror,

Ritchey noticed that the images taken using a

A Ritchey-Chrétien telescope

employs primary and

secondary mirrors that are

both hyperboloids. This

eliminates coma when

compared with a Cassegrain

telescope — which has a

paraboloid primary and

hyperboloid secondary —

and also reduces the length

required for the telescope.

ASTRONOMY: ROEN KELLY

RITCHEY-CHRÉTIEN

TELESCOPE