The Gregorian Telescope

Practical experiments with reflectors had already begun in 1639, but it was not until 1663 that they gained any prominence. In that year a Scottish mathematician,
James Gregory, at the age of 24, published a treatise entitled Optica Promota. In this he gave a description of a compound reflecting telescope employing two concave specula (metal mirrors). The larger one was to be perforated, and to have a paraboloidal surface; the smaller was to be ellipsoidal.4 The arrangement is shown in

Fig. 6. Notice that the ellipsoidal mirror s is placed beyond the

Fig. 6. Paths
of light rays in
the Gregorian
telescope.

3Refraction is the bending of a ray of light as it passes obliquely from one
medium to another of different density; dispersion is the change in the amount
of refraction for light of various wave lengths.

4If an ellipse (see Fig. 34) is rotated about its major axis, the surface of
revolution thus generated is a prolate spheroid, but telescope makers commonly use the general term ellipsoid for this figure. When an ellipse is rotated about its minor axis, the surface of revolution is an oblate spheroid. If a
parabola or a hyperbola is rotated about its principal axis, the resulting surface
of revolution is a paraboloid or a hyperboloid.

focal point F of the primary, which is also one of the foci of the ellipsoid. From this position, the secondary mirror returns the rays to form an erect image at its other focus f. High magnification could be had with this instrument, the second reflection amplifying the focal length of the primary in the ratio of fs to Fs.

 Construction of the telescope was undertaken, but whatever chance it may have had of performing creditably was lost by polishing the speculum on
a cloth lap — putty (tin oxide) being used as the polishing agent. The unyielding lap was an insurmountable barrier to parabolizing, interest apparently ebbed, and about 60 years were to elapse before a workable model was finally produced.

Cassegrainian telescope