This is a form of reflecting telescope but little known. This is
rather strange, since it is a very much better principle than the
“Gregorian,” so well known to the old observers. Herschel says it
admits of a theoretically perfect telescope. Compared with the
“Newtonian” it has its advantages and disadvantages. Its principal
advantages are, first, the shortening of the tube, which in large
telescopes is sometimes very important. Secondly, the observer has
not to _ascend_ to the eye-tube, the observations being made at the
lower end, as with a refractor. The “Cassegrain” has a flatter field
of view, owing to the action of the curve of the second reflector
causing the rays to travel twice the distance, and, adding the
element of magnifying power, the eye-pieces need not be composed of
small lenses. The adjustments are perhaps a little more trouble, as
the line of collimation must be carefully attended to, this
requiring only a little more care can soon be accomplished, and then
the definition of a good “Cassegrain” is very pleasing.
Amongst its disadvantages is the necessity for the observer to gaze
upwards as with refractors, which, when the object is at a
considerable altitude, is distressing, this is one of the reasons
why the “Newtonian” is so pleasant to use, on account of the natural
and easy position of the observer. The eye-piece being a fixture, it
is not quite so convenient to use some of the accessories of the
telescope. But there are means to overcome these drawbacks, and so
make the “Cassegrain” even more handy than the “Newtonian.”
I have mounted an 18 inch speculum of 12 feet focus in the
“Cassegrain” form, so that objects at any altitude could be observed
with the greatest possible ease. A plane was fixed near the large
mirror to receive the rays from the convex reflector and to throw
them out to the side, illuminating apparatus were fixed here, and
micrometers, &c. used, as if it were a “Newtonian”; the tube was
thus made shorter, and the flat field of a long focus realised, but
there would be a little loss of light in consequence of an extra
reflection, but this, with a large aperture (and the fact that the
“Cassegrain” gives a little more light than the “Newtonian”) can
better be spared, considering the convenience gained. The observer
is not elevated for any altitudes, and a large telescope is actually
occupying less room than a small one. It can be used with or without
the diagonal.
I have, by request, fitted the “Cassegrain” with means for two
observers to view the same object at the same time. A perforated
plane was arranged to receive a portion of the converging rays and
throw them to the side of the tube into an eye-tube, and the
remainder passing on to the eye-tube at the proper place, two images
are thus formed, and can be magnified at will and viewed
simultaneously. The perforated plate was so arranged that it could
be removed at pleasure.
[ Illustration: FIG. 1. ]
[ Illustration: FIG. 2. ]
[ Illustration: FIG. 3. ]
[ Illustration: FIG. 4. ]