The Equatorials described in this catalogue are provided with every means of adjustment. The cradle contains powerful screws to set the line of sight at right angles to the declination axis, and shifting screws to place the polar axis in the meridian and to the correct elevation, and with care and a few experiments with the adjustments, and by observations of some catalogue stars, the various adjustments will soon be correctly made, and the verniers set accordingly.
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.