University to Own Largest Telescope

A reflecting telescope, the largest of its kind in the world, is now being installed in the Observatory. Its mirror, which was purchased from the estate of Dr. A. A. Common of Ealing, England, in 1904, is five inches thick, has a diameter of five feet, and weighs over half a ton. Three months of continuous work were spent in shaping the mirror from the rough disc, and 410,000 strokes of the polishing machine were needed to give it the required smoothness. The preparation of the site was begun September 28, 1904, and the remounting has since gone on slowly, until the work is now so far advanced that in a few weeks the telescope will be in regular use.

The plan of supporting the telescope in position is the same as that devised by Dr. Common, but changes have been made in method of observation and mechanism of control. Instead of being mounted on pedestals of cast iron, cement or masonry firmly built into the ground, as most large telescopes are, it rests on a great hollow cylinder immersed in a tank of water. A tank lined with thick walls of solid concrete was first sunk in the ground, and in this receptacle, ballasted with about 10 tons of iron at its lower end, floats the steel cylinder forming the polar axis of the telescope.

The fixed axis, including the steel cylinder, points to the celestial poles; the motion mounted on its upper end is from east to west over the meridian line, and from north to south across the celestial equator. The whole structure weighs more than 20 tons, and its delicate poise enables it to be moved without jar and with the utmost ease.

As reconstructed, the telescope enables the observer to do his work wholly indoors. The observatory, instead of containing the telescope, as in the usual arrangement, simply adjoins it, and the rays of the object are reflected up through the open air and carried through a tube in the outer wall of the observing room to the eye-piece. Light from an object under observation undergoes three reflections before it reaches the observer. It is first received by the 60-inch mirror at the bottom of the tube, which throws it up to a second small mirror mounted centrally within and near the end of the tube. The second mirror returns the light down the tube to a third mirror, and by this it is reflected through the air to the eye-piece, in front of which the rays finally converge to a focus and form the image.

The instrument is controlled altogether from the observing room, where there are several levers for use in setting the telescope in motion. The following of the object under observation has been accomplished with the aid of an astronomical driving clock, and electricity is to be used for both sets of motions. Before the recorder are two long slits through each of which can be seen a white ribbon impressed with divisions and figures corresponding with degrees and minutes of the celestial are. These ribbons move up and down with the telescope, serving to guide the manipulation of the lever, so that the pointing of the tube is accomplished by simply holding the lever at motion until the required place on the ribbon is reached.

Besides guiding the tube into position, electricity is also to be employed for keeping it on the object as the latter is carried from east to west by the west-to-east motion of the earth.