On the Construction of a Silvered Glass Telescope / Fifteen and a half inches in aperture, and its use in celestial photography

by Henry Draper

When the telescope is not in use, and the dome is let down, so that there is no longer an interval of a quarter of an inch between it and the rest of the roof, it is confined inside by four clamps and wedges. Otherwise, owing to its lightness, it would be liable to be blown away. These clamps a, Fig. 41, are three sides of a square, made of iron one inch square. They catch above by a point in the wooden basis-circle of the dome b, and below are tightened by the wedge c.

When the dome is raised it is prevented from moving laterally and sliding off by three rollers, one of which is seen at f, Fig. [40]. These catch against its inner edge, and only allow slight play. At first it was thought necessary to have a subsidiary half arch at right angles to the other to hold it up, but that is now removed.

All the parts work very satisfactorily, and owing to the care taken to get the roof-circle and basis-circle flat and level, no leakage takes place at the joint, and even snow driven by high winds is unable to enter.

c. The Observer’s Chair.

Fig. 42.

The Observer’s Chair.

This is not a chair in the common acceptation of the word, but is rather a movable platform three feet square, capable of carrying two or more persons round the observatory, and maintaining them in an invariable position with regard to the telescope eyepiece.

Its general arrangement is better comprehended from the sketch, Fig. 42, than from a labored description. Below, it runs on a pair of wheels a (one only is visible) 9 inches in diameter, whose axles point to the centre of the circle upon which they run. They are prevented from shifting outwards by a wooden railroad b, b′, and inwards by the paling l, l′. Above, the chair moves on a pair of small rollers c, which press against a circular strip or track d, d′, nailed around the lower edge of the dome opening. Access to the platform is gained by the steps e, e′. Attached to the railing of this platform, and near it on the telescope, are two tables (not shown in the figure) for eyepieces, the sliding plateholder, &c.