The Telescope

by Louis Bell

It is of course always more convenient to have the telescope permanently in place and ready for action. Some observers feel that working conditions are better with the telescope in the open, but most prefer a shelter from the wind, even if but partial, and the protection of a covering, however slight, in severe weather.

In the last resort the question is mainly one of climate. Where nights, otherwise of the best seeing quality, are generally windless or with breezes so slight that the tube does not quiver a telescope in the open, however protected between times, works perfectly well.

In other regions the clearest nights are apt to be those of a steady gentle wind producing great uniformity of conditions at the expense of occasional vibration of the instrument and of discomfort to the observer. Hence one finds all sorts of practice, varied too, by the inevitable question of expense.

The simplest possible housing is to provide for the fixed instrument a moveable cover which can be lifted or slid quite out of the way leaving the telescope in the open air, exposed to wind, but free from the disturbing air currents that play around the opening of a dome. Shelters of this cheap and simple sort have been long in use both for small and large instruments.

Fig. 173.—The Simplest of Telescope Housings.

For example several small astrographic instruments in the Harvard equipment are mounted as shown in Fig. 173. Here are two fork mounts, each on a short pier, and covered in by galvanized iron hoods made in two parts, a vertical door which swings down, as in the camera of the foreground, and the hood proper, hinged to the base plate and free to swing down when the rear door is unlocked and opened. A little to the rear is a similar astrographic camera with the hood closed. It is all very simple, cheap, and effective for an instrument not exceeding say two or three feet in focal length.

A very similar scheme has been successfully tried on reflectors as shown in Fig. 174. The instrument shown is a Browning equatorial of 8½ inches aperture. The cover is arranged to open after the manner of Fig. 173 and the plan proved very effective, preserving much greater uniformity of conditions and hence permitting better definition than in case of a similar instrument peering through the open shutter of a dome.

Such a contrivance gets unwieldly in case of a refractor on account of the more considerable height of the pier and the length of the tube itself. But a modification of it may be made to serve exceedingly well in climates where working in the open is advantageous. A good example is the equatorial of the Harvard Observatory station at Mandeville, Jamaica, which has been thus housed for some twenty years, as shown in Fig. 175.