Fig. 94.—Head of 60-inch Tower Telescope.

This instrument being for solar research the mirrors are arranged for convenient working with the sun fairly low on either horizon where the definition is at its best, and can be shifted accordingly, to the same end as in the Snow telescope. There is also provision for shifting the objective laterally at a uniform rate from below, to provide for the use of the apparatus as spectro-heliograph.

The tower is of the windmill type and proved to be fairly steady in spite of its height, high winds being rare on Mt. Wilson. The great thickness of the mirrors in the effort to escape distortion deserves notice. They actually proved to be too thick to give thermal conductivity sufficient to prevent distortion.

Fig. 95.—Porter’s Polar Reflector.

In the later 150′ tower telescope the mirrors are relatively less thick, and a very interesting modification has been introduced in the tower, in that it consists of a lattice member for member within another exterior lattice, so that the open structure is retained, while each member that supports the optical parts is shielded from the wind and sudden temperature change by its corresponding outer sheath.

Still another form of mounting to give the observer access to a fixed eyepiece under shelter is found in the ingenious polar reflector by Mr. Russell W. Porter of which an example with main mirror of 16 inches diameter and 15 ft. 6 inches focal length was erected by him a few years ago. Fig. 95 is entirely descriptive of the arrangement which from Mr. Porter’s account seems to have worked extremely well. The chief difficulty encountered was condensation of moisture on the mirrors, which in some climates is very difficult to prevent.

Fig. 96.—Diagram of Hartness Turret Telescope.

It is interesting to note that Mr. Porter’s first plan was to use the instrument as a Herschelian with its focus thrown below the siderostat at F′, but the tilting of the mirror, which was worked at F/11.6, produced excessive astigmatism of the images, and the plan was abandoned in favor of the Newtonian form shown in the figure. At F/25 or thereabouts the earlier scheme would probably have succeeded well.

Still another fixed eyepiece telescope of daring and successful design is the turret telescope of the Hon. J. E. Hartness of which the inventor erected a fine example of 10 inch aperture at Springfield, Vermont. The telescope is in this case a refractor, and the feature of the mount is that the polar axis is expanded into a turret within which the observer sits comfortably, looking into the ocular which lies in the divided declination axis and is supplied from a reflecting prism in the main beam from the objective

Figure 96 shows a diagram of the mount and observatory. Here a is the polar turret, bb the bearings of the declination axis, c the main tube, d its support, and e the ocular end. Optically the telescope is merely an ordinary refractor used with a right angled prism a little larger and further up the tube than usual. The turret is entered in this instance from below, through a tunnel from the inventor’s residence. The telescope as shown in Fig. 96 has a 10 inch Brashear objective of fine optical quality, and the light is turned into the ocular tube by a right angled prism only 2¾ inches in the face. This is an entirely practicable size for a reflecting prism and the light lost is not materially in excess of that lost in the ordinary “star diagonal” so necessary for the observation of stars near the zenith in an ordinary equatorial. The only obvious difficulty of the construction is the support of the very large polar axis. Being an accomplished mechanical engineer, Mr. Hartness worked out the details of this design very successfully although the moving parts weighed some 2 tons. The ocular is not absolutely fixed with reference to the observer but is always conveniently placed, and the performance of the instrument is reported as excellent in every respect, while the sheltering of the observer from the rigors of a Vermont winter is altogether admirable. Figure 97 shows the complete observatory as it stands. Obviously the higher the latitude the easier is this particular construction, which lends itself readily to large instruments and has the additional advantage of freeing the observer from the insect pests which are extremely troublesome in warm weather over a large part of the world.

This running account of mountings makes no claim at completeness. It merely shows the devices in common use and some which point the way to further progress. The main requirements in a mount are steadiness, and smoothness of motion. Even an alt-azimuth mount with its need of two motions, if smooth working and steady, is preferable to a shaky and jerky equatorial.

Remember that the Herschels did immortal work without equatorial mountings, and used high powers at that. A clock driven equatorial is a great convenience and practically indispensable for the photographic work that makes so large a part of modern astronomy, but for eye observations one gets on very fairly without the clock.

Fig. 97.—Hartness Turret Observatory from the N. E.

Circles arc a necessity in all but the small telescopes used on portable tripods, otherwise much time will be wasted in finding. In any event do not skimp on the finder, which should be of ample aperture and wide field, say ¼ the aperture of the main objective, and 3° to 5° in field. Superior definition is needless, light, and sky room enough to locate objects quickly being the fundamental requisites.

As a final word see that all the adjustments are within easy reach from the eyepiece, since an object once lost from a high power ocular often proves troublesome to locate again.

REFERENCES