Contributed by C. H. Denniston, Pulteney. N. Y.
A Home-Made Equatorial
By Harry Clark
Instrument for Locating Stars
The ordinary equatorial is designed and built for the latitude of the observatory where it is to be used. This is necessary since the hour axis must point to the north pole of the heavens whose elevation above the horizon is equal to the latitude of the observer's station. The final adjustment of an ordinary equatorial is very tedious so that when once set up it is not to be moved. This calls for a suitable house to protect the instrument. It has been the aim of the writer to build a very simple instrument for amateur work which would be adjustable to any latitude, so easily set up ready for work and so portable that it need not be left out of doors from one evening until the next.
The instrument is mounted on a tripod or piece of iron pipe carrying a short vertical rod of 3/8-in. steel. A rectangular wooden frame with suitable bearings rotates about this shaft. The frame has also two horizontal bearings carrying a short shaft to the end of which the frame carrying the hour axis is firmly clamped. By this arrangement of two perpendicular shafts the hour axis may be directed to any point in the heavens without care as to how the tripod or pipe is set up.
The frame for the hour axis is about 12 in. long with a bearing at each end. The shaft which it carries is 1/4-in. steel, carrying the hour circle at one end, and at the other the frame for the declination axis which is similar to the other, but somewhat lighter. The declination axis is also of 1/4-in. steel, carrying at one end the declination circle and the pointer at the other.
The entire frame of the instrument is made of cherry and it will save the builder much time if he will purchase cherry "furniture" which is used by printers and can be obtained from any printers' supply company. It is best quality wood free from imperfections in straight strips one yard long and of a uniform width of about 5/8 in. As to thickness, any multiple of 12-point (about 1/8 in.) may be obtained, thus saving much work in fitting up joints. Fifty cents will buy enough wood for an entire instrument. All corners are carefully mortised and braced with small brass angle-pieces. The frame is held together by small brass machine screws. After much experimentation with bearings, it was found best to make them in halves as metal bearings are usually made. The loose half is held in place by guides on all four sides and is tightened by two screws with milled nuts. A great deal of trouble was experienced in boring out the bearings until the following method was devised. One hole was bored as well as possible. The bearing was then loosened and a bit run through it to bore the other. Finally, a piece of shafting was roughened by rolling it on a file; placed in both bearings and turned with a brace. The bearings were gradually tightened until perfectly ground.