While cleaning out some ancient boxes in the rafters of Garaj Mahal today I stumbled upon a tool I had built a long time ago and had completely forgotten after carefully storing it away. Not everyone needs a tool like this but the story behind it is, IMO, interesting and it is a classic example of what one can make with just the junk lying around in a reasonably junk-fertile shop.
I used to have my astronomical telescope set up on the back patio. To aid in observation I had written a program that would indicate the rise times and positions of all the planets as well as a few other things of interest. Trouble was that rise times were all calculated on the basis of a mathematical horizon but there are mountains surrounding the LA basin, not to mention neighborhood trees and other obstructions.
If I could map the azimuth and elevation of these obstructions I could put that data in my programs and compute true rise/set times for my location, thus saving me a lot of waiting around. To map them I needed something akin to a surveyor's transit. (Using the astronomical scope itself would have been very cumbersome.) Transits are expensive and I couldn't find any local rental places so I set out to build one for myself.
I didn't need anything as accurate as a real transit and, with young children, money wasn't plentiful so I opted to build it from what I had. The first picture shows the completed transit...
Basically, it's a framework that holds a 0.22 rifle scope so that it can be rotated in both azimuth and elevation. These angles are read off two 360 degree protractors I purchased at the local stationery store (remember when stationery stores carried stuff like that, drafting instruments, bow pens, etc.?)
The base...
is a plywood circle set on three adjustable leveling legs. Mounted on this base is a giant ball bearing found in the "someday I'll need this" bin. The upper structure that carries the scope will ride on its inner race. A plate in the middle of this race is connected to a smaller wheel under the the base that, driven by hand, will cause the upper structure to rotate. Note the ingenious bearing clamps made from screws, washers and spent 0.22 cartridge cases for spacers.
This photo...
shows the upper structure attached to the plate. The protractor that reads azimuth can also be seen along with the index post at which the azimuth angle is read.
I was lucky to have an 18:1 planetary gear assembly lying around in the same bin from which came the bearing. It had been rejected from a spacecraft antenna pointing mechanism and I retrieved it from the dumpster. This gear assembly provides the elevation motion in a nice smooth way; with the reduction precise pointing of the scope is easy. Also obvious is the elevation protractor; its index mark is on the opposite side of the aluminum support that holds the planetary gear.
The scope had dovetail mounts so I had to machine a dovetail carrier for it. This carrier is screwed to the planetary gear plate that originally carried the spacecraft antenna.
A level was needed. I was too cheap to buy a good level so I built an adjustable mount for a line level (the type masons hang on a string to lay out a level course). The level mount is made from two discarded steam chests I made for one of my steam engines...
and later discarded because I changed the valve design and that required a slightly larger steam chest.
The level is soft-mounted and two screws (not visible in the photo) on the bottoms of the steam chests allow find adjustment of the level.
Certainly not surveyor accuracy but plenty good enough for my purposes. Only the protractors, and the scope were bought so I would guess the whole thing cost less than $20 in 1970's money. No prints; I made it up as I went along and that's the most enjoyable way to build!
