What could be better than a telescopic sight, as used for an air-rifle?
You need to mount it on something that swings precisely around the exact center of the base of your theodolite... but even mounting it is little trouble, given the job for which it was made. I have a hole in the center of my base. The sight is bolted to a thin strip of wood with a cut off nail pushed through a hole in it. The nail is inserted into the hole in the base, and the sight swings around easily. It is important that the holes be just the right size, so that there is no 'wobble' in the attachment.
When you take readings, the table of the theodolite should be horizontal, and the crosshair you are using to line up on objects should be vertical. Slight errors in the former will, I believe, make only very slight errors in your results. The problems with a non-vertical cross-hair will be clear if you think about what would happen if you lined the top of it up on the top of a church spire, but the bottom of it up on a low level object.
Professionally made theolodites have a rather neat system for leveling the device.
Between the connection to the supporting tripod, below, and the main bulk of the theolodite, above, is a gimbal-like arrangement with four "screw feet", at the corners of a square. Extending two adjacent feet will raise one side of the platform.
Another (relevant!) feature of a professional theolodite is that the whole bulk, when a locking screw is loose, can rotate about the central vertical axis. And there is a spirit level on the part that rotates
You assignment, Jim, should you choose to accept it... (This computer will not self-destruct, by the way)... is to explain how the surveyor quickly and easily gets the bulk of the instrument level, using what I've just described. It is easy... once you think of "the trick"! (Easy to do, fairly easy to describe.)
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