NavList:

I have used a theodolite for finding my position, and for finding True North. If you want to use the sun, then you need a filter. I would suggest a filter in front of the object lens, rather than behind the ocular lens, as this will prevent you frying the cross-hairs when you focus the sun on them! I use a 4.0 Neutral Density Filter, which I bought from, and had edged to the diameter I required by Knight Optical (http://www.knightoptical.com). This filter reduces the brightness of the sun by a factor of 10,000. I would suggest you do not use anything less then NDF 4.0 and NDF 5.0 would probably not be too much.  I had to make a new housing for the object lens of the theodolite telescope, so that there was a thread in front of the lens into which I could screw the ND filter, for which I also made a housing. You will, of course, have your own lathe on which you can make the necessary housings. After all, no gentleman of learning is complete without his own personal mechanical workshop....

This particular theodolite is a light-weight Russian device made of aluminium, which I have found convenient by virtue of its low weight (2 kgs) for keeping myself found during expeditions into the Egyptian desert and for experiments to measure the alignment of Egyptian pyramids. The scale graduation on this theodolite is only 5 MOA, but I can easily interpolate to 1 MOA. I have never found myself challenged in measuring altitudes or azimuths of the sun to 1 MOA. I am sure that with a finer graduation, I could measure to better precision.

When using a theodolite for work at night, measuring the altitudes (or azimuths) of planets or stars, you will need some way of introducing light into the telescope so you can see the cross-hairs. I made up a system using red LEDs (so as not to ruin my night vision) to introduce a source of light which was uniform across the telescope's field of view, and was also variable so the brightness could be adjusted to optimise visibility of the star (which would swamped if the background light was too bright) and the cross-hairs (which will not be seen clearly if the background light is too dim) at the same time. The method recommended in the "old texts" - which was to wave a candle or flash-light in front of the object lens while trying to take the measurement - does not lend itself to accurate work.

For azimuth work, there are a few mathematical wrinkles which the initiated may not appreciate, but which is made accessible by John Hamilton in his pdf article  "Azimuths in Control Surveys" which you can easily find by Googling the title.

Geoffrey Kolbe