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I made a solar filter to go on the front of a theodolite. It was a bit of a
palava as I had to make a new holder for the object lens of the theodolite
telescope that had a screw thread in front of the lens. This was so that I
could screw the solar filter I made for the theodolite onto the front of
the telescope. The solar filter used a 3.0 OD (Optical Density) and a 2.0
OD glass filter stacked one in front of the other (separated by a thin
spacer) to made a 5.0 OD filter. This cuts down the light from the sun by a
factor of 100,000, which sounds a lot, but the sun is still plenty bright
when you look at it through the filter when screwed on the front of the
telescope. The NDF glass is available from
http://www.knightoptical.co.uk/acatalog/FiltersNeutraldensityfilters-Glasstype.htm

The system works very well. I made the filter so that I could obtain
azimuths during the day using a theodolite when surveying ancient
megalithic sites. Also there is a method which has been proposed by which
the Old Kingdom Egyptians aligned their pyramids to the cardinal points and
I am doing some experiments to verify the method. However, I have also
taken a number of altitudes of the sun with face-left and face-right on the
theodolite to see if there was any noticeable non parallelism in the
filters. There was none that I could detect at the half minute level.

I have been pleased at how easy it is to get good altitudes using a
theodolite. When using it with the sun, however, it is a good idea to put a
small mask around the telescope so that you do not inadvertently stare at
the sun when putting your eye to the telescope.

This is an expensive way to make a solar filter for a theodolite and it
helps that I own a precision engineering company, so turning and threading
the lens and filter holders is not a problem for me. For taking altitudes
of the sun, it would be much easier to use a sheet of aluminised mylar
filter sheet that can be used for cameras or astronomical telescopes and
simply wrap it around the front of the telescope and hold it on  using an
elastic band. This method was not suitable for my requirements, however,
where I needed to be able to attach the filter and take it off without
upsetting the levelness of the theodolite.

Geoffrey Kolbe

At 12:24 12/02/2008, you wrote:
>
>
>I have finally found a really simple** way to prove to kids that you
>can locate yourself using the sun (in the surroundings of a campsite
>without the sea for a horizon) ... and that is to use a theodolite.
>
>It seems to work very well, I buy a cheap theodolite, spend a week
>fixing it so that I can measure the position of the sun, another
>realising I hadn't so much calibrated as bodged.
>
>Then eventually the sun comes out, I point the theodolite at the sun
>and focussing the sun on my hand, I align the sun so that it is
>visually located within the shadow of the telescope and .... hey
>presto, I confirm I am only 1 nautical mile away from real position.
>
>... but ... how do I know whether this is just a lucky observation?
>
>Obviously I can't simply look through the telescope and check the sun
>is centrally located with the cross hairs ... or can I? I wouldn't
>have thought I could just take my sextant filter and use them on the
>back of the theodolite .... or could I ... and surely I would not see
>the cross hairs.
>
>What if I bought some solar filter and put that on the front of the
>telescope? Would I (and children) be able to see the cross hairs then?
>
>.... and is there any danger to the theodolite from solar heating if I
>keep pointing at the sun?
>
>==============
>
>** SIMPLE WAY:- I tried using a false horizon with a few children and
>quickly discovered that it was virtually impossible to use. The basic
>problem is that whereas children can easily find one object in a
>telescope and then you only need ask "can you see the sea, or the
>sky", and adjust the angle appropriately, with a false horizon,
>you are trying to bring two points into view when there are 14 ways
>they can be hidden:
>
>1/2 the object can be out of view because you are pointed too high/
>low,
>3/4  too far left/right,
>5/6 the sextant is tilted toward the left/right
>7/8 or that the angle is too great/small.
>9-12 the sextant may be correctly angled to see the sun/reflection,
>but the child has moved to left/right or forward/back and can no
>longer see the reflection.
>
>and without physically being able to look down the same eyepiece as
>the children, it is impossible to work out in which way they are going
>wrong and therefore impossible to help them find the "two suns".
>

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