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It was the first starry night since I had calibrated the sextant, and
so I took it out last night to check the calibration on the North
Star, something I couldn't do with a liquid false horizon because I
found it impossible to find the reflected star image.

Unfortunately, I hadn't thought about just how high the pole star is
in the sky, and what with the odd screw adjuster sticking out from the
device I wasn't able to even view the star let alone get a good fix on
it.

I did however, try a few lower stars, and for the record, it is quite
possible to view the cross hairs, by shining a small torch into the
front of the telescope. WIth the x30 optics, I got a very clear image
of the star with a orange glow background (my torch had a red LED
setting which I preferred to white)

Now I realise that although the midday sun is well within the
theodolite's viewing angle (~30) in the summer the sun is going to be
a lot higher, ~70 at midday/midsummer and there is no way I will be
able to spot it with the theodolite as it is.

Is there any way to view the image at 90? .... Don't tell me a false
horizon! ARRRRRRRHHHHHHHHHGHHHGHGHGHGHGHGHGHHGHGHGHGHHGHGHGHGHH!

Is there any optics I can buy to put on the front of the telescope to
view at 90degree?

Maybe I should just buy a small LCD video camera and a handheld
monitor and use it on the theodolite ... or even a sextant!

On Feb 12, 5:00 pm, Geoffrey Kolbe 
wrote:
> 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 
fromhttp://www.knightoptical.co.uk/acatalog/FiltersNeutraldensityfilters-...
>
> 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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