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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Using a theodolite as a (solar) sextant
From: Mike L
Date: 2008 Feb 13, 03:33 -0800
From: Mike L
Date: 2008 Feb 13, 03:33 -0800
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 Kolbewrote: > 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". --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---