Measuring devices: Sextant

Posted on su 15 huhtikuu 2018 in Articles

Sextant is used for measuring the angles between two ( far away ) objects.

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Sextants are quite simple but as they are really accurate measuring devices, one should always treat them with care and respect!  



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Notice that one needs to use targets lower or upper edge if target is moon or sun as their center point is hard to define with eye: edge is a lot easier.

So, with the sextant one can measure angle between to objects. Normally this is done with horizon and the target. But horizon is not always seen, so one can also use thing called artificial horizon. More expensive sextants have that built in but one can either create it themselves or use reflections from water or other reflective surface.

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Errors with sextants

Index error is easy to see: just point sextant towards horison and also align the reflected image to show horizon. Now, your sextant should show 0 angle. Otherway round is to put your sextant into 0 deg and see if horizon looks to be in same level in both.

Index mirror error

The parts marked with red should align in the index mirror image and in the device's frame. If not, the index mirror is positioned wrong.

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In this case index mirror in leaning too much forward.

Secondary mirror errors

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If one sees as double or part of the image is cropped out, adjust the mirror where you see this faulty image ( secondary mirror ).

Mirror is not in direct angle to the frame if tilting the sextant means that aligned pictures disalign. Image will move but for example horizon line should stay aligned in direct view and reflection.

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Other error come from example temperature changes, how one reads the angle of sextant ( user error ), how measurement is done lot higher than from sea level and so on.



This is an good story about sextant saving lives ( read README ):

https://github.com/timonoko/sextant