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On 17/01/2014 5:43, Bill Morris wrote:
> The first attached photo shows the placement of the edges and the second
> the view when they are lined up. In the case of the Soviet SNO-M and
> SNO-T sextants two vanes are provided. They are nothing more complex
> than two pieces of angle aluminium with the top edges filed nearly to a
> knife edge, leaving a narrow untouched land about 0.5 mm wide.

I have been thinking about these vanes... If the directly-viewed vane
were rotated 90 degrees, that would allow you to position your eye more
exactly at the height of the vane, by lowering your sight until the top
edge appears. It seems it would be easier to do this with an edge that
is (I'm guessing) about 15 mm 'deep', rather than the 0.5 mm when viewed
head on as in your picture.
This would make it harder to compare to the other vane though, so a
compromise would be to rotate it 45 degrees. This results in a
horizontal edge still 0.707 times (=cos(45 degrees)) the width of the
view head on, while also giving the above benefit.
Crazy idea, perhaps!

> I am mystified as to how anyone could confuse the direct and reflected
> images and thus need the vanes or other objects to be of different
> colours.

This I don't quite understand either. Perhaps it's easier to explain the
procedure when one can talk about 'the red and the blue marker cap',
rather than 'the one on the right, which is actually on the left, but
looks like it's on the right'?

Luc

   

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