NavList:

For what it’s worth here’s how I would look at the alignment procedure:

Perpendicularity of the index mirror is achieved by ensuring that the plane of the sextant’s arc coincides with its image when reflected in the index mirror. This is true for all sextants whether the index mirror is offset or not. In general, without other constraints, it is not possible to determine whether 2 planes coincide by lining up just one point on each.

Sighting along the top of the dice as described in other posts presumably ensures that the whole plane of the top of the real and reflected dice coincide. This may not be necessary however. If the real and reflected dice were offset slightly it would be enough to check that the top face of one is a smooth continuation of the top face of the other. This may offer advantages compared to exact superposition.

For sextants whose index mirror surface lies on the index arm axis, perpendicularity can be achieved in another way. In this case coincidence of the planes will make the sextant arc and its image in the index mirror lie on the same circle. Hence perpendicularity is achieved by making the edge of the real arc and its image be smooth continuations one of the other.

Note we are talking about objects and images under reflection in a plane mirror coinciding in space and none of it depends on where the eye is placed. If variation is seen as the eye is moved then parallax is operating. The object and image are then not at the same location in space and the adjustment is off,

Robin Stuart

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