NavList:

   

Reply

"Royer, Doug" wrote:

> Whatever the procedure
> states in an owner's manual reguarding how to accomplish  the task is what I
> follow without giving it to much thought.

It is very easy to see. Forget the sextant and place two cylinders side by side
on a table on a line parallel to the front edge. Look from two feet away, move
your head up and down and the cylinders will remain aligned. Now push the one
cylinder two inch back. When you move your head up and down, the cylinder
farther away will follow the motion of the eye. It will seem higher, when
looking from above. But when the eye is level with the cylinders, they will be
aligned again.

Conclusion: It depends on the length of the light paths between the eye and the
cylinders. If they are not the same, you get parallax.

Now take your sextant and study the light paths in the set-up for the
perpendicularity check. The only additional complication to the above is that
one light ray gets reflected near the front edge of the mirror. Everything else
remains the same. Clearly, when the mirror surface is aligned with the pivot,
both rays are always the same length regardless of where you position your eye
in the plane through left cylinder and mirror edge.

But when the surface is not aligned with the pivot (never mind whether that's
the front or back of the glass), one path will be shorter. Then you get the
effect of parallax unless you align your eye with the cylinders parallel to the
reference plane. The role that the table played in the above simple
demonstration is now assumed by the plane through the centre of the telescope
and parallel to the limb. After all, this is the plane with regard to which we
want to check the perpendicularity of the index mirror!

I said all this already in my post this morning, but skipped over the
explanation of parallax itself, assuming that everybody on the nav list knows
that effect.

When describing my experiment, I also indicated that with a back silvered mirror
the effect might not disappear but rather become the opposite of what one would
expect from a front silvered mirror. There is a tendency that the mirroring
surface will lie on the other side of the pivot, swapping long and short path.
The effect is hard to quantify, because the frame of the mirror gets in the way.
This may be a simple reason why it's not noticed as frequently as with a front
silvered mirror.

Herbert Prinz

   

Reply