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>> I am coming around to your way of thinking on sextant mirrors.
>
> How gratifying it is to find a sinner who sees the light by thinking about
> it and convincing himself...

Careful George. "If you make people think they are thinking, they will love
you; but if you make people really think, they will hate you."   I do not
know the source of the quote, but have always liked it and valued people who
do not resent being made to think.

> As a general rule the mid-point of the reflecting plane of the index mirror
> is indeed aligned with the pivot axis of the index am. But it's in no way
> crucial that the axis should be placed exactly there, in the reflecting
> plane or across the middle of the mirror's area.. If it wasn't, then as you
> shifted the index arm, that mirror would be displaced sideways a bit, and
> in the end you might lose a bit of light because one edge would move in and
> shrink the width of the light-path to the horizon mirror. But it wouldn't
> alter the sextant readings, not a jot.

Understood, and a nice breakthrough in my thinking.  Somehow I felt it had
to be that way because traditional sextants were designed so the axis was in
the center of the mirror vertically AND in line with the rear-silvered
surface.  So I was always suspect of front-silvered mirrors in the same
mount as rear-silvered, but took it on faith until now.

> Alex Eremenko has commented that in his Russian SNO sextant that is indeed
> the case. The pivot line lies behind the front-silvered face of his index
> mirror, not on it, as I recall. And the only disadvantage that has resulted
> is that with such a geometry, a favourite trick for checking
> perpendicularity of the index mirror doesn't work. That trick is to look at
> at the continuity of the direct and reflected arc as seen in the index
> mirror. It only applies if the effective reflecting plane is exactly
> aligned with the pivot. Because, in the past, all sextants were made that
> way, that restriction on the use of the method was never made clear. Alex
> has to use a different method to check perpendicularity.

My Astra IIB Deluxe, like Alex's SNO-T, has a front-silvered index mirror.
Like Alex I have to use gauge blocks to align the index mirror.

Thanks again

Bill

   

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