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> Large mirrors collect more light and transmit more though the optics which are
> an advantage in taking twilight sights.

Joel

The above has always bothered me (especially as a photographer).  Indeed a
9' by 9' panel will "collect" and reflect more light more light than a 1' by
1' panel. The problem I have is that flat mirror is not a lens.  The scope
only sees one portion of it, no matter how large it is, so the mirror cannot
"collect" it and send it all to the scope.  In theory, as long as the index
mirror is tall enough to reflect the whole body when on the diagonal (that
being when the index arm is set at the largest number on the arc) it has
done its job.  Black or blue above or below is of no theoretical concern.

Of course it should be wide enough to cover the critical areas of the the
horizon mirror/glass.

As an analogy, I could use a 1' x 1' panel a foot away from the subject to
reflect sunlight for a close-up photograph a penny in the shade, or a 9' x
9' panel.  If they are at the same distance and angle, the penny will
receive the same amount of reflected light from either.  The 9' x 9' will of
course light a much larger area, but in this case it is overkill as I am not
looking at those areas, only the penny.

One could argue that if I move the reflective panel(s) closer to the penny
it will receive more light, or away and it will receive less light.  This is
true, as the panel(s) have become the main light source, so will obey the
inverse-square law.

But with a mirror and a point source (or close to it) at infinity that no
longer holds true.  All I need to see is the point source.  If I had the
wall of a building in the shade with the Sun behind it, a *large*
plate-glass mirror at my vantage point and out of the shadow might reflect
Sun and skylight onto a large area of the shaded building wall and shadow.
A hand sized mirror very little.  But both would have a hot spot on the wall
where they reflected the Sun's disk.  And in the final analysis, that disk,
or star/planet point source is all that concerns us.

Where has my reasoning gone off track?

> The second adavantage is that celestial objects tend to move around on the
> horizon mirror due to the motion of the vessel or of the sight taker. They
> also seem to move more when taking high altitude sights. The larger size
> mirrors provide more surface area for the body to dance on before being lost
> from view.

This I fully agree with,  It is also nice to have the index mirror wide
enough to play with faint reflections on the glass side of the horizon
mirror/glass.  Another slight advantage I see to a taller index mirror comes
into play when shooting high angles with a light source(s) behind me.  As
the apparent height of the index mirror, as viewed through the scope off the
horizon mirror, becomes less and less, the portion of the horizon mirror
that is not reflecting the index mirror becomes free to reflect anything
light behind it.  It can become bothersome.

Bill

   

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