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-------------- Original message from Bill <billyrem42@EARTHLINK.NET>: --------------


> > 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 blu! e 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 da! nce 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