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    Re: Suitable Sextants - Mirrors
    From: Joel Jacobs
    Date: 2005 Oct 12, 11:36 +0000
    Hello Bill,
     
    Your analysis though interesting, fails to take into account that a sextant's mirrors are not used in a static state, and hence size does make a difference. Consider that the platform is moving directionally, and rolling and pitching all at the same time. There also is motion introduced by the user when he rocks his arm to align for perpendicularity. BTW, my empahsis was on the horizon.
     
    Back in the early 1970's the U.S.Navy came up with a set of suggested improvements for sextant design. They included larger miror size, and having a  scope with a larger objective lens, transmitting more light rays for twilight sights. These were incorporated by most manufactures of serious instruments whose mirrors now are all about the same size. 57 x 42mm and 57mm dia.
     
    Joel Jacobs
     
    Joel
     
     
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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
       
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