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    Re: How does the AstraIIIb split mirror work?
    From: Ken Muldrew
    Date: 2004 Apr 23, 09:36 -0600

    On 23 Apr 2004 at 8:46, Joel Jacobs wrote:
    > Ken, and Jim T,
    > You may be right, but I have a problem with your explanation because
    > of the double reflecting principal of a sextant's mirror system.
    > We know that when we look through a sextant's telescope, unless it is
    > set a zero elevation, the clear portion is viewed directly ahead, and
    > the reflected image we see is actually one half the number of degrees
    > of arc sighted. That means the eye is seeing an image in the telescope
    > that is above the viewer's line of sight. If this is correct, then how
    > can the result of seeing the image in both panes, be caused by
    > differences in focal length? IMO, the image is that which is reflected
    > off the index mirror to both sides of the horizon mirror and is from a
    > single source so that the focal length is equal.
    The important thing that changes with focal length (remember that the
    focal length refers to the objective lens of the scope, the actual
    focus is set at infinity) is the depth of field. The field of view
    (FOV) can also change, but the Astra sight tube, for example, so
    severely restricts FOV that this isn't really an issue. You can
    easily verify the fact that you can (apparently) see right through
    solid objects that are in front of the focal depth by putting your
    finger in front of your scope and looking at something far away. You
    don't see a total obstruction in the middle of the field, instead you
    see a dimming but everything in there. The reason you can see
    everything is because the lens in larger than your finger (if you
    have a small objective and large fingers, you'll have to use
    something smaller, maybe a pencil) and it gather off-parallel rays of
    light and redirects them into your eye. This effect fully explains
    why you can see the straight-ahead view through the horizon mirror
    and why you can see the reflected view through the horizon glass.
    Ken Muldrew.

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