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    Re: How does the AstraIIIb split mirror work?
    From: Ken Muldrew
    Date: 2004 Apr 23, 10:12 -0600

    On 23 Apr 2004 at 16:30, George Huxtable wrote:
    > In reply to my comments about a split-horizon mirror, as follows-
    > >> The reflection from the silvering is getting on for 100%. But even
    > >> an unsilvered glass surface reflects light to some extent, just as
    > >> you can see in a window-pane. Light can be reflected in this way
    > >> from both surfaces of the unsilvered part of the horizon mirror. So
    > >> you still see an image of the Sun in that part of the glass, but a
    > >> significantly dimmer one than in the silvering.
    > Ken Muldrew replied-
    > >This is hard to believe. Surely the front surface reflections would
    > >play havoc with the image if they were significant.
    > Why so, Ken? As long as the front and back surfaces are plane-parallel
    > then reflections in them will be precisely aligned with each other,
    > and quite undistinguishable.
    Because you would have equal-brightness reflections off both the
    front surface and the back surface. The only way to keep track of
    which was the front-surface reflection would be to track it from the
    mirror side to the glass slide and then keep in mind whether it was
    the lower or the upper reflection. As Jim notes, and as all of us
    observe regularly, there is a single bright reflection over the
    glass, and though it dims when moving from the mirror side to the
    glass side, it doesn't dim by 92% (or 96%? I can never remember
    whether a glass air interface reflects 4% or 8%).
    > >Isn't the phenomenon that
    > >Jim's talking about simply due to the fact that he's using a
    > >telescope? Sincd the telescope is focussed at infinity and the
    > >horizon mirror is right in front of it, the lens will gather light
    > >from off-parallel rays coming from behind the obstruction. These can
    > >then be refracted back into the field of view. Although this part of
    > >the image will be dimmer, it's still there. For example, if I stick
    > >my finger in front of my telescope (or binoculars, closing one eye so
    > >that it's a monocular), I can see the whole scene as if I'm looking
    > >through my finger. The part of the image that should be blocked is
    > >just dimmed, not extinguished. The greater the magnifying power of
    > >the telescope, the less the depth of field, so the further removed
    > >the horizon mirror is from the focus. With a sight tube (maximum
    > >depth of field), the mirror presents a more-or-less complete blockage
    > >because the eye can keep it in relatively good focus while still
    > >looking at infinity. With a 6x telescope, it will look like the
    > >mirror isn't there.
    > I ask Ken to explain how Jim could possibly see an image of the Sun
    > that way, looking AROUND the horizon mirror. Any such extraneous
    > light-path around the horizon mirror would give him a view of the
    > horizon only.
    The simple experiment I describe above (placing one's finger in front
    of the objective lens so that the center of the image should be
    blocked) illustrates the principle. When you do it, you see the whole
    image (though there is noticeable degradation of the image in the
    center). One might ask how you could possible see AROUND your finger
    to see that part of the image that should be blocked. The reason is
    because the objective lens is larger than you finger is wide, so the
    lens gathers light from off-parallel directions (around your finger)
    and brings them back into your eye. You can do the same experiment
    without a telescope, but now you need an object that is smaller than
    the pupillary opening of your eye. A piece of wire held just in front
    of your eye will not prevent you from seeing detail at infinity that
    should be blocked by the wire (of course, you have to close your
    other eye for this experiment). The explanation is the same.
    > So my explanation still stands, even if Ken Muldrew finds it hard to
    > believe.
    I remain unconvinced, but this is clearly not as simple as I thought,
    so I'm open to argument.
    Ken Muldrew.

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