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    Re: How does the AstraIIIb split mirror work?
    From: George Huxtable
    Date: 2004 Apr 23, 20:33 +0100

    About viewing the Sun by reflection in the unsilvered part of a split
    horizon mirror.
    This is a matter that we need to resolve, Clearly, there's still much
    misunderstanding about. I had said-
    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.
    I replied
    >> 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.
    and he now answers-
    >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.
    NO! As long as the glass is plane-parallel, the two images, by reflection
    from front and back surfaces of the glass, from a distant object align
    precisely with each other. There is no need to decide which is which, and
    no easy way to do so. Ken seems to think, perhaps, that the horizontal
    displacement between the front and back surfaces will give rise to a
    corresponding displacenment between the reflections, but this is not the
    >As Jim notes, and as all of us
    >observe regularly, there is a single bright reflection over the
    True, for the reason given above.
    >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%).
    With glass of refractive index 1.5, the reflection for light falling
    perpendicular to a glass-air interface is 4% from each surface. You have
    reflections from the two surfaces to add together, so 8%. And the light is
    falling on a tilted mirror in this case, at an angle of incidence of about
    30deg (depending on the details of the sextant's geometry) which increases
    the reflection further, but I have no figures for how much. The eye is
    notoriously bad at estimating different brightnesses. I do not think there
    is any conflict with what is observed.
    Ken argued that a partial blockage of the path of light to the objective of
    a telescope, by an object close to that objective such as the horizon
    mirror, would simply dim the image of what could be seen behind the
    blockage, and not obscure it.
    There's no dispute between us about that.
    I challenged Ken-
    >> 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.
    And he has answered-
    >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.
    We agree completely about all of that. If the horizon mirror didn't block
    off all the light coming from the horizon behind it, then the observer
    would see a view of the horizon, if a dimmed one. But he hasn't answered my
    challenge. We agree that in those circumstances he could see the horizon
    through light passing around the blockage he refers to. But how on earth is
    Jim managing to see the Sun, except by reflection in the horizon glass (and
    in the index mirror too, of course)?
    An indication that Ken is struggling with his arguments comes in this
    anecdote from an earlier mailing-
    >For example, if you're out in the dark taking a
    >sight and somebody suddenly switches a porch light on behind you,
    >then you will suddenly see a bright reflection of your telescope and
    >face in the clear horizon glass (two reflections, actually, you can
    >try it and see) and your observation will have to wait until the
    >light is switched off.
    I haven't tried that, but has Ken? It seems most implausible to me. Because
    of the way that the horizon mirror is tilted, it would reflect light into
    the telescope, not from the observer's face and telescope, but from
    somewhere above his forehead, perhaps from his porch-light. To see what Ken
    claims to have seen, you would need a reflecting surface perpendicular to
    the view-line of the telescope, or nearly so. I would suggest reflection
    off a horizon shade, perhaps, except that in the dark he is unlikely to be
    employing such a shade. Is there any other shiny surface, at right angles
    to the telescope line, and in its view? Or was he, perhaps, looking out
    through a window? Otherwise, his account of viewing his own telescope and
    face seems quite inexplicable. Unless there is something very odd about his
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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