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    Re: How does the AstraIIIb split mirror work?
    From: Ken Muldrew
    Date: 2004 Apr 23, 15:14 -0600

    On 23 Apr 2004 at 20:33, George Huxtable wrote:
    > About viewing the Sun by reflection in the unsilvered part of a split
    > horizon mirror.
    > 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 case.
    Because the light coming down from the index mirror is not
    perpendicular to the horizon glass, the reflections from the front
    surface and back surface will be vertically displaced, due to the
    horizontal displacement. The reflected rays will be parallel, but
    displaced vertically. I just got a piece of plexiglass (9mm thick,
    but close enough) and verified that. By changing the angle of
    incidence, one can change the separation. One reflection is
    significantly brighter than the other, but they are both clearly
    visible. I don't see that in a sextant.
    > 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)?
    The off-parallel rays from the reflected image are also hitting the
    lens and being refracted back into the viewer's eye. It is as if you
    could move your eye over to the edge of the objective lens nearest
    the sextant and peer into the horizon mirror from this oblique angle
    to see a reflected sun that, to the normal line of sight, is behind
    clear glass.
    > 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.
    Last week I was doing a star-star sight in my backyard and my
    daughter turned the porch light on. The index glass was immediately
    flooded with reflections, completely obsuring my view. I was annoyed
    at the time and my memory of what was reflected may well be faulty. I
    withdraw this claim. I'll have to check more carefully. I apologize
    for writing as if I was more sure of this effect than I actually am.
    But I'm still confused about the front and back surface reflections.
    Ken Muldrew.

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