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    Re: How does the AstraIIIb split mirror work?
    From: Joel Jacobs
    Date: 2004 Apr 23, 08:46 -0400

    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.
    George Huxtable says  >>Some old sextants would take advantage of this, by
    arranging to reduce the brightness of the ray from the Sun, by moving the
    telescope exactly parallel with itself so as to look only at the image in
    the unsilvered part. >>
    I'm uncertain of this, but I take "by moving the telescope exactly parallel
    with itself " to mean moving the telescope on its rising piece "in or out"
    from the frame and consequently, towards or away from the silvered portion
    of the mirror. If that's what is being described, contemporary sextants,
    still have that feature.
    Adding to what George said, the fork of the scope is grooved to allow
    lateral movement so that you can adjust for differences in clarity. When you
    move the scope out, favoring the clear side, it helps see a non-distinct
    horizon more clearly or if moved in towards the frame, it will help increase
    the visibility of faint stars.
    In Jim T's experiments some of the differences he is observing may be due to
    the fork or the groove in the fork being different lengths.
    Elsewhere, Doug Royer added some comments about the product RainEx. That
    product came along well after I was no longer navigating. I've used it on
    windshields with good results and see its advantages on mirrors. Never
    thought about it, but it is a good suggestion.
    Joel Jacobs
    ----- Original Message -----
    From: "Ken Muldrew" 
    Sent: Thursday, April 22, 2004 9:55 PM
    Subject: Re: How does the AstraIIIb split mirror work?
    > > Jim Thompson asked-
    > >
    > > >How does the body appear across the entire circular view of the
    > > >telescopes when using a split mirror?
    > George Huxtable answered:
    > > I don't know about the Astra IIIb, but here, I hope, is a general
    > > answer to Jim's question.
    > >
    > > 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.
    > This is hard to believe. Surely the front surface reflections would play
    > havoc with the image if they were significant. 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
    > in front of it, the lens will gather light from off-parallel rays coming
    > 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.
    > 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
    > 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.
    > a 6x telescope, it will look like the mirror isn't there.
    > Ken Muldrew.
    > > Some old sextants would take advantage of this, by arranging to reduce
    > > the brightness of the ray from the Sun, by moving the telescope
    > > exactly parallel with itself so as to look only at the image in the
    > > unsilvered part. Indeed, this is the only purpose I can see in having
    > > that unsilvered area of glass at all. Otherwise, in a split-horizon
    > > sextant the horizon mirror could just as well be truncated at the edge
    > > of the silvered part.
    > >
    > > On the BBC reenactment, in the Endeavour replica, of Cook's passage
    > > North of Australia, the navigators, who were observing lunars for
    > > longitude, had modern sextants, both split-horizon type and
    > > whole-horizon type. They commented, when trying to locate the Moon
    > > high up in a bright daytime sky, that it was very difficult in a
    > > whole-horizon sextant, but the greater contrast of a split-horizon
    > > instrument allowed the Moon to be picked up much more readily.
    > >
    > > George.
    > >
    > > ================================================================
    > > 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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