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    Re: How does the AstraIIIb split mirror work?
    From: Jim Thompson
    Date: 2004 Apr 23, 09:01 -0300

    Here is a summary of messages to date, and my new information in response to
    suggestions made by list members:
    
    1. The device.
    
    The AstraIIIb split mirror has a mirror in the right field of view, a clear
    glass on the left, and a black vertical blocking device that narrows the far
    left field of view slightly.  I have 3 sighting attachments: sight tube, and
    3.5x40 and 6x30 telescopes.
    
    2. The problem.
    
    The effect we are trying to explain is this: During daytime I find the sun
    with the 6x30 telescope and bring it down to the horizon.  The sun's image
    and the horizon appear to be present in both halves of the field of view as
    I rock the sextant, much like the whole horizon mirror I have been using.
    The whole horizon effect is still present to a lesser extent with lower
    magnification, even a sight tube.  Why is the whole horizon effect present
    using a split mirror, and why does the intensity of the effect vary with
    magnification?
    
    3. Observations.
    
    3a. Looking at the horizon (telephone lines against blue sky) with the index
    arm set to near zero degrees but just off zero to separate the horizon
    lines, and holding the sextant still, I see:
    
    3a1. Sight tube: the mirror is clearly visible on the right.  The left
    blocking device is not visible because it is outside the tube's field of
    view, toward the left.  The horizon is visible in the left and right halves
    of the field of view and the horizon image in each half seems to stop at the
    center (but I need reading and distance glasses now -- there could be a very
    slight fade either side of center from each half).  [NB: see 3a1 below!).
    2a2. 3.5x40 telescope: the mirror is visible on the right but its image is
    slightly fuzzy. The left blocking device is visible somewhat indistinctly as
    a dark line on the left. The horizon mirror's left clear-glass image of the
    horizon is blocked at the center from the right half of the field of view,
    but the index mirror's mirror-image continues from the right, fading to
    disappear just left of center as I look further toward the left of center.
    3a3. 6x30 telescope: the mirror on the right is merely a slight smudge on
    the right when I hold the sextant relative to my eye at a certain horizontal
    angle.  If I hold the sextant still and rotate my head/eye toward the left,
    the mirror's smudge covers more of the field of view from the right, and I
    can make out the mirror's vertical margin.  The left blocking device is not
    visible on the left.  The horizon mirror's mirror image is sharp on the
    right and fades left of center to disappear just before the left edge of the
    field of view.  The horizon mirror's clear window image is sharp on the left
    and fades toward the right, but varies depending on my eye angle.  If I
    first center the vertical margin of the mirror smudge in the center and look
    right ahead toward the center, then the clear window's horizon line crosses
    to the right of center, more than 3/4 of the distance across the field of
    view.  If I hold head and sextant still but turn my eye to the far right,
    the horizon line shrinks to a very short line on the left.  If I bring my
    eye back left to 1/4 from the right edge of the field of view, the clear
    window's horizon line expands to the center of the field of view.  If I
    bring my eye further back to 1/3 from the right edge, the clear window's
    horizon line expands to just right of center.
    
    3b. Here is what happens to the sun's image when I swing the sextant through
    azimuth left and right:
    
    3b1. Sight tube: The mirror is clearly visible on the right.  The horizon is
    blocked from the right half of the field of view.  The sun's image looks the
    same as with the 3.5x40 telescope.
    3b2. 3.5x40 telescope: The mirror is identifiable on the right but its image
    is slightly fuzzy.  The horizon is visible only in the left half of the
    field of view.  The sun's image does this: First I turn the sextant slightly
    in azimuth to put the sun's image in the far right field of view.  The sun's
    image is very bright and clear.  As I turn the sextant toward the left in
    azimuth along the horizontal, the sun's image moves leftward toward the
    center.  The sun remains bright and clear in the center.  As I continue to
    turn, the sun moves past center toward the left side of the field of view.
    As it moves to the left side it becomes increasingly dimmer but remains
    clear until it fades from view just before it reaches the far left field of
    view.  This image is somewhat similar to the "whole horizon" mirror.
    3b3. 6x30 telescope: the mirror on the right is merely a slight smudge on
    the right when I hold the sextant at a certain angle.  The horizon crosses
    the whole field of view, disappearing just left of the right margin.  The
    sun's image is brightest in the right half of the field of view, and less
    bright in the left half but equally less so all the way across to the left
    margin.  As noted in 3a3, the horizontal angle of the sextant to my eye is
    critical.  If my head/eye is twisted left relative to the sextant then the
    mirror is less visible as a smudge in the right field of view, and
    consequently the sun's image is less bright on the right.
    
    4. Explanations and more questions.
    
    4a. Joel and George proposed that the sun's image could be reflecting off
    the clear portion of the split mirror, since it grows dimmer toward the left
    side of the field of view.  Paul suggested that I cover the clear window on
    the horizon mirror with dark paper, so only the horizon mirror's mirror can
    project rays to the telescope.  I tried that with the index arm set to near
    zero degrees but just off zero to separate the horizon lines, while viewing
    the horizon (telephone lines against blue sky) and holding the sextant
    still:
    
    4a1. Sight tube: Blocking the clear window made the mirror's horizon line
    jump into view in the left half, but the background was slightly darker than
    in the right half.  When I alternated removing and replacing the blocking
    paper, I could see that the mirror's horizon was probably still present in
    the left half of the field of view when the clear window was not blocked,
    but it was so dim that it was very hard to see.
    4a2. 3.5x40 telescope: The mirror's horizon line was clearly visible to the
    right of center, but it was also visible past center toward the left.  The
    horizon image to the left of center faded gradually going to the left until
    it was not visible about 3/4 of the distance from center to the left edge of
    the field of view.
    4a3. 6x30 telescope: Without the blocking paper the mirror's horizon was
    clearly visible right of center, and although dimmer was also clearly
    visible right across the field of view to the left margin.  When I added the
    blocking paper to the clear window, the mirror's horizon line was clearly
    visible all the way to the left margin, but the background in the left half
    was a little darker.
    
    4b. Ken suggested that the "whole horizon" effect is more striking when I
    view the body with increasingly more powerful magnification, because the
    blocking device and mirror nearly disappear as depth of field increases.
    Magnification has a lot to do with this phenomenon, because just flicking my
    eye across the field of view dramatically affects what my brain sees (3a3).
    
    4c. The "blocking device" in the horizon mirror's left margin must be there
    to prevent clear window horizon light from polluting the body's image as it
    fades toward the left.  I am not sure about this, however.
    
    Jim Thompson
    jim2{at}jimthompson.net
    http://jimthompson.net/boating/Celestial_Navigation.htm
    
    
    

       
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