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    Re: How does the AstraIIIb split mirror work?
    From: Joel Jacobs
    Date: 2004 Apr 23, 08:42 -0400

    Jim,
    
    Just a few things.
    
    1. The sextant must always be perpendicular to the plane of the horizon. If
    you tilt it only a little bit it will effect what you see, and change your
    perception. It also will change what you think is index error.
    
    Unless all your tests were made with a perfectly vertical instrument it is
    hard to judge the cause of the variations.
    
    2. With a change in scope power you get a change not only in depth of field,
    but you get a dramatic change in FIELD OF VIEW. This will also cause changes
    in what you perceive.
    
    3. As scope power increases the objects should appear sharper and
    vice-versa.
    
    4. I found Paul's suggestion noteworthy. However, I don't understand how if
    you block the clear portion of the horizon mirror from the mirror's front,
    how you can have an object seen on a non-reflective surface.
    
    Did you block it from the front or the back?
    
    5. If you are viewing the object by placing your eye off to one side of the
    center of the ocular lens, you also will be introducing additional
    variables.
    
    I applaud your thoroughness and inquisitiveness.
    
    I submit all factors must be accounted for, and taken into consideration as
    they relate too one another. If you've done that, I'm at a loss to explain
    some of the differences you are observing.
    
    I'm sending some additional comments in a separate message.
    
    Joel Jacobs
    
    
    ----- Original Message -----
    From: "Jim Thompson" 
    To: 
    Sent: Friday, April 23, 2004 8:01 AM
    Subject: Re: How does the AstraIIIb split mirror work?
    
    
    > 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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