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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@jimthompson.net
http://jimthompson.net/boating/Celestial_Navigation.htm

   

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