NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Eye problems and IE, IC
From: George Huxtable
Date: 2006 Jul 11, 08:26 -0500
Bill comes across as a careful and intelligent observer. His recent
observations are troubling him, and deserve our careful scrutiny. But
he needs to be aware that in working to fractional minutes in that
way, he is getting near to the limits of what's possible in terms of
the instrument he is using, and his eyes.
First, we can dispose of one discordant result as due to faulty
arithmetic. Where he refers to "Later tests done with sun Hc nominally
32d, left eye" and gets (in minutes of arc): Vertical average on the
arc 31.625, and vertical average off the arc 30.405, I make the
resulting index error to be 0.6' on the arc, not as Bill derives
0.11'. Which would make it completely concordant with his first
vertical set.
It would be interesting to know what magnification telescope is in
use.
The fact that different results are obtained with differing sextant
orientations leads to a strong suspicion of flexing under gravity
somewhere. Not very likely to be in the frame itself, more likely to
be in the fixing of the mirrors in their mountings, or in the fixing
of the mountings themselves with respect to arm and frame. If Bill
pushes the mirror-glasses slightly, at the corners (with a gentle soft
tool) is there the slightest sign of any motion? Clamping the handle
in a firm vice, can any shift be detected in a reflected image when
that is tried?
I'm aware that Bill has discounted gravity-flexure because of
differences in behaviour seen by left or right eye, but he has not
spelled out those differences for us in detail, so I am disinclined to
accept those assurances at face value. More information about that
please, better specified. One test for flexure would be to make the
same measurement with the sextant vertical (in its normal
orientation); then inverted.
Now for the odd effects that Bill reports in his vision, in which
circles are distorted in different ways by different eyes. How does he
tell this? If he looks with just one eye, is a circle, to him, clearly
an uncircle? By about what sort of percentage? Is it only when looking
with both eyes that he becomes aware of the discrepancy? And about how
much discrepancy, as a percentage of the diameter, is he aware of?
I am speaking here as a layman, but one with some experience of
optical defects. The human eye-system is clever; it projects an image
on to the retina, the pixels of which map to the brain as a perceived
image. Part of learning, from infancy, is correlating that image with
real life. If circles had always been ellipses on his retina, due to
some astigmatic defect in his optics, the brain would have mapped that
ellipse as actually corresponding to a circle. So if Bill in now
conscious of circles reading-out in his head as ellipses, it could be
because of a recent change, in optics or retina, that his brain hasn't
adapted to yet. Either that, or perhaps he wears specs with an
astigmatic correction, that his brain has adapted to, so he sees
distortion when he takes them off; as magnification in one plane, or
another.
But it shouldn't matter much to Bill's sextant problem, as I see it.
Bill is not using his eye-telescope combination to measure anything,
such as the height of a distant building, when using his sextant. He
is simply using it as a null-sensor, to determine coincidence between
the two views, of the top of the building in one view, and the bottom
of it in the other. It's the sextant, not his eye, that measures that
height. That's why you can switch to a telescope with a different
magnification, or even no telescope at all, and get the same result.
Now, about irradiation. There are special problems when using the Sun.
A lot of dark shade is needed to make it tolerable. Often, the darkest
shade for the horizon mirror is less black that that for the horizon
mirror, because the reflected horizon is much less bright than the Sun
is. Is that the case here? Or, with a full-view mirror, the two images
may not be equally bright, or may be differently coloured. Is that the
case here? If the two Sun images that Bill is comparing are not the
same brightness, for any reason, then the effect of irradiation
enters. This is a defect in every human eye, which causes brighter
objects to appear to have their boundary, with adjacent dimmer
objects, extended, so that they look slightly bigger. It differs from
one person to another, and for all I know, may well differ from one
eye to another. Is that part of the difference that Bill sees when he
shifts from one eye to the other?
The effects of irradiation could be removed by using for index
checking the horizon or a distant building, without shades. Perhaps,
for the Sun, a single shade, cobbled-in (securely !) just in front of
the telescope objective, instead of the normal shades, would equalise
the brightness (and also remove shade errors).
In reply to a question from Fred-
> Third, have you tried changing the position of tangency to try to at
> least get the SDs to agree?
Bill replied-
"Tangent" is by definition "tangent." It is not, nearly touching. It
is
not overlapping. It is tangent... "
Well, in cases where irradiation can have an effect, tangency is not
so clear-cut a matter as Bill seems to think it is.
I am puzzled about the changes in index error and side error that are
reported when adjusting the focus of the telescope eyepiece. Are these
observed effects clear-cut and reproducible? It's just that a point,
seen directly, should be exactly coincident with its reflection, when
observed through two mirrors which are exactly parallel in both
planes, no matter what the direction of observation is. So I don't see
how such changes to the telescope can change those errors. I wouldn't
be so foolish as to claim that these effects can't happen, in the face
of claims by two respected observers. But I ask, seriously, whether
that evidence is strong, reproducible, and watertight. And if it is,
can anyone suggest how it comes about?
George.
contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
=================================
Bill wrote-
|
| After a long series of tests, I have determined I have a problem,
and don't
| know how to adjust for it. I feel like I am overlooking something
obvious,
| but just can't get a handle on it.
|
| My left eye distorts a sphere, with the vertical axis longer than
the
| horizontal axis. My right eye distorts a sphere with the vertical
axis
| shorter than the horizontal axis.
|
| When doing IE checks with my left eye (which I use for observations
although
| being right-eye dominant as the left eye is sharper for some reason)
and two
| sun images my vertical and horizontal sextant positions yield two
different
| sets of results. (Before suggesting frame rigidity, I can reverse
the effect
| using the right eye.)
|
| Sun Hc nominally 63d, left eye
|
| Vertical average on the arc: 32!08'
| Standard deviation: 0!10'
| Vertical average off the arc: 29!17 = 30!83'
| Standard deviation: 0!10'
| 4 SD from Almanac: 63!2'
| 4 SD observed: 62!9'
| IE: 0!625' on the arc
|
| Horizontal average on the arc: 31!65'
| Standard deviation: 0!1'
| Horizontal average off the arc: 28!75 = 31!25'
| Standard deviation: 0!10'
| 4 SD from Almanac: 63!2'
| 4 SD observed: 62!9'
| IE: 0!20' on the arc
|
| Vertical minus horizontal on the arc: 0!43'
| Vertical minus horizontal off the arc: -0!42'
|
| If I achieve tangency with the left eye and rotate the sextant
(handle up or
| down) the images will separate. If I achieve tangency with the
right eye
| and rotate the sextant (handle up or down) the images will overlap.
Again, I
| don't think I can blame frame rigidity for the discrepancy.
|
| Later tests done with sun Hc nominally 32d, left eye
|
| Vertical average on the arc: 31!625'
| Standard deviation: 0!08'
| Vertical average off the arc: 29!595 = 30!405'
| Standard deviation: 0!10'
| 4 SD from Almanac: 63!2'
| 4 SD observed: 62!03'
| IE: 0!11' on the arc
|
| Horizontal average on the arc: 31!875'
| Standard deviation: 0!10'
| Horizontal average off the arc: 28!90 = 31!10'
| Standard deviation: 0!11'
| 4 SD from Almanac: 63!2'
| 4 SD observed: 62!975'
| IE: 0!39' on the arc
|
| Vertical minus horizontal on the arc: 0!25'
| Vertical minus horizontal off the arc: -0!305'
|
| NOTE: All sun IE checks were made without changing focus, as this
will
| change IE and side error with my scope.
|
| It makes some sense to me that as the sun begins to squish
marginally, the
| images appear closer to a sphere to my left eye, so vertical and
horizontal
| observations become closer.
|
| Using a natural horizon, flat-roof buildings, radio towers and tree
lines 3+
| miles away across the Wabash River valley I am observing -0!1 IE
(off the
| arc) +/- 0!1 minute (no filters). So this whole thing is driving me
up the
| wall. My clusters are tight.
|
| Are there some other tests the list might propose?
|
| More to the point, is there some way to mathematically null out the
eyeball
| problem and possible problems with shades (shades on sun but often
not on
| the horizon) factor when using a sphere (sun or moon) and a natural
(flat
| line) horizon with my left eye?
|
| Along the same line, if I am deriving distance from the top of an
object
| with its base below the water line (Bowditch table 15), should I
regard the
| top of the object as a point source, or is my vision causing the
object to
| appear taller, hence I am observing too large an angle between the
top and
| horizon? The question being, will that cause me to observe a larger
angle
| therefore calculate my distance closer to the object; so I need to
derive my
| own "constants" for Bowditch T15 etc. for my eyes?
|
| As a check, I ran a series of tests using shoreline objects of known
height
| whose bases were visible, and I always overshoot the angle needed to
make
| the trig work.
|
| Any thoughts would be appreciated.
|
| Bill
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To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
From: George Huxtable
Date: 2006 Jul 11, 08:26 -0500
Bill comes across as a careful and intelligent observer. His recent
observations are troubling him, and deserve our careful scrutiny. But
he needs to be aware that in working to fractional minutes in that
way, he is getting near to the limits of what's possible in terms of
the instrument he is using, and his eyes.
First, we can dispose of one discordant result as due to faulty
arithmetic. Where he refers to "Later tests done with sun Hc nominally
32d, left eye" and gets (in minutes of arc): Vertical average on the
arc 31.625, and vertical average off the arc 30.405, I make the
resulting index error to be 0.6' on the arc, not as Bill derives
0.11'. Which would make it completely concordant with his first
vertical set.
It would be interesting to know what magnification telescope is in
use.
The fact that different results are obtained with differing sextant
orientations leads to a strong suspicion of flexing under gravity
somewhere. Not very likely to be in the frame itself, more likely to
be in the fixing of the mirrors in their mountings, or in the fixing
of the mountings themselves with respect to arm and frame. If Bill
pushes the mirror-glasses slightly, at the corners (with a gentle soft
tool) is there the slightest sign of any motion? Clamping the handle
in a firm vice, can any shift be detected in a reflected image when
that is tried?
I'm aware that Bill has discounted gravity-flexure because of
differences in behaviour seen by left or right eye, but he has not
spelled out those differences for us in detail, so I am disinclined to
accept those assurances at face value. More information about that
please, better specified. One test for flexure would be to make the
same measurement with the sextant vertical (in its normal
orientation); then inverted.
Now for the odd effects that Bill reports in his vision, in which
circles are distorted in different ways by different eyes. How does he
tell this? If he looks with just one eye, is a circle, to him, clearly
an uncircle? By about what sort of percentage? Is it only when looking
with both eyes that he becomes aware of the discrepancy? And about how
much discrepancy, as a percentage of the diameter, is he aware of?
I am speaking here as a layman, but one with some experience of
optical defects. The human eye-system is clever; it projects an image
on to the retina, the pixels of which map to the brain as a perceived
image. Part of learning, from infancy, is correlating that image with
real life. If circles had always been ellipses on his retina, due to
some astigmatic defect in his optics, the brain would have mapped that
ellipse as actually corresponding to a circle. So if Bill in now
conscious of circles reading-out in his head as ellipses, it could be
because of a recent change, in optics or retina, that his brain hasn't
adapted to yet. Either that, or perhaps he wears specs with an
astigmatic correction, that his brain has adapted to, so he sees
distortion when he takes them off; as magnification in one plane, or
another.
But it shouldn't matter much to Bill's sextant problem, as I see it.
Bill is not using his eye-telescope combination to measure anything,
such as the height of a distant building, when using his sextant. He
is simply using it as a null-sensor, to determine coincidence between
the two views, of the top of the building in one view, and the bottom
of it in the other. It's the sextant, not his eye, that measures that
height. That's why you can switch to a telescope with a different
magnification, or even no telescope at all, and get the same result.
Now, about irradiation. There are special problems when using the Sun.
A lot of dark shade is needed to make it tolerable. Often, the darkest
shade for the horizon mirror is less black that that for the horizon
mirror, because the reflected horizon is much less bright than the Sun
is. Is that the case here? Or, with a full-view mirror, the two images
may not be equally bright, or may be differently coloured. Is that the
case here? If the two Sun images that Bill is comparing are not the
same brightness, for any reason, then the effect of irradiation
enters. This is a defect in every human eye, which causes brighter
objects to appear to have their boundary, with adjacent dimmer
objects, extended, so that they look slightly bigger. It differs from
one person to another, and for all I know, may well differ from one
eye to another. Is that part of the difference that Bill sees when he
shifts from one eye to the other?
The effects of irradiation could be removed by using for index
checking the horizon or a distant building, without shades. Perhaps,
for the Sun, a single shade, cobbled-in (securely !) just in front of
the telescope objective, instead of the normal shades, would equalise
the brightness (and also remove shade errors).
In reply to a question from Fred-
> Third, have you tried changing the position of tangency to try to at
> least get the SDs to agree?
Bill replied-
"Tangent" is by definition "tangent." It is not, nearly touching. It
is
not overlapping. It is tangent... "
Well, in cases where irradiation can have an effect, tangency is not
so clear-cut a matter as Bill seems to think it is.
I am puzzled about the changes in index error and side error that are
reported when adjusting the focus of the telescope eyepiece. Are these
observed effects clear-cut and reproducible? It's just that a point,
seen directly, should be exactly coincident with its reflection, when
observed through two mirrors which are exactly parallel in both
planes, no matter what the direction of observation is. So I don't see
how such changes to the telescope can change those errors. I wouldn't
be so foolish as to claim that these effects can't happen, in the face
of claims by two respected observers. But I ask, seriously, whether
that evidence is strong, reproducible, and watertight. And if it is,
can anyone suggest how it comes about?
George.
contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
=================================
Bill wrote-
|
| After a long series of tests, I have determined I have a problem,
and don't
| know how to adjust for it. I feel like I am overlooking something
obvious,
| but just can't get a handle on it.
|
| My left eye distorts a sphere, with the vertical axis longer than
the
| horizontal axis. My right eye distorts a sphere with the vertical
axis
| shorter than the horizontal axis.
|
| When doing IE checks with my left eye (which I use for observations
although
| being right-eye dominant as the left eye is sharper for some reason)
and two
| sun images my vertical and horizontal sextant positions yield two
different
| sets of results. (Before suggesting frame rigidity, I can reverse
the effect
| using the right eye.)
|
| Sun Hc nominally 63d, left eye
|
| Vertical average on the arc: 32!08'
| Standard deviation: 0!10'
| Vertical average off the arc: 29!17 = 30!83'
| Standard deviation: 0!10'
| 4 SD from Almanac: 63!2'
| 4 SD observed: 62!9'
| IE: 0!625' on the arc
|
| Horizontal average on the arc: 31!65'
| Standard deviation: 0!1'
| Horizontal average off the arc: 28!75 = 31!25'
| Standard deviation: 0!10'
| 4 SD from Almanac: 63!2'
| 4 SD observed: 62!9'
| IE: 0!20' on the arc
|
| Vertical minus horizontal on the arc: 0!43'
| Vertical minus horizontal off the arc: -0!42'
|
| If I achieve tangency with the left eye and rotate the sextant
(handle up or
| down) the images will separate. If I achieve tangency with the
right eye
| and rotate the sextant (handle up or down) the images will overlap.
Again, I
| don't think I can blame frame rigidity for the discrepancy.
|
| Later tests done with sun Hc nominally 32d, left eye
|
| Vertical average on the arc: 31!625'
| Standard deviation: 0!08'
| Vertical average off the arc: 29!595 = 30!405'
| Standard deviation: 0!10'
| 4 SD from Almanac: 63!2'
| 4 SD observed: 62!03'
| IE: 0!11' on the arc
|
| Horizontal average on the arc: 31!875'
| Standard deviation: 0!10'
| Horizontal average off the arc: 28!90 = 31!10'
| Standard deviation: 0!11'
| 4 SD from Almanac: 63!2'
| 4 SD observed: 62!975'
| IE: 0!39' on the arc
|
| Vertical minus horizontal on the arc: 0!25'
| Vertical minus horizontal off the arc: -0!305'
|
| NOTE: All sun IE checks were made without changing focus, as this
will
| change IE and side error with my scope.
|
| It makes some sense to me that as the sun begins to squish
marginally, the
| images appear closer to a sphere to my left eye, so vertical and
horizontal
| observations become closer.
|
| Using a natural horizon, flat-roof buildings, radio towers and tree
lines 3+
| miles away across the Wabash River valley I am observing -0!1 IE
(off the
| arc) +/- 0!1 minute (no filters). So this whole thing is driving me
up the
| wall. My clusters are tight.
|
| Are there some other tests the list might propose?
|
| More to the point, is there some way to mathematically null out the
eyeball
| problem and possible problems with shades (shades on sun but often
not on
| the horizon) factor when using a sphere (sun or moon) and a natural
(flat
| line) horizon with my left eye?
|
| Along the same line, if I am deriving distance from the top of an
object
| with its base below the water line (Bowditch table 15), should I
regard the
| top of the object as a point source, or is my vision causing the
object to
| appear taller, hence I am observing too large an angle between the
top and
| horizon? The question being, will that cause me to observe a larger
angle
| therefore calculate my distance closer to the object; so I need to
derive my
| own "constants" for Bowditch T15 etc. for my eyes?
|
| As a check, I ran a series of tests using shoreline objects of known
height
| whose bases were visible, and I always overshoot the angle needed to
make
| the trig work.
|
| Any thoughts would be appreciated.
|
| Bill
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
