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    Re: Sextant and Eye Optics
    From: Bill B
    Date: 2006 Dec 16, 18:07 -0500

    Bill, you wrote:
    > "Indeed being off axis does matter."
    Frank replied:
    > That would imply that you could only measure index error in the exact center
    > of the field of view. Collimation of the line of sight does not matter when
    > the angle is zero. That is, the error in angular measurement resulting from
    > being off-axis is zero when the angle you're measuring is zero.
    I make no bones about being a relative dummy, but a few areas concern me.
    First is the "defraction spike" which if memory serves Frank attributed to
    the laser beam passing through the scope.
    I find that if I use just the laser and scope, there is no defraction spike.
    If I adjust the scope on the sextant so it sees only the glass on the
    horizon mirror/glass there is no sharply defined, if any, refraction spike.
    If I use a zero-mag tube or no tube, and shoot through the glass there is no
    If I split the glass/mirror juncture with with the laser beam (no scope)
    there are sharp defraction spikes at close distances (10 ft).
    If I split the glass/mirror juncture with with the beam (using a scope)
    there are defraction spikes that can be focused with the scope at greater
    As a side thought, I wonder if a whole-horizon mirror will produce spikes?
    "That would imply that you could only measure index error in the exact
    center of the field of view."
    If I keep the entry point of the laser beam in the center of the rear of a
    sight tube, or the rear lens of the scope, and it passes through the center
    of the other end before striking the glass/mirror I measure the beams as
    65.5mm apart.  If keep the entry point centered and use either extreme of
    the glass/mirror, my measurement increases by >5 mm at 15 ft with the scope.
    More so with the sight tube.
    Apples to oranges, if I superimpose one star's images at the bottom of the
    field of view, will they still be superimposed if I use the top of the filed
    of view?
    I believe I can see that it will not matter with a zero-mag sight tube if I
    am parallel to the  line of sight but above or below it with my laser beam
    and the mirrors are parallel.  I do not have that much faith in the optics
    of the scope.
    Looking up "collimation" two definitions are:
    --To adjust accurately the line of sight of (a telescope).
    --Aligning lenses along line of sight to minimize aberrations
    If we were to take a photograph through most sextant scopes it would look
    more like a Monet cityscape.  Sharp in the center but progressively
    out-of-focus and distorted as it moved toward the edge.
    As to moving the beam laterally, but still parallel to the line of sight,
    there are physical limitations (as well as scope optics) to consider. To
    split the laser beam into two parallel paths/points (and get a sharp
    defraction spike) it must be be centered at the horizon glass/mirror
    boundary.  One could adjust the scope so the boundary was not in the center
    of the field of view, but again the optical quality of the scope is of
    concern to me.
    It seems likely to me that once set up the beam being off axis will affect
    the distance between resulting parallel beams, but they will still be
    parallel if the mirrors are parallel. So what does distance between beams
    matter for this test so long as they are parallel?  I will be able to test
    that when Alex and I test on a longer range.
    My concerns are repeatability and optics.
    By shooting down the center of the scope on axis (using masks etc.), I have
    been able to reset over 30 times at 40ft and 80ft and come within +/- 0.5mm
    of the same mean value (65.5mm).  Off-axis resets are all over the map.
    I have had to change focus between my 40 ft target and 80 ft target.
    Considering what I and others have perceived as shift in side error with a
    change of focus when the points are not superimposed already, I have serious
    concerns about precise collimation of the lenses in the barrel of my scope.
    The closer I stay to dead center, the more comfortable I am.
    Long and short of it, and perhaps illogically, I feel I am reducing
    experimental error by staying on axis and as close to dead center as I can
    As a side benefit, I am coming very close to a measurement of the distance
    between the index and horizon lines of sight.
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