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    Re: "Vernier acuity" of horizon IC tests
    From: Bill B
    Date: 2009 Jul 07, 02:54 -0400

    > Close to the eye the line between the silvered and unsilvered protions of the
    > horizon mirror is out of focus. At arms length, it is better focussed, as in,
    > say, the foresight of a rifle. A sighting tube, by increasing depth of focus
    > by means of the eye stop, has the same effect.
    Some excellent points.  I might also question differences in scopes, their
    effective depth of field may vary based on the diameter of the objective
    lens and the exit.  My 3.5x scope has a mask at the exit.
    A trick I borrowed from Ken G's catalog with my first cardboard sextant
    (sight tube) was adding a mask with a small opening at the eye end to
    increase depth of field, especially when using the practice bubble horizon
    as I could not focus on all the elements without an increase.
    > Frank also wrote:
    > ?By contrast, when a telescope is attached or when the instrument is held
    > close to the eye, the horizon on the direct side of the field of view fades
    > away slowly and merges with the reflected view on the other side of the
    > field of view. We align these horizon images by superimposing them. This is
    > not a hyperacuity task, and so the results are limited by normal
    > resolution.?
    > and you responded:
    > ?It seems logical to me, that if a 7x telescope makes no improvement in the
    > eye's ability to align two images of a horizon, that's because either the
    > telescope is a very bad one, or, more likely, that the horizon is an unsharp
    > one (in which case, magnification will make it no sharper).?
    > A x 7 telescope is likely to be a Keplerian rather than a Galilean. ...
    > If we accept the reality of vernier acuity, and I think we must, we would
    > expect vernier acuity using the naked eye to be at least as good as merging
    > images using a telescope, as Frank has postulated earlier; and this is what an
    > appeal to experiment might show.
    A question based on depth of field.  If we double the magnification, does
    the "blending" area between the glass and mirror automatically double as
    well?  Could it get proportionately smaller (good) or proportionately larger
    (bad) depending on the scope(s).
    If hand held, performance can deteriorate as magnification increases.  Try a
    pair of 10X or greater binoculars on the water in a 30-40 ft foot boat with
    significant seas or swells.
    Another point is the target itself.  The crispness of a horizon against a
    sky is a fuzzy variable (groan).  Recalling visual perception classes from
    the psychological vantage point (some 40 years ago) and photographic lens
    resolution tests, it strikes me contrast between the line and the field is
    of some importance.
    Some of the tests with my "real" sextant (a bit more weather resistant my
    cardboard unit) were done employing David Burch's/Starpath "touch and leave"
    method. Record when you first perceive alignment, and when you perceive
    alignment is "broken."
    I used a distant phone line against the sky, and tripod mounted my sextant.
    Most of my belongings are in storage, so I have to work from memory, but my
    limits were burned into my memory.  Over several days I did perhaps 100+
    observations (split between turning the drum CW for one set then CCW for the
    next set to test for slack as well) with my Astra IIIB and the 3.5x scope.
    The mean was about 0.2 minutes between touch and leave, with a n-1 standard
    deviation much less than 0.1 minute in a given drum-rotation direction.  I
    do not recall the difference between CW and CCW rotation of the micrometer
    drum, but it was nothing to be concerned with if one follows SOP (always
    rotate the drum in the same direction for IE/IC and observations).
    My conclusion was that since I was doing touch and leave (a range of
    alignment with one end being *not* aligned, *my* man/machine limit was
    likely 0.2' or better under ideal conditions.
    Bill B.
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