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    Re: Sextant parallax and index error determination
    From: Fred Hebard
    Date: 2007 Nov 21, 09:24 -0500

    Frank Reed brought up interesting uses for a level with a laser
    guide.  One was to determine parallax.  The following may not be as
    accurate and ingenious as the method Frank outlined, or maybe not
    more accurate than your method as it's one I just cooked up, but....
    Set the laser on beam not fan, then point it through the reverse path
    of the optics, into the eyepiece of the telescope.  Set the index-
    error-corrected zeroed sextant to real zero (where two stars would be
    superimposed).  Place a flat object a known distance away,
    perpendicular to the beam and measure the distance between the two
    beams.  Voila, parallax.  This could be checked by converging the two
    beams with the micrometer and noting the angular difference from
    "real zero."  Also one could vary the distance to the object.  One
    might need to check for telescope focus and also point through the
    telescope center and pretty much exactly to where the telescope is
    aiming through the horizon mirror.
    One would first have to check that the telescope was on straight.  To
    do this, place the level on the sextant arc at the height of the
    telescope, beam to a point and mark the point.  Then point through
    the telescope ocular.  It should hit the same point.  If the level
    cannot be placed at the telescope height, measure the difference
    between the two.  Dominoes or dice from a good set can be used to
    raise the telescope off the arc to clear the index arm.
    On Nov 21, 2007, at 6:55 AM, Isonomia wrote:
    > I measured my EBBCO parallax yesterday the easy way by taking a
    > measuring of an edge at 5m and recording the angle (12' I seem to
    > remember) and came up with a figure of 74mm which I then used to
    > measure the distance to an object 30m away (saved the hassle of
    > measuring out the distance with a 5m tape in the rain!)
    > With my sextant the parallax at 254m is 1', so if you want an error of
    > less than 0.1' the horizon must be at least 2.5km away!
    > Mike
    > On Nov 20, 11:42 pm, Greg Rudzinski  wrote:
    >> Question for NAVLIST- Is the sextant parallax baseline measured from
    >> the center of the scope to the center of the index mirror? How far
    >> should the visible horizon be when used as a reference for
    >> determining
    >> index error? Bowditch offers no explanation.
    >> Greg Rudzinski
    > >
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