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    Re: Index mirror adjustment with offset mirror
    From: Gary LaPook
    Date: 2014 Jan 15, 10:54 -0800
    * A Davis manual, same
    * A SNO-T manual, which gives the correct procedure using the placement
    of 'diopters' on the arc. But these diopters appear to be L-shaped

    ====================================


    They are but you just use the top edge for your reference just like
    using the top edge of a stack of coins.

    gl



    From: Luc Van den Borre <luc{at}nuclide.com>
    To: garylapook---.net
    Sent: Wednesday, January 15, 2014 3:57 AM
    Subject: [NavList] Index mirror adjustment with offset mirror


    I recently bought my first sextant, a second-hand Cassens & Plath 'Bobby
    Schenk'. While trying to adjust the perpendicularity of the index mirror
    using the usually recommended procedures, I found myself getting
    increasingly confused. I think I have the correct procedure figured out,
    but I hope someone can check my reasoning below, because it would be
    surprising that all the well known manuals are incorrect or incomplete
    on this common procedure...
    
    As far as I understand (after much drawing of diagrams), the trouble is
    with sextants whose index mirror is not centred on the index arm pivot
    axis. On my sextant I'd estimate this offset to be 4 to 5 mm, in the
    direction of the eye when looking into the mirror. This changes the view
    enough that the most commonly referred to procedures don't describe what
    I am seeing, and give confusing and/or wrong instructions (1).
    
    Here's what I see when the mirror is misaligned (top angled forward
    slightly, towards the eye).
    
    When I look into the index mirror and angle the sextant in such a way
    that the mirrored section of the arc appears in the lower right corner
    of the mirror, the mirrored arc appears slightly _higher_ than its
    continuation to the right of the mirror.
    
    If I now tilt the sextant so that the mirrored section of the arc
    appears in the top right corner, it appears _lower_ than the part of the
    arc seen beside the mirror.
    
    My first impression was that the mirror wasn't flat! I then figured that
    I should probably aim for a good average, with the arcs lining up in the
    centre of the mirror. Or perhaps I should align the arcs at a location
    slightly higher on the mirror, along the reflected axis of the
    telescope. All wrong of course.
    
    If I am not mistaken this effect is not seen in sextants that have an
    index mirror exactly on the centre of the arc. In these less exhausting
    instruments, I imagine, a well-aligned index mirror will always show a
    continuous arc, whatever the orientation the sextant is held in (at
    least, I have convinced myself this is true for reasons of symmetry). A
    misaligned index mirror will show a stair step effect commensurate with
    the angle of misalignment, consistent whichever way you orient the
    sextant. I hope this is correct so far, I haven't got such a sextant to
    check this on.
    
    On my sextant however, due to the offset of the mirror even a
    _correctly_ aligned mirror will show a stair step effect growing in size
    towards the top of the mirror. Only at the bottom of the mirror, in line
    with the top plane of the arc, will the stair step effect disappear. But
    this becomes hard to observe since the mirror doesn't extend down far
    enough - hence the need for placing dice or some such on the arc. The
    top plane of these objects being parallel to the plane of the arc, but
    easier to view in the mirror.
    
    When the index mirror on my sextant is misaligned with the top angled
    backward, the mirrored part will always appear too low, though more so
    at the top of the mirror.
    
    The most confusing situation is when the index mirror is angled forward
    just a little bit. This make the mirrored arc appear too high at the
    bottom, but too low at the top of the mirror. When the angle of
    misalignment becomes larger, one effect overtakes the other, and the
    mirrored part appears always too high.
    
    In summary, the correct procedure (in short form) according to me is:
    
    1) place dice or some such on the arc (to construct a plane parallel to
    the arc)
    2) change your view so you are sighting along the top surface of the
    unreflected die
    3) adjust the mirror so the top surface of the reflected die lines up
    with its sibling
    
    * This procedure will work correctly for sextants with mirrors offset
    any distance (including zero) from the index arm pivot
    * Only not use dice if you're certain the mirror is not offset, or if
    the sextant is constructed in such a way that you can sight in parallel
    to the top plane of the arc.
    * It's not correct to use the top _edge_ of the dice. One must be
    looking along the top plane.
    * A stair step effect between the unreflected and reflected arc does not
    necessarily mean the mirror is out of alignment.
    
    Did I get this all correct?
    
    Luc
    
    
    (1) I consulted:
    * Bowditch, ed. 2002, p 267 (which is clearly wrong for this sextant),
    * Bruce Bauer's often recommended 'The Sextant Handbook' (which led my
    to wrongly try and line up the arcs in the center of the mirror, though
    this is followed by the correct procedure using dominoes),
    * Tom Cunliffe's 'Celestial Navigation', which only works for non-offset
    index mirrors
    * A Tamaya manual, not helpfull
    * A Davis manual, same
    * A SNO-T manual, which gives the correct procedure using the placement
    of 'diopters' on the arc. But these diopters appear to be L-shaped (see
    picture at
    http://www.cruisersforum.com/forums/f121/questions-about-a-russian-sextant-112909.html
    ), with a sharp edge at the top instead of a flat surface, which now
    makes me doubt again if I got everything correct.
    
    
    
    
    
    
    View and reply to this message: http://fer3.com/arc/m2.aspx?i=126390


       
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