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    Re: Artificial horizon question
    From: Brad Morris
    Date: 2009 Apr 21, 11:37 -0400

    John wrote:
    "The biggest problem I had, and in fact always have with the A.H., is that I 
    can't seem to bring the two images very close together laterally, despite 
    adjusting the mirrors of my Davis Mark 15 beforehand. It is hard to tell 
    whether the images are level with one another when they are so separated.  
    The lateral space varies suddenly and without apparent reason; that is I 
    can't figure out what I am doing that causes them to converge and separate.  
    Moving my eye from side to side behind the telescope doesn't help; also the 
    images seem to move towards and away from one another as I move the index 
    arm, but not in a predictable manner."
    If the lateral space varies, then the images are going in opposite directions. 
     They converge and separate, which describes, at least to me, the exact 
    motion you would see when taking a lunar observation and (at the risk of 
    confusion, yet one more time) the motion of the objects you would see when 
    you check your telescope for parallelism.  In particular, as John manipulates 
    his index, the sighted objects move in an unpredictable manner.  Each time he 
    touches the index arm, he is disturbing the plane of the arc, which slightly 
    moves the optical path, creating this apparent motion of the objects in the 
    Best Regards
    -----Original Message-----
    From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of George Huxtable
    Sent: Tuesday, April 21, 2009 9:21 AM
    To: NavList@fer3.com
    Subject: [NavList 8011] Re: Artificial horizon question
    Brad Morris wrote-
    "Well, I seem to have miscommunicated my answer, based upon the
    misinterpretation of it through various postings."
    That was a kind way of putting it. I had failed to follow his meaning, which
    was my fault, not his.
    And with hindsight, I think Brad had it right when he wrote-
    "I don't think John's problem is parallelism.
    I do think his problem is holding the sextant steady.  When you take a
    lunar, the moon and the star brush each other as you wobble the scope.  One
    goes left, the other goes right.  And hence the reason why I said the plane
    of the arc in the same plane as the two objects.  In this case both objects
    are the same object, just on a different optical path."
    Indeed, yes. When you measure an altitude above a horizon, the limb of the
    Sun brushes along the horizon, and you take the point where it comes lowest
    as indicating the sextant is vertical, where the reading should be taken.
    With an artificial horizon there's no such horizon line. One Sun image
    brushes past another, and only when they are aligned, one above the other,
    is the sextant truly vertical. So, in a way, that makes it easier, to get it
    vertical, and the setup is somewhat forgiving about whether it's exactly
    vertical or not.
    So when John sees a sideways displacement between the two Sun images, all he
    has to do is twist his wrist, a bit, holding the sextant handle, and one
    image should slide sideways past the other until they brush when the plane
    of the sextant is truly vertical. Does this describe the effect that is
    bugging him?
    If so, that renders my other other suggestions about his doubled image
    superfluous. Anyway, he's explained the cloche-type structure of his
    wind-shade, which is the right geometry for the job, and avoids the problem
    I had postulated, which could happen with a flat cover.
    By the way, my suggestion, of placing one Sun image above the order, then
    reversing their order and averaging, to avoid correcting for semidiameter ,
    wasn't very clever. All very well if the Sun is on the meridian, its
    altitude unchanging. But if it's rising or falling, it becomes quite
    complicated to correct for that movement when you want to average. Better to
    follow the procedures suggested by others.
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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