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    Re: Artificial horizon question
    From: George Huxtable
    Date: 2009 Apr 20, 18:45 +0100

    I wonder whether the root cause of JKP's doubled images, sideways, is the
    result of sextant misalignments, as Brad has suggested.
    I tentatively offer an alternative possibilty. He is seeing two reflections,
    one from the surface, and one from the shiny bottom of the oil-pan.
    That's the reason for using an opaque liquid, such as treacle (molasses to
    Americans) or old engine oil. Or why, if a glass reflector is used, black
    glass is preferred. Otherwise, John could try introducing a non-shiny
    surface to sit on the bottom of the tray, such as fine sandpaper, fine sand,
    cloth, matt black paint. The aim is to ensure that any reflection comes off
    the top surface, and isn't confused with another from the bottom.
    John has no call to apologise in describing his experiences. He has
    described well what he sees, and we can all learn by trying to interpret
    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.
    ----- Original Message -----
    Sent: Monday, April 20, 2009 3:59 PM
    Subject: [NavList 7998] Re: Artificial horizon question
    Since the topic of artificial horizons has returned, let me bore you all
    with a newbie's experience in learning to use one.
    Last night I made my first attempt at a star sight with the Davis artificial
    horizon.  The pan was filled with mineral oil and sat on a table, uncovered.
    I used no "house" as the weather was absolutely calm.  I found the star's
    reflections were pretty faint (I am in a city) but could be picked up by
    bringing my eye very close to the surface of the pool.  I then had to slowly
    back away and simultaneously straighten up while keeping my eye on the
    reflected image of the star in the pool until I was standing confortably,
    some six or seven feet back from the table, and could still see the
    reflection.  WIth the sextant set at zero I looked through the scope at the
    star and brought the instrurment down while swinging the arm so as to keep
    the star in the index mirror until I was again looking into the artificial
    Sirius and Arcturus were bright enough for this method, but Betelgeuse's
    reflection just could not be seen once I began to back away from the table.
    I remembered reading some postings in this list's archives about black glass
    plates, etc., so I took the dark blue shade that comes with the Davis A.H.
    and dropped it into the bottom of the pan of mineral oil.  It helped
    somewhat, but not really enough.  I may go out looking for some black glass
    this week.
    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.
    Can anyone suggest why I might be having this problem and what I could do
    about it?
    -John P.
    Navigation List archive: www.fer3.com/arc
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