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    Re: Artificial horizon question
    From: Brad Morris
    Date: 2009 Apr 20, 13:19 -0400

    Hi John
    The lateral displacement of the reflected and direct images has to do with the 
    plane of the arc and the angle it forms with the two images.  That is, the 
    plane that contains the arc of your sextant does not also contain the two 
    You can see this quite readily if you can get to a location where you can see 
    two objects far away, when the angle is about 90 degrees on your sextant.  If 
    you wiggle the sextant, you will see the two objects brush each other as they 
    move in your scope.
    You can actually test the parallelism of your scope relative to the sextant 
    arc by this method. By wiggling (Bowditch called this a "vibratory motion") 
    the sextant, you should be able to see the objects brush each other on either 
    side of the scope.  If they touch only on one side and not the other, then 
    the scope is not parallel to the arc.  It takes a bit of practice but is well 
    worth the effort when done.  In particular, since artificial horizons yield a 
    doubling of the angle, and the parallelism becomes more significant as the 
    angle grows, you should minimize this error.
    Best Regards
    -----Original Message-----
    From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of JKP@obec.com
    Sent: Monday, April 20, 2009 10:59 AM
    To: NavList@fer3.com
    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.
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