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    Re: Artificial horizon question
    From: Gary LaPook
    Date: 2009 Apr 20, 23:48 +0200

    JKP@obec.com wrote:
    Actually you should touch the top of one image to the bottom of the 
    other image rather than superimposing them, you get a clearer match 
    point. Depending which way you do this you will either get an upper limb 
    or a lower limb observation. Since the reflected image in the artificial 
    horizon is inverted, the top edge of that image is actually the lower 
    limb. So touching the top of that image to the  bottom of the image in 
    the sextant index mirror results in a lower limb observation.
    You then have to apply sextant corrections and it is important in which 
    order you apply them. First apply the index correction. Then divide by 
    two since the measured altitude is twice the actual altitude.  If you do 
    this in the wrong order you will get the wrong answer. An example should 
    make this clear Let's say you measure 71 degrees between the top edge of 
    the reflected image with the bottom edge of the sextant image, a lower 
    limb observation, and the index correction  is minus one whole degree. 
    Subtract the one degree leaving you 70 degrees which you divide by two 
    to obtain the sextant altitude of 35 degrees. But if you divide by two 
    first, 71 divided by two equals 35 � 30' then subtract the one degree of 
    index correction you end up with the incorrect altitude of 34� 30'. Then 
    apply refraction and then semi-diameter. Obviously you don't apply dip.
    > Brad,
    > Tnhnaks for your advice.  I'll see if I can check the parallelism as you describe.
    > GTeorge, Thanks for your advice as well.  I will either get a piece of black 
    glass (and devise a means of leveling it as I have read in past posteings) or 
    at least try some sort of non-reflective treatment for the pan bottom.
    > I don't particularly suspect that I'm getting a double image from the oil 
    pan, however.  When sighting the sun, I use a colored cover over the 
    artificial horizon pan, and different colored filters between my eye and the 
    sun, and I get two distinctly colored images, say green from the sky and 
    yellow from the pan.  It is these that can't seem to be brought very near 
    each other horizonatally.  That is, until for one luck second they slip right 
    over one another, and if I'm quick with the micrometer I can get my shot 
    beforfe they agaion wobble apart.
    > By the way, I had also meant to ask:  Should I ignore altitude correction 
    when using an artificial horizon?  Clearly no correction for semi-diameter 
    should be needed, because I am superimposing full discs of the sun and its 
    reflection, center-over-center, rather than touching the upper or lower limb 
    to the horizon.  But I guess I can't grasp the principle behind altitude 
    correction sufficiently to decide whether it counts in this situation.
    > -John
    > >
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