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    Sextant Error and Whole Horizon Mirrors
    From: Chuck Griffiths
    Date: 2002 Jul 9, 14:43 -0400

    Bruce Stark's recent posting in which he mentions the need for lunarians to pay
    close attention to mirror adjustment brings up a problem with errors I've run in
    to.
    
    While I've been measuring lunar distances, I've noticed that I have an error in
    my sextant that I can't remove or account for. This is easiest to see measuring
    interstellar distances because the angular distance doesn't change with time. If
    I move the images from one side of my whole horizon mirror to the other it
    appears that I have a mirror error. This error manifests itself by making the
    measured angle smaller when the images are on the area of the mirror closer to
    the frame and larger when the images are farther from the frame.
    
    Just to be perfectly clear I'll try to describe the problem one more time. If
    the image of two stars are brought into coincidence in the middle of the field
    of view of the horizon mirror (while carefully rocking the sextant to swing the
    arcs of course) the images will move apart or closer together if the images are
    moved from the center of the field of view to the edge of the field of view.
    Watching for this is a challenge as other lunarians will testify. I've had the
    best luck reclining in a chair with something to brace my arms on.
    
    I thought, at first, that this was due to the index mirror not being
    perpendicular to the frame. I've adjusted my sextant several times using both
    the image of the arc and using small cylinders placed on the arc (Celestair
    sells a product for this, but I just used something similar). After each
    adjustment I removed the resulting side error. But this apparent movement of the
    images from one side of the mirror to the other persists. I haven't taken enough
    measurements at different angles to say how consistent it is over the whole arc
    but it's usually in the neighborhood of about .2 arc minutes. I imagine that the
    problem is related to the fact that I adjust the index mirror at one point on
    the arc and as the index arm moves the pivot must not be perfectly parallel with
    the arc.
    
    I would be very appreciative if other list members with whole horizon mirrors on
    their sextants could comment on whether their sextants also demonstrate this
    error and whether they were able to remove it by adjusting the index mirror.
    
    Chuck Griffiths
    
    
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