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    Re: Off Center Sextant Scope Observations
    From: Frank Reed
    Date: 2007 Dec 02, 22:22 -0500

    George H wrote:
    "However, as long as everything  IS kept to the same plane, symmetrical left
    and right, it doesn't matter a bit whether the viewline is a bit up or down,
    the laws of reflection in a plane being quite unaffected. "
    
    I agree with this completely on theoretical grounds. But I can say that I
    "think" that I have seen exceptions in plastic sextants. Where it
    originates, I can't guess. It's within the expected accuracy limits for
    plastic sextants (one or a few minutes of arc), so I haven't seen any reason
    to worry about it. I have not seen any "up-down" change in alignment using a
    quality sextant. Therefore, I agree with your advice: don't worry about
    where you make contact in the field of view.
    
    On a related note, if you're worried about collimation, which only becomes a
    significant factor at large angles and requires some care in aligning images
    "letf-right", the main thing to remember is that you want to minimize the
    angle. So if you're measuring the angle between two stars, for example,
    after you make the initial alignment bringing the reflected and direct image
    together, you should wobble the sextant around so that the side-by-side
    images appear at various points in the field of view. If you don't see any
    change in separation, then you have nothing to worry about --any collimation
    effect is below your ability to detect. If you see a slight up-down
    separation when the images are on the right side of the field of view, no
    separation at some point near the middle (doesn't have to be centered
    exactly), then a slight up-down separation again on the left side of the
    field of view, then you should do your exact alignment at that point in the
    field of view where there was no separation. BUT if the separation decreases
    steadily across the field of view until you get to the edge, THEN you have a
    significant collimation problem (the minimum is outside the telescope's
    field of view) and you should find a way to adjust the alignment of your
    scope (e.g. using a laser level, as I've described previously and
    demonstrated at Mystic Seaport). Again though, if you don't SEE any change
    in separation, then this is something you simply don't have to worry about
    (because it's below your ability to notice with the magnification you're
    using, or because the angle you're measuring is small enough that it isn't
    an important effect).
    
     -FER
    www.HistoricalAtlas.com/lunars
    PS: it's obvious if you know it already, but not if you don't: "left-right"
    motion of the image here refers to motion in the field of view towards and
    away from the frame of the instrument. It's left-right literally only if the
    instrument is held in the usual vertical orientation. And so "up-down" is
    motion parallel to the frame.
    
    
    
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