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    Re: "Vernier acuity" of horizon IC tests
    From: Frank Reed
    Date: 2009 Jul 6, 23:39 -0700

    Bill M, you wrote:
    "In the SNO-T horizon mirror, there is a narrow band of paint overlap, about 
    half a millimetre, on the back of the mirror extending on to the clear glass. 
    This is true ( or should be true) of most sextants, as the paint protects the 
    edge of the silvering. Exceptions  might be those instruments with sealed 
    mirror cells. Its relevance is not clear to me."
    Unfortunately, the relevance is that it apparently no longer qualifies as a 
    vernier acuity task if there is a gap between the two sides of the field of 
    view. I'll have to try out the full range of sextants I have available 
    tomorrow and see if there's a significant difference for the different type 
    of horizon glass cells.
    But clearly, for that sextant, your results do not demonstrate ANY evidence of 
    vernier acuity. It's normal acuity. So, in this case, better results would 
    necessarily be obtained with a telescope than without. This "strange trick" 
    may be a lot less common in practical cases than I would have guessed.
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