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    Re: Error of Perpendicularity
    From: Alexandre Eremenko
    Date: 2006 Apr 17, 15:19 -0400

    I forgot to mention in my previous message that
    the angle measured in the numerical example was assumed 120d,
    about the largest angle available on modern sextants.
    (Larger angles produce larger errors).
    A simple consequence of this formula is that
    the "tolerance" in this angle is about 15'
    for "practical purposes", even for the Lunars.
    This partially explains why sextant is such a good
    and robust instrument: the errors in the final result mostly
    depend quadratically on the errors in adjustment.
    And a square of a small number is a VERY small number.
    This applies to the "side error" as well (perpendiculatiry
    of the horizon glass). Some experts even recommend not to adjust
    the side error completely, leaving some 0.5'. This simplifies
    bringing two star images together when measuring IC or
    star distances.
    An exception is the INDEX error which is simply added to
    the result. That's why one has to determine the index error
    very carefuly and precisely.
    On Mon, 17 Apr 2006, Alexandre E Eremenko wrote:
    > Dear Robert,
    > The only book I know that contains a COMPLETE
    > theory of sextant is Chauvenet. (I am sure that there
    > are others, but this is really complete, very concise,
    > and written in good English:-)
    > In particular, on p. 115 (vol. 2) the following formula
    > is derived:
    > Err=-2 L^2 sin1" tan(h/4),
    > where Err is the resulting error, in seconds
    > L is the deviation of your mirror from
    > perpendicularity (in seconds) and h the angle you measure.
    > In more modern notation this becomes:
    > Err=-0.00057 L^2 tan(h/4)
    > where both Err and L are in minutes now.
    > For example, if L=5'
    > Then Err=0.008 or 05", which is completely negligible.

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