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    Re: Index Correction
    From: Bill B
    Date: 2007 May 04, 16:44 -0400

    > From: "Robert Eno" 
    > It is common practice to check for IC before and after a round of sights. In
    > my experience, they are seldom the same particularly when taking sights under
    > non-standard temperatures.
    > Question: what correction would you apply?  Generally what I do is a
    > combination of fudging and black magic but it usually entails simply taking
    > the two ICs and averaging them to arrive at a single IC that can be applied to
    > the entire round of sights.
    > Please: I am not looking for involved statistical computations. I am trying to
    > determine what other navigators do in the field/at sea.
    Good question and one I am wrestling with currently.  Sextant, Astra IIIB
    In speaking with Alex, he maintains that a well designed sextant will not
    change IE based on temperature if you let it acclimate to the ambient
    temperature.  That is, after a 20-minute period (in the shade if doing sun
    shots) it should have the same IE/IC at 20F as 50F as 90F.  The IE should
    not change over a round of sights.  Alex has the silver finish on his SNO-T
    I get mixed results.
    One day I went out and did IE checks for 25 minutes sitting in the sun
    (after sextant had achieved ambient temperature and therefore stabile in
    theory).  Over that period the IE changed from 0!0 to 0!6 off the arc, with
    half the change happening in the first 7 minutes.
    Temperature was 76F, very clear sky, elevation about 47d, 3 kt breeze.
    The next day I went out at the same time, same chair, same place, same
    shirt, temperature 74 F, 12 kt breeze, *very* slight overcast.  At most the
    IE changed (again to off the arc) 0!1 over 27 minutes.
    I used the Sun's limbs for IE.  4SD was within 0!15 of Frank's site in a
    given three-minute set, closer if factoring in 1.55" per diameter increase
    for irradiation. Sigma was less than 0!1 for all off and on the arc averages
    from a given three-minute set, so I feel confident in that data.
    My thinking to date:
    I have noticed the shift off the arc in lengthy sun observations before.  I
    suspect heat from the sun and low ventilation on the first day allowed the
    black-finished sextant to heat up during the observations and be unstable.
    The slight reduction in energy due to the haze and wind kept it relatively
    stable the second day.
    So tough to predict.  Best guess is to do an IE check after letting it come
    up to ambient temperature, then at the end of the round of sights.  If they
    differ, average the IE and use that for IC.
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