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    Re: Index Correction
    From: Robert Eno
    Date: 2007 May 4, 19:38 -0400

    Thanks Bill. This is more or less what I do.
    This afternoon, I took my sextant out for a round of noon sun sites and some
    post noon sites. After letting the sextant sit outside for about 15 minutes
    at minus 3 C, I commenced my IC check. IC was determined to be +0.3.  Took
    several observations for noon and afterwards, six post noon observations.
    Total elapsed time was about 30 minutes. IC check at the end of the round of
    sights amounted to -0.4.
    For the noon shots, I took +0.3 because the shots were taken literally a few
    minutes after the index error check. For the remainder, which occurred about
    10 minutes after the noon shots, I used the average of the two ICs. Results
    were good.
    Alex's contention that a well designed sextant should not change IC once it
    has been allowed to acclimatize for a set period of time is something I can
    agree with in principle, however, it does not jibe with my own observations
    over the decades. I own a C.Plath. You cannot get a better design than that.
    It has not been dropped, abused or insulted in any way, yet I often get a
    slow change of IC over a round of sights. More correctly, side error creeps
    into the sextant -- mostly under non-standard conditions -- which in turn,
    affects index error. Mind you, the change over time -- at least in my
    sextant -- is less than one minute of arc, so I should not complain but it
    is, nonetheless, aggravating.
    Further to the above, I can tell you for a fact, that the IC at plus 20 C
    will be radically different than the IC at minus 20. No if, ands or buts.
    Thanks for the response Bill. It is nice to know that at least one other
    person out there has faced the same situation.
    ----- Original Message -----
    From: "Bill" 
    Sent: Friday, May 04, 2007 4:44 PM
    Subject: [NavList 2819] Re: Index Correction
    >> 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
    > Deluxe.
    > 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
    > (tropical).
    > 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.
    > Bill
    > >
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