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    Re: Index Correction
    From: Alexandre Eremenko
    Date: 2007 May 7, 16:06 -0400

    You wrote:
    > I doubt 15 minutes is enough time for a metal sextant to equilabrate
    > from room to winter air temperature, assuming it's not in the sun,
    > unless it is very windy.
    This is what some books recommend.
    (I did not try to estimate the time necessary myself).
    Then perhaps Robert will repeat his experimnt with his
    sextant, with waiting period 20-25 minutes?
    > Fred
    > On May 6, 2007, at 4:59 AM, eremenko@math.purdue.edu wrote:
    >> Dear Robert,
    >> What you describe sounds unusual and strange to me.
    >>> 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.
    >> It is unlikely that this is due to temperature, because
    >> 15 minutes is enough for temperature to level.
    >> I would try to investigate the reasons of this unusual
    >> behavior of the sextant. There is little doubt that
    >> this is not normal for IC of a metal sextant
    >> to change by .7' in 30 minutes.
    >> How exactly do you check IC (do you make several sights
    >> for it or just only one sight?)
    >> If you did not do any observations in between do you think
    >> IC would be the same in 30 minites?
    >>> 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.
    >>> Robert
    >>> ----- Original Message -----
    >>> From: "Bill" 
    >>> To: 
    >>> 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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