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    Re: Index Correction
    From: Fred Hebard
    Date: 2007 May 6, 09:20 -0400

    Alex,
    
    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.
    
    Fred
    
    On May 6, 2007, at 4:59 AM, eremenko---.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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