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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@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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