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    Re: Index Correction
    From: Robert Eno
    Date: 2007 May 6, 11:38 -0400

    G'day Alex,
    I guess the question in your first e-mail was answered already but I will go
    over it again.
    My sextant is a C.Plath. Difference in IC was 0.7 minutes of arc over a 30
    minute period. Outside temperature was minus 3C.
    I usually do an IC check by two methods: 1) using the sun's limbs and 2) a
    distant island with a range marker.
    A couple of explanations are possible: as Fred suggests in posting 2837,
    perhaps I need to leave the sextant out for longer than 15 minutes in order
    for it to reach an equilibrium with the outside temperature. I have
    certainly found this to be the case when taking a round of sights when the
    temperature is minus 20 C and below.
    Under ideal conditions, like when I have lots of time to play with, I will
    leave the sextant out for about 30 minutes, then take a round of practice,
    practice sights (that is, I don't record them, I just "work" the sextant),
    then I take the real round. The sights that I spoke of were taken during my
    lunch hour so time was of the essence.
    But have experienced this phenomenon in a variety of metal sextants
    including Cassens and Plath and Frieberger, so I do not believe that changes
    in IC are all that unusual. Granted, sometimes the determination of index
    error can be somewhat objective, however as I indicated in my e-mail, what
    is actually happening here is that side error creeps into the sextant and
    with it, a change in the index error. I am not exactly looking for solutions
    to this problem. I have long accepted that this is the way things work. My
    main question to the group is how do others manage it?  Thus far, a few
    people have agreed that averaging out the before and after IC seems to be
    the way to go. Having said this, I can tell you that while the frame of my
    sextant is solid bronze, the frame of the index mirror is constructed of
    aluminium. Could this be the reason for the change in IC over time?
    --- Original Message -----
    Sent: Sunday, May 06, 2007 4:59 AM
    Subject: [NavList 2836] Re: Index Correction
    > 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?
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