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Geoffrey,

Ok, I understood.
Actually this is not exactly what we call "index error"
(at least for marine sextants)
but "combined index+arc+bubble+prismaticity-of-filters+
personal+
whatever systematic error".

Alex.

On Fri, 6 Apr 2007, Geoffrey Kolbe wrote:

>
>
> Alex, the index error of a bubble sextant can be determined from an unknown
> position.
>
> By taking sightings on bodies in opposition, the separation of the
> resultant position lines will, on average, be twice the index error.
>
> In my posting #2531, I advocated taking four sightings, a North-South pair
> and an East-West pair, on bodies at the Cardinal points. The North-South
> pair effectively give you your latitude and the East-West pair your
> longitude. The position lines from the four sightings should form a square
> box if index error is the dominant error. If the box is not substantially
> square, there is a non-systematic error somewhere which is of the same
> order or greater than the index error.
>
> See http://www.pisces-press.com/desert/14thMarch.jpg for an example of what
> I mean. In this example, the box is substantially square, giving us
> confidence that the dominant error is indeed index error. The size of the
> box is about 10' on a side, from which we deduce that the uncorrected index
> error was 5'. The position lines are all 'away from' the body in question,
> so the sextant altitudes are 5' too small. I need to add 5' to my altitudes
> to correct the index error.
>
> A subsequent round of sights at a different location a few days later show
> the results when this 5' is factored in. See
> http://www.pisces-press.com/desert/19thMarch.jpg
>
> Geoffrey Kolbe
>
>
> At 20:24 06/04/2007, you wrote:
>
>
>
> >Geoffrey,
> >
> > > It is easy to determine the Index Error of a marine
> > > sextant, but not of bubble sextants.
> >
> >Easy or not, but I infer from your message that you
> >can find the IC of your bubble sextant SOMEHOW.
> >
> >I don't see any way to do this at all,
> >except taking an altitude from a known position.
> >
> >This can be done once in a while, but certainly not
> >before or after an observation, if the purpose of
> >the observation is to find this position.
> >
> >Alex.
> >
> >
> >
> >
>
> >


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