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Peter Fogg wrote:
>>>>On Behalf Of Lu Abel
>>
>>Do you speak from experience or just theory about "plastic?" [sextants]
>>
>
> When I've had the chance to examine them I've found them disconcerting to
> use - for example, the index error was different each time I checked on one
> instrument, before and after taking a sight. It didn't inspire confidance. I
> know and accept that they have been used to cross oceans with, and possibly
> they are ideal for the task of teaching students.

These were Davis sextants or one of the others?   (Although I don't
believe there are any other brands of plastic sextants being
manufactured these days, I recollect there was a British one that came
out about the same time as the Davis sextants).

I can just speak from experience with my Davis.  I think most of us were
taught to check Index Error every time we take a round of sights.  I
check my IE both before and again after taking a round and have never
found a difference.  One thing I have noticed with my Davis is that
there is a bit of backlash error on the micrometer drum (maybe on the
order of 0.2').  Again, I was taught to always bring a body to the
horizon in the same direction (ie, always bring it down or always bring
it up) to avoid backlash error, so I consider this an annoyance (if I
overshoot bringing a body down I can't just bring it back up, I have to
go well back and try to bring the body down again).

And, let's not forget the original premise:  We are speaking of a BACKUP
system for navigation, not the primary.

Lu Abel

   

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