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You're quite right of course about index error adjustment, and that you
need a smooth, clear and distant horizon for making an accurate
adjustment.  The 16' I was referring to was when I first received the
sextant, and just checked it quickly from my back deck against a close
terrestrial object.  When I actually took it for a clear horizon shot,
it was out by over 20'(!).

I've brought it down to pretty much zero error now, however if it's ever
out slightly from now on, I'll just allow for this in my calculations.

Lisa

George Huxtable wrote:
> Lisa Fiene wrote-
>
>
>>When I received my sextant from the Ukraine (I'm in Australia), the
>>index error was 16' at only 200 metres!  So, it needed quite a bit of
>>adjusting, let me tell you.  All's well now though.
>
>
> It's unfortunate that a sextant should be supplied that's so far out of
> adjustment. But it only involves adding or subtracting the measured index
> error, using a distant reference, so it's no more than a matter of
> preference whether it's reduced to nearly-zero, or not. In my opinion it's
> a waste of time worrying away at the sextant's mirror-screws to exactly
> zero the index error. In the end, some error will remain, or arise, so it's
> only a question of allowing for a small number or a larger number
>
> But more worrying still, in Lisa's reference to "at only 200 metres"! What,
> I ask, has 200 metres to do with the question?
>
> To adjust the index error, using a terrestrial object, then that object
> needs to be MUCH further away than 200 metres. This is because through the
> telescope your viewpoint is slightly shifted vertically from your viewpoint
> through the mirror, by, at a guess depending on the sextant, about 80mm
> (say, 3 inches).
>
> A perfect sextant, which has had its index error adjusted to zero using an
> object at 200 metres, will have a resulting index error, for objects at
> infinity, of as much as 1.4 arc-minutes.
>
> For most purposes, a (smooth) horizon is far enough away to use for
> adjusting index error, but for those that wish to work to ultimate
> accuracy, say for lunar distances, then a celestial body is marginally
> better.
>
> George.
>
> ================================================================
> contact George Huxtable by email at george@huxtable.u-net.com, by phone at
> 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
> Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
> ================================================================
>
>


--
Kind regards
Lisa Fiene
***************************

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