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Frank,

On Sun, 30 Apr 2006, Frank Reed wrote:

> By the way, as a hypothetical, how  could you use a sextant with a dead
> section, from whatever cause, close to zero  degrees?
> The sextant would certainly
> be a nuisance, but it would still  work.

That's exactly what I am doing now with my SNO:-)
(See my previous messages on unexplainable errors near 0).
This section is actually only used for index correction.

So I cannot trust my index correction, neither from Sun nor from
stars, measured in the standard way.

I use the stars few degrees apart. There are several convenient pairs
visible from my balcony at this time of the year, and I am sure
one can find such pairs everywhere in the sky.

I use two pairs, one is Castor-Pollux, 4d30'.3 apart, another
delta-zeta Orion, 2d44'.2 apart. I use essentially the same method as
with the Sun: bring the pair together once on the arc, another time
off the arc. The average gives me (the negative of) IC,
and the difference twice the distance for control. My sextant permits
to measure up to -5d (off the arc).

This gives excellent results. The span in other star distances
taken everywhere on the arc is usually no kore than 0'.2 now,
with IC determined by this method. As my sextant holds its IC
pretty well, I think the method can be used.

Of course, if the arc is spoiled on a larger interval, say 10 degrees
around zero or more, one has to determine the IC from usual star distances
which fall on the unspoiled part.

Alex.

P.S. One can probably use some convenient electric wires
for the same purpose.

   

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