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Frank

I think the power-line is quite possible with a 1 inch or better cable, a
good scope, and a dry high-pressure system.  I had been playing with sextant
parallax and the thickness of the power line needed before your post.

At 3 statute miles, a 1 inch power line would be >1" to the naked eye, and
parallax error created by the distance between the center of the index and
horizon mirror would be < 3".   If I figure my height of eye at 8 ft. on a
sailboat, then the horizon is 3.8 statute miles away, then my parallax error
is still >2" with a horizon that is probably harder to see sharply than a
power line.

I currently use closer power lines to test the precision (repeatability) of
the the operator/instrument combination, and gear backlash.

Bill

> The standard imaging resolution of the eye is just what you would expect from
> diffraction limits and the spacing of the cones. Vernier acuity, which goes
> waay beyond normal resolution, apparently involves only certain very specific
> discrimination tasks. Do these have direct relevance to sextant use? Some
> should, but it depends on how specific these tasks are. For example, it is
> clear that the eye can distinguish discontinuities in straight lines at very
> high angular resolution. So if I were to use a distant power line (or other
> narrow straight feature) to get the index correction of my sextant, I should
> be able to get very accurate results.

   

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