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There have been these comments about the same passage in the article by
Gordon, as follows-

From Douglas Denny, "page 129,  refers to methods of determining index
error, and specifically mentions inter-stellar angles as a means of checking
sextant accuracy.  He concludes it is impracticable as the superimposition
of stars cannot be made accurately enough,  and "the error in the readings
is unacceptably large".  He places the error as large as 5 minutes of arc,
which even I find surprising and considerably larger than I would have
thought."

From Frank Reed, "Gordon attempted to get IC for his sextant using star
sights. His results are amazingly poor --I can't imagine any sextant user
today not being amazed by a range of FIVE minutes of arc."

But is that really what Gordon wrote? These are his words.-
"To superimpose the direct and reflected images of a star.  ... With the
2.5x telescope on the Hughes sextant, a star image will superimpose over a
range of 5'."

His meaning in those words is by no means crystal-clear, but I interpret
them as stating that some overlap occurs between the images over a range of
5' adjustment of the sextant arc. I take it that he is trying to put one
star-disc above another, then reverse them, rather as one would do with a
Sun disc, and split the difference. He goes on, to refer to "the mean of
readings as one image is brought alternately up and down" and to "the point
of tangency of imperfect images", which reinforces that view. In which case,
that 5' range would comprise two whole disc-diameters of a star image, and I
would expect a careful observer to be able to assess the index error to
within an arc-minute or so. It would allow us to assess the resolution of
his telescope as just "poor", rather than as "grotesquely-bad".

===============

On another matter, Frank wrote- "If you look at the horizon through a
sextant with its telescope removed (wear your eyeglasses), you can get an
excellent value for IC --as good as you can get with a 7x telescope using
other tests."

I would like to see evidence to back that claim. I've no doubt that it's
true sometimes; indeed, often. All that would imply is that the horizon was
not a sharp line that day. That applies to horizons in UK waters most days.
If the horizon isn't sharp to start with, magnifying it won't improve it.
It's not some mysterious trick of the human eye. In case of doubt, compare a
horizon line with something that really is sharp, such as an overhead
power-line.

In other respects, I'm generally in accord with Frank's views about that
paper.

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.


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