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Bill, Frank, Alex and all

The posts about using lasers for IE checks and the
effect of misalignment along the axis of the telescope
have triggered me off ....

If your sextant has a telescope using lenses with
'spherical' surfaces, as I assume most do, its design
will be based on the classical 'dioptre' based optical
theory. This theoretical treatment produces some
simple design algebra, but I believe that it only
works for what is called the paraxial ray which runs
down the centre of the optical components. It's not a
great surprise if different results follow from set
ups that have different ray paths, away from that
central axis. But then if you have a more expensive
telescope.....

And to refer back to some of the hail of previous
posts on index error.... Anyone doing a little
scholarly work in a visual optics textbook will also
come across descriptions of the process by which
change of focus can indeed, in some eye circumstances,
change the size of the image on the retina. Maximum
power in the image is not always the point of accurate
focus. Accurate focus is needed for accurate rendition
of the image size on the retina. General advice is
always to focus in from the 'short sighted' side.

Then you take the sextant out in the dark when the
eye's focusing mechanism doesn't always have enough
power in the image to operate properly.  Those of us
who use spectacles can test this by going out in the
middle of a dark night ( moon perhaps too full at the
moment ), wait for your eyes to adjust, then look at
the stars ( no instrument ) with and without the
specs. You will find that the specs don't make any
difference at all....

In summary, the sextant may be a beauty, but you have
to consider its telescope and after that the eye.

I'm no optician, but following the summer's furore I
briefly borrowed Tunnicliffe ' An Introduction to
Visual Optics', published by Brit. Assoc. Dispensing
Opticians, for the limited research I'm reporting
here.

Michael Bradley







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