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Frank Reed wrote-

>The other option (for getting shade error on a dense shade):
>Get a piece  of good welder's glass or equivalent, dark enough to look at the
>Sun straight  on. With no shades down, hold the welder's glass up to your eye
>and look through  the sextant. Because of its location, optical imperfection
>in this glass will  not matter. Now adjust the sextant for an ordinary IC test
>bringing the Sun's  direct and reflected image into limb-to-limb contact.
>Lower the sextant, set  aside the welder's glass, and swing in that dense
>shade.
>Whatever gap or overlap  is now present between the direct and reflected Sun
>images is due entirely to  the dense shade. Adjust it away and there's your
>shade error.
>
>If you're  worried about holding the welder's glass in front of your eye
>where it might be  heated by the focused light from the sextant's
>telescope, it
>also works fine if  you hold it directly in front of the objective lense
>of the
>telescope.
>
>Does that all make sense?

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

And I have to reply in response to "Does that all make sense?", -

No, it doesn't, not really. It misses the point. It's exactly the same
proposal that Henry Halboth put forward, of using a special shade over the
object lens of the telescope, and suffers from the same objection.

It fails, at the point where it proposes-

>Lower the sextant, set  aside the welder's glass, and swing in that dense
>shade.
>Whatever gap or overlap  is now present between the direct and reflected Sun
>images is due entirely to  the dense shade. Adjust it away and there's your
>shade error.

At the point where you have set aside the welder's glass, and swung in the
dense shade in the index-mirror view, you still have the full brilliance of
the Sun in the horizon-mirror view. Unless you introduce a similarly dense
shade into that direct view, your eye will be quite unable to take it. But
if you do that, you are measuring, not the error of the upper shade, but
the combined error of two shades (just as Lecky described), which was NOT
the object of the exercise.

It may be that shades on modern sextants are so good that attempting to
check them is unnecessary and irrelevan, for the purposes of ordinary
navigationt. Are they good enough to cope with the rigour of the sub-minute
testing that Alex has been indulging in, I wonder? I would not be so
confident, however, about my cheapo Ebbco sextant. The laminated shades on
that may be its weakest point. I just don't know.

What this discussion has illustrated is that measuring refraction errors of
the darkest shades is not quite the simple matter it's cracked up to be.

George.





===============================================================
Contact George at george@huxtable.u-net.com ,or by phone +44 1865 820222,
or from within UK 01865 820222.
Or by post- George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13
5HX, UK.

   

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