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Jim Thompson wrote:

> I just repeated this on the setting sun, 6x40 scope.  The sun was
> about 8 degrees above the horizon. I brought the sun's image down to
> the horizon and then blocked the clear window from the front
> (telescope side).  Holding the sextant vertically, the sun's reflected
> image disappeared at the center when I turned my head and sextant
> horizontally to swing the image across the FOV. The sun's reflected
> image disappeared before center on the right if I tipped the sextant
> from vertical toward the right, but persisted almost all the way
> across to the left if I tipped the sextant from vertical toward the
> left. So as others have pointed out, verticality is critical.

Hi Jim,
I tried this (bringing the sun down to a neighbor's roof, though) and I
think the whole issue is now clear to me. With my 5x36 scope centered
on the horizon mirror (I pull the eyepiece back as far as I can so that the
mirror is plain, if not well focussed, and then move the scope back and
forth on its mount until the split line is in the center of the field of view;
the scope is then re-focussed to infinity), and the glass part totally
blocked between the scope and the horizon glass, I can rotate the
sextant so that the reflected sun moves all the way across the field to the
right but when rotating the sextant so that the reflected sun moves to the
left, the image of the sun is blocked close to, but not quite at, the left
edge of the field of view. If I move the telescope out from the sextant (I'm
using a Tamaya sextant, so the scope mount should be the same as the
Astra) so that the split would appear on the right side of the field of view
(closest to the sextant body) if I was able to see it, the sun vanishes at
the center of the field of view when the sextant is rotated (but again, the
sun can be rotated to the right all the way to the edge of the field). If I
move the telescope in closer to the sextant body, then I can rotate the
sun from one side of the field of view to the other.

When the telescope is centered, and the sextant is rotated so that half
the sun vanishes, removal of the paper does not affect the image
whatsoever. I think it is clear that in this sextant, at least, reflection from
the glass surface plays no role at all in normal sighting with the
instrument. I think these reflections are so much dimmer than the rays of
light that enter the telescope from the edge of the field of view (reflected
by the mirror, not the glass) that the mirror-reflections are completely
responsible for the image sweeping past the apparent edge of the mirror.
The illusion is due to the reduced depth of field with a higher power
telescope.

That being said, I am now convinced that I was completely wrong about
the double reflection from the glass portion of the horizon mirror. Only
nearby objects give a double reflection; celestial bodies should give a
single reflection, as George and others have been patiently explaining to
me. I did an experiment last night that convinced me otherwise, but there
must be artifact that I haven't appreciated. I accept that the mirror will
give a single image, I just think that it is far too dim to be noticed.

Ken Muldrew.

   

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