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From: (no name) (no email)
Date: Fri Oct 15 2004 - 10:49:38 EDT
In a message dated 10/14/04 12:26:18 PM Eastern Daylight Time,
writes:
> I have no clue why they designed it like this.
> It seems they did it intentionally, and they knew the adjustment
> will be trickier, that's why they supplied the visors as standard
> accessories.
>
> The closest to SNO-T Western sextant is Freiberger.
> Question to Freiberger owners: does it also have this feature?
>
I have a Freiberger Trommelsextant and a Davis Mk 25. I took a piece of coat
hanger wire and bent a 3 in diameter circle in it forming a base which rests
on a table. I bent the wire up from the circle then horizontally cutting it
off as it passed over the center of the circle. I ground the cut end to a
point and painted it black with a magic marker. I placed first one then the other
of the sextants on the table with the point just over the sextant index
mirror. With my eye a foot or two above the table, I repeatedly swung the index
arm from -5d to +120d moving the sextant until the point was over the spot on
the index mirror holder which did not shift in relationship with the point.
That should be the center of the index arm axis. I marked that spot on the
mirror holder. I could find the spot to within 0.5 mm.
For the Davis sextant the point is 2.5 mm behind the front surface of the
mirror. The mirror is silvered on its rear and is 2.0 mm thick
For the Freiberger sextant the spot is 4.0 mm behind the front surface of the
mirror. The mirror is silvered on its front.
For the Davis sextant the effect that Alex mentions is quite small (or
nonexistent) and difficult to see. I set the index at 55d so that I am comparing
the 0d and 120d areas on the arc. The index mirror frame, the edge of the
mirror between the front and silver surface, and the mirror clip all get in the way
and make it hard to compare the two parts of the arc as the viewing angle is
changed from one corner of the index mirror to the other.
For the Freiberger sextant the effect that Alex mentions is easily
noticeable. I set the index at 30d so that I am comparing the 0d and 120d areas on the
arc. Because the mirror is silvered on its front and because the mirror frame
is behind the front of the mirror and almost invisible it is easy to compare
the two parts of the arc when the viewing angle is changed from one corner of
the index mirror to the other.
Bill Murdoch
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