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In answer to my own question, I found the following
http://www.lewisandclarknavigation.org/InstrumentCalibration.htm
About midway down the page, the author has drawn the optical path for a foresight 
and a backsight observation.

It appears that the paths are parallel but not co-linear.  The optical path for 
the backsight mirror to the index
mirror is to the inside of, but does not cross the optics of, the foresight horizon mirror.

Best Regards
Brad



-----Original Message-----
From: navlist-bounce@fer3.com [mailto:navlist-bounce@fer3.com] On Behalf Of Brad Morris
Sent: Monday, March 22, 2010 9:27 AM
To: NavList@fer3.com
Subject: [NavList] Re: Back sights

In adjusting the octant or sextant for index error, there is nothing to stop you 
from adjusting or determining
it on land and then using that value at sea.  Being as land based as Chauvenet, 
all my star-star sights are on
a stable platform.

There is another dilemma concerning the BackSight Octant that I am attempting to 
resolve in my head.  The
octant has only one arc.  That is, the scale on the arc is singular.  There aren't 
two scales, one for foresights
and the other for backsights.

For a foresight, for a given angle, the nonius will  indicate that value.  But the 
backsight is
to be used to measure the same angle.  I believe that the index mirror does not 
move, because the nonius
itself cannot move, else it would indicate a different angular value.   What 
therefore, is the arrangement
of the backsight mirror relative to the foresight mirror?

The backsight mirror must be parallel to the foresight mirror, in order to obtain 
the same angular value.
The optical path from the backsight mirror to the index mirror must then somehow 
cross through the
foresight horizon mirror.  Is the backsight mirror slit further away from the plane of 
the arc, permitting
the optical path to go through the clear part of the horizon mirror?

If the paths are parallel but not co-linear, then the index mirror must be larger 
to support this offset.

Is there some graphical optical path explanation existent, one perhaps for a real 
octant we can examine online?

Best Regards
Brad



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