NavList:

Petra Monta you wrote: The Davis Mark 3 has a sight tube with 11 mm internal diameter, about 165 mm from the horizon mirror.  So if the eye is at the extreme left or right portion of the sight tube (or telescope mounting ring; but those are generally larger, maybe 25 mm or more), then looking at the center of the horizon mirror, the line of sight is canted away from the plane of the sextant by roughly 5.5/165 radians or about 2 degrees.  With a 25 mm ring it would be 4 degrees (!).

With the eyepiece removed and plane of my Mate’s Three Circle sextant vertical, if I move my eye one way horizontally, the reflected image of the Sun moves too far from the clear part of the horizon mirror for realistic observation.  If I move my eye the other way, I lose the reflected image.  In such a situation, the only way I can bring the Sun back to a realistically observable position close to the silvered/unsilvered line of the horizon mirror is to roll the sextant, which of course makes the Sun appear to climb above the horizon.  Bringing it down again would lead to an overreading error, but on solid earth for a simple sextant altitude, the amount of roll required to produce this effect is, I suggest, obvious.  I concede that this might not be the case when holding the sextant at a slant when taking a lunar from a rolling plarform.  Might this be one reason why early quadrants had such a tiny viewing hole?  It’s possible I’ve entirely missed the point (not unusual).  Please will you suggest a good place to read about collimation error in terms of Newtonian optics.  DaveP