A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Alexandre Eremenko
Date: 2017 Mar 6, 17:26 +0000
Tony: You wrote:
when the sextant arm is set to high degrees (~70° and above) the index mirror appears in the horizon mirror as a narrow strip. At 120° the index mirror is no more than ~6~8mm wide as seen through the ghost ring and does not occupy all the reflective half of the horizon mirror. The center line of that strip is located slightly above the middle of the horizon mirror - i.e. parallel to its' horizontal diameter but ~5~7mm above it.
This is so. You encounter this when you take Lunars at large distances, or horizontal angles. (Normal altitude observation cannot be more than 90°).
Does that mean that ideally the axis of the scope (or the center of the ghost ring) should be aligned so that the narrow strip of the index mirror - when set to 120° - is seen exactly in the center of the viewing field? I can draw a sketch of what's going on if my description is not clear.
Ideally yes. But most modern sextants do not have any adjustment for the telescope position (except moving it to and from the frame, parallel to it.). My SNO-T allows me to take angles up to 140° , and I did actually do some at more than 135. The view of the index mirror is so narrow that it does not fit the whole Sun/Moon disk. The image of the mirror is not exactly at the center but close enough to it. And I was able to get a decent result.
When angles are relatively small - the index mirror is wider (in vertical direction) than the field of view and I can use monocular simply put to the ghost ring (by hand). But at high angles I have to tilt the scope slightly up to get the correct image. Now even a scope-less sight is difficult to take if big angles are involved.
With scope-less sights, some sighting van or a "zero-magnification tube" is needed. Otherwise you may get big errors because you look under an angle to the horizon glass.