NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Greg Rudzinski
Date: 2016 Apr 2, 11:28 -0700
Performance conclusions of the Davis MK 3 sextant:
1. Lack of a scope limits use to bright stars, planets, Moon, and Sun.
2. Only a 2' vernier scale precision on the index arm.
3. Horizon mirror has no means to adjust out side error. The fixed horizon mirror side error requires making a correction table through the arc.
4. Index error must be checked before and after each observation.
In spite of the above instrument limitations it is still possible to obtain +/- 3' accuracy which makes this ($40 new) plastic sextant a good value for small craft cruiser looking to back-up the GPS. The next post will review the Ebbco Special which is the next step in plastic sextants for the value navigator.
Greg Rudzinski
From: Greg Rudzinski
Date: 2016 Mar 28, 12:51 -0700
The Davis MK 3 plastic sextant can be improved by adding solar film filters for index mirror shading and sight tube shading (see image). Solar film shading the sight tube is of big help when determining index error by direct Sun observation vs. natural horizon. Solar film shading the index mirror will show a sharper Sun image than the stock shades.
Improve Davis MK 3 accuracy by recording arc error through the arc every 10°. Intercepts for observations made from a GPS position will serve as a good enough quick arc error to start with. For better arc error determination use a metal sextant as a standard to compare on simultaneous observations. An assisting fellow navigator is recommended. Otherwise a quick swap between sextants has to be done. Then there are lunar distances and star to star distances if time permits.
Greg Rudzinski
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