A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Position-Finding
From: Frank Reed
Date: 2017 Nov 14, 10:26 -0800
Robert, you wrote:
"The reason I understand is that normally there is clear glass in the horizon path because there is a slight induced error in the optical path thru the horizon mirror (the 2 faces of the mirror glass are not perfectly parallel. By having the same piece of glass in both optical paths that error cancels out."
But as you suggested, and as Brad also noted, this is no issue for modern glass. Some 280 years ago when the first reflecting quadrants were being crafted, no two items of any manufacture were ever identical. Prismatic error was a real concern. But how long ago did that problem fade into history? Surely it's been no concern for 50 years... Maybe 75 years? 100 years?? But sextants without glass on both sides in the horizon view tend to create uncertainty and anxiety in navigators. In other words, they freak out! And it's not just the Davis Mk 3. The C.Plath Navistar design from the late 1970s, which also was targeted at the low-cost end of the market, had a half-mirror with no clear glass side. See the photo below...