A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brad Morris
Date: 2017 Jan 19, 16:39 -0500
The best I can demonstrate here in Florida (where I'm visiting) is to show a 28mm wide angle digital camera image of an Ebbco sextant with the scope removed. The Ebbco as the standard is set to exactly 20° 00.0' (see image). The building cupola (300 yards away) is observed with the 28mm DSLR through the scope mount while also showing an unobstructed view of the cupola. 1749 pixels are counted between images. Lens calibration graph has the multiplier at .6864 minutes of arc per pixel ( see graph). The lens was calibrated using a Plath metal sextant as the standard last year. The Camera Hs = 20° 00.5' vs. the Ebbco set at exactly 20° 00.0'.
If I had a metal sextant to use as a standard then it would replace the Ebbco and the Ebbco would be used with scope in to measure the angle of the two cupola images as seen with the 28mm lens. It is best to do this on a level table with sextant frames parallel.
Precision with this method is only going to be 1' which is good enough for a plastic sextant. 0.1' arc error determination will require the expensive machine as mentioned earlier in this thread.
Haven't had any trouble with repeatability. The final practical test for my other calibrated Ebbco Special was to have my dock neighbor take sights with it while crossing the Atlantic. His intercept results were better than 3' on all but two of his daily Sun observations vs. GPS.
When I return to California then I'll post images of two sextants working together for calibration purposes.
From: Brad Morris
Date: 2017 Jan 18, 12:24 -0500