A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Greg Rudzinski
Date: 2016 May 24, 10:13 -0700
A better way to calibrate a Bris sextant has been discovered. Remove the scope from a metal sextant with near zero index error. Hold Bris sextant where scope would be and with some trial and error line up the desired Bris reflected Sun image with the sextant horizon mirror Sun image and record the fixed reflected angle value. This is much easier and less time consuming than waiting for the various Bris images to set on a natural horizon and then perform a sight reduction in reverse. It is a great deal simpler to compare sextant to sextant directly. No timing, sight reduction, or trips to the beach. The Bris can now be calibrated in the back yard.
On custom made Bris sextants using microscope slides it is advised to observe the reflected Sun in the exact same spot on the slide each time. To assist in this a pair of black dots can be placed as goal posts with the Sun to be observed between them. See attached image of a custom Bris using a 52mm UV camera lens filter as a base for the microscope slides. The slides are spaced 0.7mm and then spaced as a pair 3mm fom the UV filter. A second UV filter can be sandwiched to the first to encapsulate the slides from dirt and weather.