A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Michael Bradley
Date: 2016 May 6, 15:09 -0700
There's no need for concern about possible retinal damage in the two polariser shade sextant applications reported by myself and Greg ...
'My' application is in the light path from the horizon to the eye, where a light filter of one sort or another in a light horizon shade is used to improve the contrast of the horizon.
The dense horizon filter is left in place to protect the retina, when required, for index error checking using the double grazing sun technique, when the horizon line of the sextant is pointed directly at the sun.
'Gregs' application is in indeed in the potentially dangerous index mirror light path, but is being observed by a camera, not a human eye.
These 'circular' polarisers such as the Phot-R device I've used actually consist of a single, rotatable, linear polariser with a secondary circular polariser also in the light path, designed for use on the front of a camera lens. The purpose of the second circular polariser is to ensure the beam splitter inside the camera delivers light to both the camera image sensor and the exposure metering system inside the camera. Camera users have to avoid simple linear polarisers, and like to see the name 'Circular Polariser' to re-assure them that the exposure metering system in their camera will work with the advertised polariser. The 'circular' part of the device is of little use to the sextant user, but such polarisers have the advantage to us of being mass produced to a good optical quality, in very neat packages, at a low price, for the camera market..
FWIW, there is no sign of a change in index error when using the polariser, regardless of the rotational position the polariser is set at.
Greg is right to mention the C&P sextant as having a polarised horizon shade as standard. It was using my newly aquired C&P, enthused by the quality of the sextant image, that set me off looking to gain the advantage of that improved horizon image quality in a lighter sextant. These days my preferred index error sanity check on shore is to to turn the sextant on its side and observe vertical stick like features such as lighthouses, beacons, building spires, antenna and the like. The C&P is a lovely thing, but a heavy beast to try to hold on its side and it gets in the way on my small boat.
I hope you're reassured that Greg and I are not damaging ourselves, Francis...