NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Sextant optics
From: Jim Thompson
Date: 2004 May 2, 08:02 -0300
From: Jim Thompson
Date: 2004 May 2, 08:02 -0300
The thread on the AstraIIIb's split mirror was very helpful, but did not thoroughly review the subject of sextant optics, and raised more questions in my mind. We have not yet thoroughly described issues of depth of field, field of view, magnification, and glass and lens manufacture with respect to modern sextants. I have been prowling around the Internet looking for online information regarding sextant optics to fill in the gaps. My guess is that these are closely guarded industrial secrets, or that there is information buried in specialized print publication that has not made its way to the Internet yet. And we do not have optics specialists on this list. I am not in a geographic position that allows me to explore the print literature on this subject. Meanwhile I did turn up this distracting web page, where Celestaire appealed to the US Government to redefine sextant tariff law for the AstraIIIb: http://www.law.emory.edu/fedcircuit/july97/97-1005.html "Celestaire ... appeals the order of the United States Court of International Trade (defining) marine sextants as ?optical navigational instruments? ... dutiable at 5.6% ... rather than as ?other non-optical navigational instruments? ... which are not subject to a tariff." It demonstrates how lawyers can argue the definition of a sextant, including whether they are metal instruments or not. The central argument by Celestaire seemed to be that sextant optics do not primarily aid human vision, but are subsidiary to the purpose of measuring angles. "It is uncontested that sextants are navigational instruments; we are only asked to determine whether they are optical ones or not." The court concluded that "the immediate purpose of the sextant is to allow the user to see the sun and the horizon at the same time ? an act which the user could not otherwise do. The intermediate purpose is to accurately measure angles which can only be estimated by the naked eye. It is only the ultimate purpose, to determine navigational position, that does not have to do with enhancing human vision ...". The Court found in favouring of continuing the tariff, "Because a sextant aids or enhances human vision ... through the use of its non-subsidiary split-image mirror...". So the court reaffirms that split mirror optics indeed aid human vision, but ... precisely how? Jim Thompson jim2@jimthompson.net www.jimthompson.net Outgoing mail scanned by Norton Antivirus -----------------------------------------