A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brad Morris
Date: 2014 Apr 2, 10:53 -0400
I've had a look at the referenced pentaprism mounted on the sextant. That's a clever idea, as it twists the reference path.
The beauty of a pentaprism is that the light bundle out is always 90° from the light bundle in. This immediately eliminates most of the setup problems in aligning the prism to the optical path of the sextant. It still requires that the plane of the sextant and the plane of the pentaprism be parallel, but any machinist can achieve that within a few thousandths of an inch. Thus the cosine error will be negligible.
I then set out to see how the sextant would be used, with the artificial horizon and the pentaprism in the optical path. My mind has been churning over the position the sextant must be in and how I would then see the celestial object in both optical paths (index path & horizon path). I must confess I cannot see how to make it work. Which path will you direct to the AH? Which path directly towards the celestial object?
I think this is along the right track. That is, a known deflection of the horizon path will change the angle distended on the sextant. Greg needs that to extend his AH work. Can the idea be reworked a little to make it truly functional?
Here are a couple of links describing pentaprism attachments for sextants---search for "penta" within the pages. Only de Hilster's site shows a photo, though; the prism is evidently in the horizon-mirror path.Cheers,
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