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    Re: Compact Lightweight Metal Sextants - Freiberger Yacht vs. Astra IIIB
    From: John Karl
    Date: 2012 Aug 10, 19:23 -0700


    Yes, Bill Morris is right (as usual). The Freiberger Yacht sextant has the clear-glass left-side of the horizon mirror missing. And this has the effect of not showing the images from the index mirror on that left-side field of the view (FOV). What may surprise some readers is that in sextants with Galilean scopes and half-silvered horizon mirrors, you really never use the silvered half of the horizon mirror when shooting sun or bright moon sights. The horizon is not visible on the right side of the VOF, but as Bill says, the unsilvered left half of the mirror reflects plenty of sun light to make both the horizon and the sun visible there. So, perhaps without thinking about it, you make the sight on the left half of the VOF (even though you paid for the whole FOV). But with the Yacht sextant, with its left half missing, you get a region about ½ d wide to make the sight. That’s about one eighth of the normal 4d FOV.

    In small vessels this FOV is really important in rough seas, and in general, important for comfortable and pleasant viewing. In general, the optics in most sextants is pretty miserable. They have severely limited FOV, extraneous light in the scope, and shades reducing the FOV in many sextants. Just compare the sextant’s view to that of even a modest cost binocular. The only sextant that does the optics right is the Navy Mark II, it has a very wide, high quality, FOV. It suffers only slightly from lack of intensity in difficult star shots because of its smaller aperture. You can read more about this topic (and the effects of prism scopes) in my book, and in the 2008 issue of Ocean Voyager (the summer issue of Ocean Navigator).

    The smaller size of the Yacht sextant is really nice, specially in small boats, and in air-travel luggage to various passage legs. But my main objection is that its micrometer drum is too small and lacks a vernier scale. Even worse, its shinny surface reflects light, making it very difficult to read in many circumstances. The sextant also has no lighting. (In addition, the arc error has no bragging rights. Mine ranges up to 48’.)

    Happy force four winds,
    JK

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