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Re: How does the AstraIIIb split mirror work?
From: Trevor Kenchington
Date: 2004 Apr 26, 20:34 +0000
From: Trevor Kenchington
Date: 2004 Apr 26, 20:34 +0000
Ken Muldrew wrote: > The reflection past the middle of > the field of view is a combined image from both the mirror and the glass > until the object is rotated past the point where the scope can no longer > get reflected light from the mirror. Then the image comes solely from > reflection off the two glass surfaces. The transition is imperceptable with > a 5x scope. > > George Huxtable was entirely correct in this matter and I thank him for > sticking with it until I was able to understand the phenomenon. George is usually entirely correct in most matters but I'm not convinced that Ken is not giving him, and George himself is not claiming, a little too much credit this time around. From the above quote of Ken's message, plus an overview of the rest of this thread, it looks to me as though there are _two_ optical mechanisms at work -- their relative contributions depending on the depth of field of the sextant/eye optical system, the light level (e.g. Moon versus Sun) and maybe yet more factors. (Are some horizon mirrors given low-reflectivity coatings? Certainly some will have an excessive coating of finger grease and other contaminants that will increase the surface reflection from the clear half.) One mechanism is clearly George's suggestion of reflection from the surfaces of the glass. But that does not preclude Ken's mechanism of light gathered from the silvered half of the mirror being perceived as coming from the other half, due to the boundary between the two being grossly out of focus when seen through a telescope focused on infinity. Trevor Kenchington -- Trevor J. Kenchington PhD Gadus@iStar.ca Gadus Associates, Office(902) 889-9250 R.R.#1, Musquodoboit Harbour, Fax (902) 889-9251 Nova Scotia B0J 2L0, CANADA Home (902) 889-3555 Science Serving the Fisheries http://home.istar.ca/~gadus