A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: How does the AstraIIIb split mirror work? PERSONAL SUMMARY
From: Joel Jacobs
Date: 2004 Apr 28, 08:29 -0400
From: Joel Jacobs
Date: 2004 Apr 28, 08:29 -0400
Earlier, Bob Enro asked for a summary of the mirror discussion from someone erudite such as George H. I don't mean to imply I qualify on that account, but I received an off group question yesterday which answered, serves as my own summary, and since George H. also suggested I might have a comment, I submit this. QUESTION: "Following the split-mirror discussion on the forum - on my previous, plastic, sextants which had full mirrors, I sat the sun on the horizon; now I see the sun in the RH part and the horizon in the LH part. But the horizon doesn't stretch across to the mirrored, RH, side, so it involves more skill (I first wrote "guesswork") to establish the point where the sun's LL sits on the horizon -am I correct? Bowditch says the sun shd be visible, centred, acrss both parts of the H mirror...one of your posts implied the same where there is no side error. If my sun is not centered but definitely in the RH side, and I have no side error, do I just move the scope by sliding the rising piece? I will experiment later today." JOEL'S ANSWER: I would be hard pressed to disagree with Bowditch. You look through the optics with the scope in the center position, and with a normal sextant, that has its error minimized, you should see only one reflected object when the sextant is held at the vertical. It will appear half on the silvered side and half on the clear side. If you rotate your wrist, from right to left, as you should, the body will make an arc, from right to left across both the silvered and unsilved side. The low point of the arc tells you when the sextant is perfectly vertical, and is the altitude you want to measure. If the sextant is not vertical the object seen will be displaced to one side or the other. If side error exists you will see a portion of the same two objects. With no error you should see only one. Moving the telescope, towards or away from the frame favors seeing only the object on the silvered side when the scope is next to the frame. Or seeing only the horizon when the scope is most distant from the frame. This depends on how far the scope is meant to slide. The position of the scope, in or out, is to either maximize the horizon when it is indistinct or maximize the object in poor light conditions. No coatings were used on back silvered mirrors. Coatings are a big deal on front silvered mirrors, and care should be taken in washing and drying them or they may quickly haze. It is my opinion that the image reflected on both sides of the horizon mirror is that of the image sent from the index mirror. The clear prortion of the horizon glass we all agree has some, but to a lesser extent, reflective properties, hence the dimmer image. The horizon you see on the left side is not a reflected image and therefore will NOT appear on the rightside. I have just acquired an unusual and elegeant sextant, one made by Leupold & Stevens for the U. S. Maritime Commission in 1950. One nice feature is the longer then usual adjutsment that can be made in postioning the scope. The distance it slides is about 5/8" which I think is more than most. My preferred postition is centered. There, by tilting the sextant, I can see a single image of the whole sun, on both sides of the horizon mirror. Joel Jacobs ----- Original Message ----- From: "George Huxtable"
To: Sent: Tuesday, April 27, 2004 12:09 PM Subject: Re: How does the AstraIIIb split mirror work? > Trevor Kenchington's contributions are always perceptive; as when he said- > > >.... I'm not convinced > >that Ken is not giving him, and George himself is not claiming, a little > >too much credit this time around. > > >.... 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 is quite right. The two effects must occur together, to some extent. > In an early sextant without a telescope, observed directly by eye through a > peep, then the greater depth-of-focus of the eye would vastly reduce any > such apparent "spreading" of Sun light-image from the silvered part to the > unsilvered part; wouldn't it? > > A sextant may be fitted with a mechanism (called a "rising-piece, perhaps?? > Joel would know.) that can shift its telescope bodily away from the frame, > keeping parallel with itself. If it could be moved far enough to shift the > silvered part of the mirror completely out of its field of view, only > reflection in the unsilvered glass would then remain to give a dimmed image > of the Sun. I suspect that is precisely the purpose of such a mechanism; to > provide a degree of controlled dimming of the Sun. What else is it for? My > own cheap-and-cheerful plastic Ebbco has no such elaboration, of course. > > In the case of a sextant with a truncated horizon-mirror (i.e. no > clear-glass portion to look through, just a short mirror to look past) then > presumably such a "rising-piece?" would be able to dim this "ghost-image" > of the Sun right down to nothing, if the telescope shifted far enough. You > could try it for yourself by grinding-away the unsilvered part of your > horizon mirror (though I doubt if many will...). > > George > > ================================================================ > contact George Huxtable by email at email@example.com, by phone at > 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy > Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. > ================================================================