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Re: Bris Sextant
From: Frank Reed CT
Date: 2005 Nov 7, 16:55 EST
From: Frank Reed CT
Date: 2005 Nov 7, 16:55 EST
George wrote: " It's important, for a sextant, that both lines of view (to Sun and to horizon) are in the plane of its frame, which is at right angles to the axis" And Alex you replied: "It is important because you have a frame with the scale attached to it. And measure the angles using this scale." I'm pretty sure there's a real collimation issue here. You even mentioned the technique for dealing with it in a post last month. To get the line of sight perpendicular to the glass, you rotate the instrument back and forth around the vertical axis. This is the same procedure that's used with a true sextant to detect errors of collimation in the telescope --but on a radically different scale. If you take your Bris "sextant" and hold it at 45 degrees to the line of sight (rotated about the vertical axis), you should find that the angles are very different from their normal values. That's certainly what I see when I play with the Bris sextant mockup that I assembled... I haven't worked the math, but I wouldn't be surprised if the standard equation for sextant telescope collimation error applies without modification. Since this device depends on "hand" collimation, I would expect errors of collimation of 5 degrees or so to occur relatively often. A 5 degree "yaw" yields a 10 arcminute error in the measured altitude for an altitude around 45 degrees. I suspect that this is the fundamental limitation of the Bris "sextant's" performance. "It seems to me that in Bris sextant NOTHING is important:-)" For the most part, I agree with that. There's no index error, no arc error, etc. But there is collimation error. There is probably also error arising from non-uniformity of the glass. Also, I notice with my mockup that the reflected images are all doubled (presumably from the non-zero thickness of the glass). This effectively "blurs" the reflected image though with careful observation it should be possible to deal with this. "You just glue three pieces of glass very approximately. The only thing that is important is that the assembly is rigid, the glass panes do not move about each other." If anyone else is building these, I have a suggestion for experimentation. I've been gluing mine together with water-soluble glue. Makes re-designs easy. My current model has four panes at rather wide angles. -FER 42.0N 87.7W, or 41.4N 72.1W. www.HistoricalAtlas.com/lunars