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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Geometry of SNO-T
From: Herbert Prinz
Date: 2004 Oct 14, 01:20 -0400
From: Herbert Prinz
Date: 2004 Oct 14, 01:20 -0400
Bill wrote: > I had posted a question a while back, and have received no response. Since > Alex seems to have a good grasp of this now, I will ask him and others to > address it. > > "If the front-silvered mirror is no longer on the axis of the rotation, will > this affect the sextant's performance?" > It may have a small impact on performance, but it will not directly affect the measured altitudes. 1. It does not directly affect the measured angle, because the mirror remains parallel to its previous (or ideal) position. All that matters are the angles of the light rays through the instrument and they are still the same. Alexandre Eremenko did in fact mention this in passing in an earlier message today. 2. There is a potential indirect effect in that it becomes more difficult to ensure the perpendicularity of the index mirror. This has been discussed. From my experience with an Astra with a front silvered mirror which is displaced from the pivot by ca. 5 mm, I can say that in actual practice this is not a problem. But let's assume for the sake of argument that you have a perpendicularity error of 5' that you cannot get rid of. Then the error of any measured angle over 10 deg would be smaller than 0.1'. Smaller angles suffer more, but they are hardly ever used. 3. If the sextant was originally designed for a centred mirror surface which has been later replaced by an off centred one, then the light path that goes through the centre of the telescope no longer passes through the centre of the index mirror. Consequently, a few square millimetres of mirror surface are lost. -------- Earlier today, Cliff Sojourner had asked "is there any effect from index of refraction of the glass in" "front of the silvered surface?" And I answered "Only in so far as the refraction of the glass shifts the apparent reflective surface from the back towards the front surface of the mirror." I should add that this is only true for a glass with perfectly parallel faces. (Which we hope we always have.) If, however, the glass is prismatic, in other words, the front surface is not perfectly parallel to the mirror surface, then the measured altitude will be in error. This error is not constant and can therefore not be compensated by index correction. I do not know whether this is a real world problem. Herbert Prinz