A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Back sights
From: George Huxtable
Date: 2010 Mar 24, 19:48 -0000
From: George Huxtable
Date: 2010 Mar 24, 19:48 -0000
Brad's arguments are at fault here, and Bill Noyce is right. Earlier, Brad referred us to Hans Heynau's website, at- http://www.lewisandclarknavigation.org:80/InstrumentCalibration.htm which shows two sketches, 5a and 5b, of the light paths for fore and back observations. These sketches correctly show a difference in the angles of the light path between the two mirrors, with respect to the frame, in the two cases. Indeed, that simply has to be so, because when making a back observation, the fore horizon mirror remains in place, and would otherwise block the light path to the back horizon mirror. I don't know what that angle difference needs to be, but let's just guess at 10º. Brad correctly points out that a zenith star has to be seen at 90º on the scale, in both fore view and back view, so the index arm and index mirror must stay unaltered, with respect to the frame, in the two modes.. But how can this fit in with the light path between the relevant mirrors differing by 10º? Answer: the whole instrument has to be tilted quite differently, with respect to the horizontal, when making a back observation. If the whole octant is rotated anticlockwise through 10º, that will return the zenith star to the centre of the view from the back horizon mirror. The resulting tilt of the frame isn't shown on Heynau's sketch 5b, so it's a bit misleading in that respect. And next, the back horizon mirror needs to be so angled to the frame that when the frame is tilted by that 10º angle, and the horizon appears in view, the twice-reflected zenith star appears to be aligned with it. I suggest that if Brad makes himself a more careful drawing than that sketch, and sets up the angles with a protractor, it will become obvious. What's bugging him is, perhaps, the notion that the inverted A-frame of a sextant/octant must be somehow balanced, in the nodding direction, about the vertical, but there is no such requirement; neither in fore observations or back observations. It just depends how the geometry of the mirrors has been designed and set up. George. contact George Huxtable, at email@example.com or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ----- Original Message ----- From: "Brad Morris"
To: Sent: Wednesday, March 24, 2010 3:36 PM Subject: [NavList] Re: Back sights | Hi Bill | | Angle of incidence = angle of reflection for planar mirrors. | | If the paths are not parallel from either of the horizon mirrors to the index mirror, then the angle of the beam out of the index mirror will be different for the non-parallel paths from horizon mirrors to index mirror. Since non-parallel paths will not place the object on the horizon (or the star on the limb of the moon) for the same indication of the nonius of the vernier to the arc, a different reading will be had once the contact is made. There is only one index arm, one index mirror and one arc. The angular setting must therefore be the same (excluding index error), to achieve the same reading for the same angle, with both foresight and backsight observations. | | In other words, set the nonius of the vernier to 90 degrees. Assume index error for both the foresight and backsight to be zero. Now observe a zenith star using the backsight and foresight observation. For both observations, the index arm does not move. Yet how can this be read as 90 degrees, if the optical path from the foresight horizon mirror is not parallel to the optical path of the backsight horizon mirror. | | I think the paths to be parallel, with an offset between them. This means that the optical paths strike the index mirror in different locations. However, since the index mirror is a planar surface, then the angle of incidence and angle of reflection are the same. | | Best Regards | Brad | | | From: firstname.lastname@example.org [mailto:email@example.com] On Behalf Of Bill Noyce | Sent: Wednesday, March 24, 2010 11:21 AM | To: NavList@fer3.com | Subject: [NavList] Re: Back sights | | | I don't think the optical paths need to be parallel for the fore-sight and the back-sight. The only requirement for the scale to read right is for the back-horizon mirror to be at an angle of 90 degrees from the fore-horizon mirror. It can then be placed almost anywhere, as long as the back peephole then allows the index mirror to be seen in the back-horizon mirror. Remember that the genius of the octant (and sextant) is that it measures the angular difference between the horizon mirror and the index mirror, regardless of how the instrument is oriented. | -- Bill N. | ---------------------------------------------------------------- | NavList message boards and member settings: www.fer3.com/NavList | Members may optionally receive posts by email. | To cancel email delivery, send a message to NoMail[at]fer3.com | ---------------------------------------------------------------- | | ________________________________ | "Confidentiality and Privilege Notice | The information transmitted by this electronic mail (and any attachments) is being sent by or on behalf of Tactronics; it is intended for the exclusive use of the addressee named above and may constitute information that is privileged or confidential or otherwise legally exempt from disclosure. If you are not the addressee or an employee or agent responsible for delivering this message to same, you are not authorized to retain, read, copy or disseminate this electronic mail (or any attachments) or any part thereof. If you have received this electronic mail (and any attachments) in error, please call us immediately and send written confirmation that same has been deleted from your system. Thank you." |