NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: shooting indoors
From: George Huxtable
Date: 2010 Mar 24, 14:00 -0000
From: George Huxtable
Date: 2010 Mar 24, 14:00 -0000
I'll argue, a bit, with Brad here. If the glass surfaces are non-parallel then that will lead to prismatic deviation in angle; that's true But that doesn't depend very greatly on the angle at which the object is being viewed through the glass. It can apply, in a very similar way, whether or not the glass is at right angles to the line of view. Same is true of the shades. It depends, mostly, on the age of the glass window. Modern float glass is remarkably plane-parallel and shows little prismatic error. Older glass is not, and shows a bit of ripple as you view a scene through it. If any prismatic effect of the glass happens to be the same, or nearly so, for the horizon view and the index view. then the overall error in a sextant reading will be minimised. What the sextant will be specially susceptible to, is a varying ripple over different parts of the window surface. But as long as the glass is plane-parallel, there's nothing to worry about. Another effect that might need considering, when the glass isn't at right-angles, is that of the wedge-shaped bit of air, between observer and window, which won't quite match the refraction from the similar wedge-shaped bit of air outside the window if the air temperatures differ. Although this might deviate the light path a bit, it will deviate both light paths the same amount, so will have no effect on as sextant reading. George. contact George Huxtable, at george@hux.me.uk 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 12:45 PM Subject: [NavList] Re: shooting indoors | Hi Scott | | There may be some degree of prismatic error, depending upon the angles that you are viewing objects thru your glass. As light passes thru regions of differing indices of refraction, the light will be refracted towards or away from a normal to the interface as a function of Snell’s Law. The same prismatic error can occur in your sextant’s shades, should they not be perpendicular to the optical path. Think of your glass doors as yet one more “shade”, which must be considered in the optical path. | | Another issue you may have is the Dip Short. If the distant lakeshore is visible, then you do NOT have a horizon, you have a dip short horizon. There are ways to deal with this, you are not alone with this issue. Bowditch, and others like Norie, will provide the appropriate correction. | | Best Regards | Brad | | From: navlist-bounce@fer3.com [mailto:navlist-bounce@fer3.com] On Behalf Of Scott O'Connor | Sent: Tuesday, March 23, 2010 8:04 PM | To: NavList@fer3.com | Subject: [NavList] shooting indoors | | | Maybe a dumb question. I live where it is very cold much of the year, but can get sextant shots through a sliding glass door to a distant lakeshore as horizon. Does shooting through the glass affect accuracy of the sights, from a scientific basis? Thanks for the input. | ---------------------------------------------------------------- | 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." |