NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Off Center Sextant Scope Observations
From: George Huxtable
Date: 2007 Nov 23, 10:21 -0000
From: George Huxtable
Date: 2007 Nov 23, 10:21 -0000
Greg Rudzinski asked- | | What happens to an image of a celestial body when viewed off center in | a sextant scope? I have a bad habit of using the lower half of the | field of view when taking lower limb observations of the sun. I | suspect that this increases the observed altitude slightly. ==================== Response from George- I think not; as long as its central in the side-to-side field of view. It doesn't matter if it's central between top and bottom. What follows will assume the sextant frame is vertical, as when taking an altitude. For a lunar, it should be read differently. What's important is that the viewline should be in the plane of the sextant's frame, which is why a telescope needs to point in the right direction to avoid collimation error, and why the mirrors need to be perpendicular to that plane. Otherwise an out-of-plane error ensues, which is negligible for small measured altitudes, but matters for large ones. If you make a contact to one side or the other of the centre-line of the view, you are creating a bit of skewness. Some telescopes are fitted with crosswires to guide the eye to the centre line, where any contact should be made, but that's not possible for a Galilean; one of its disadvantages. However, as long as everything IS kept to the same plane, symmetrical left and right, it doesn't matter a bit whether the viewline is a bit up or down, the laws of reflection in a plane being quite unaffected. So Greg's worries, about observing the contact in the bottom of the view, are groundless, he will be pleased to learn.. These points should become clearer if you experiment a bit with extreme skew angles, by removing the scope and looking through the mirrors with the naked eye from various skewed directions, off from the line of the scope. George. contact George Huxtable at george@huxtable.u-net.com or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---
