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: Frank Reed
Date: 2007 Dec 02, 22:22 -0500
From: Frank Reed
Date: 2007 Dec 02, 22:22 -0500
George H wrote: "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. " I agree with this completely on theoretical grounds. But I can say that I "think" that I have seen exceptions in plastic sextants. Where it originates, I can't guess. It's within the expected accuracy limits for plastic sextants (one or a few minutes of arc), so I haven't seen any reason to worry about it. I have not seen any "up-down" change in alignment using a quality sextant. Therefore, I agree with your advice: don't worry about where you make contact in the field of view. On a related note, if you're worried about collimation, which only becomes a significant factor at large angles and requires some care in aligning images "letf-right", the main thing to remember is that you want to minimize the angle. So if you're measuring the angle between two stars, for example, after you make the initial alignment bringing the reflected and direct image together, you should wobble the sextant around so that the side-by-side images appear at various points in the field of view. If you don't see any change in separation, then you have nothing to worry about --any collimation effect is below your ability to detect. If you see a slight up-down separation when the images are on the right side of the field of view, no separation at some point near the middle (doesn't have to be centered exactly), then a slight up-down separation again on the left side of the field of view, then you should do your exact alignment at that point in the field of view where there was no separation. BUT if the separation decreases steadily across the field of view until you get to the edge, THEN you have a significant collimation problem (the minimum is outside the telescope's field of view) and you should find a way to adjust the alignment of your scope (e.g. using a laser level, as I've described previously and demonstrated at Mystic Seaport). Again though, if you don't SEE any change in separation, then this is something you simply don't have to worry about (because it's below your ability to notice with the magnification you're using, or because the angle you're measuring is small enough that it isn't an important effect). -FER www.HistoricalAtlas.com/lunars PS: it's obvious if you know it already, but not if you don't: "left-right" motion of the image here refers to motion in the field of view towards and away from the frame of the instrument. It's left-right literally only if the instrument is held in the usual vertical orientation. And so "up-down" is motion parallel to the frame. --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---