NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Backlash
From: Bill B
Date: 2005 Nov 20, 20:46 -0500
From: Bill B
Date: 2005 Nov 20, 20:46 -0500
> The problem of sidewise shake hadn't occurred to me! The sun-diameter > experiment, with the sextant upright all the time, was poorly designed for > catching such an effect. I think there must be lateral, as well forward and backward thrust when the worm gear is turned, so even in an upright position it will occur. Another interesting error I believe I have nailed down comes from the scope. From the article Frank pointed out a few months ago, our night focus can be different than our day focus. I put tape by on the main barrel and focus ring of the scope, and put witness marks where optimal day focus occurred (aided by sunspots) and optimal focus on a star occurred after adapting to the night sky. And sure enough, in test after test, the night witness marks (approx. 1/4" apart from day to night) almost lined up at night time after time, as did the day marks. So what? Well, I would adjust side error to perfection, and after a day shot that night it was off. Refocus, close again. What I was able to see, and Alex confirmed I believe, is that in both the Astra and low-power SNO-T scopes changing the focus changed the side error. In essence, the reflected body was rotating around the direct image (or vice versa). We can only conclude it would change index error as well. Now I have semi-permanent tape arrows affixed to the scope for day and night focus. A bit off topic, but the article also indicated that as the eye dark adapts and the pupil opens ( 2mm to 7mm) what was a pinpoint becomes a much larger "disco mirror ball" (my words). Now for star-to-star distances with bright stars I observer for several minutes, then expose my eyes to a well-lighted white wall, then shoot for a few more minutes. It really seems to help me. Bill