A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Randall Morrow
Date: 2014 May 15, 07:27 -0700
Though I am new to navigation by Nav-list standards, and less knowledgeable than most, I am very good at taking sights with an artificial horizon. Recently I started placing the mirror on the ground instead of on a waist high table, and the average intercept of my last 15 consecutive star/planets sights was 0.3 nm. (none > 0.6nm) My results were pretty good before, but still less consistent than I would like. I believe this last change of putting the mirror on the ground made the star images smaller and thereby decreased ambiguity. Accuracy of index correction sights is not certain and of course, neither is the spirit leveling process. AH sight accuracy does not necessarily translate to sights taken at sea but the extra practice ashore certainly can’t hurt.
AH can also open up our fascinating hobby for those who are not fortunate enough to live near the ocean. So here as a public service, after 4 years and 5,500 observations, I offer a “Guide to AH Use”. (mature audiences only please)
1)Forget the liquids. Mercury is hard to come by and store and no other liquid allows you to take star or planet sights. With a mirror even the faintest navigational stars can be sighted. I have shot 37 of the 57 so far.
2)Get a first surface mirror. Regular mirrors will produce ghost images that will affect accuracy by increasing the diameter of the star image. I chose ¼” mirror thickness to keep vial weight from distorting the glass. They are available from Firstsurfacemirrors.com.
3)A hefty metal base will sit still in windy conditions and shake less when you turn the adjustment screws. I used a ½ inch thick aluminum disc.
4)Use 40 threads per inch for adjustment screws. It makes dialing in the level faster because you don’t keep over and under shooting. A buddy of mine turned these on his lathe for me but you can probably find them on-line.
5)Make a precision level. It’s not really difficult. Buy a 10-20 second vial from LevelDevelopments and stick it in a base you make yourself. The step by step directions I have posted can be done in 2 hours or so and believe me I have no special skills or experience. Anyone can do this with household tools. The resulting level does not even need to be rotated end-to-end to improve accuracy. If you make two levels as I did, you can be leveled out and ready to shoot in 2 minutes. You can grind your own vials but trust me, the $50 for the professionaly made vial is worth every nickel.
6)Use 3 X scope. The 7 X causes to much motion of the images and increased magnification can blur the image in the mirror increasing the diameter and decreasing accuracy.
7)Put the mirror on the ground, not on a table. Increasing the distance between the telescope and the mirror improves focus, decreases image size and increases accuracy.
8)Rock the sextant. Don’t try to superimpose the star images. Instead, rock the sextant and watch the sky image pass back and forth through the AH image. The eye can more readily see the alignment as the moving image sweeps back and forth across the fixed one.
9)Be fussy about what you think is good alignment before you call “mark” on the sight. You will likely have to hold your breath or brace your elbows against your chest to stabilize the sextant as you make the last turns of the micrometer. If it doesn’t feel right when you take the scope from your eye, do it again. You can also brace the arc of the sextant against your chin if you take off your glasses.
10)Use your best eye. I shoot with my left eye and I find I have to tip my head up and down to get to the perfect focus point in these no-line progressive tri-focal glasses. The blur created with poor glasses focus will increase the diameter and decrease accuracy. (Vision disclaimer: my eyes are 59 years old)
11)Do very accurate index correction checks, which means use the sun method. The human eye will not be able to superimpose 2 pinpoint star images with the needed accuracy. My Astra instrument seems to remain fairly stable so I check this once per week.
Kind regards, Randy
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