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    Artificial horizon construction
    From: Randall Morrow
    Date: 2011 Nov 18, 10:56 -0800

       There is nothing new under the heavens.   While reading through old Nav-list posts for tidbits of wisdom and lore I ran across a March 2011 posting by P.G. which includes a photo of a mirror a.h, nearly identical to first one I had made for myself.   The plastic cutting board was probably came from the same store.   I had just cut one end off the board to make it square and had chosen to use 3 leveling screws instead of 4.     When I joined the Nav-list at the end of September in 2011 and rambled on and on for advice to use a mirror horizon, I didn't know someone had already invented this wheel.   The mirror a.h. works really well, and many of my recent observations have intercepts of less than a mile.   (33 consecutive observations over several nights with a mean intercept of 1.31 miles using a plastic sextant)
       
    For the benefit of other land locked navigators, I offer the here the sum of what I have learned from my experiences, (and Nav-list help), about the mirror artificial horizon:  

    1) Use a metal base, as thick and heavy as you can find.  
    The base I use is about 8 inches in diameter 1/2 in thick aluminum but 1/4 inch mild steel plate would do.   The weight makes it stay put when you are turning the adjusting screws.   I chose a round shape but it wouldn't matter, the three leg system works either way.

    2) Use a first surface mirror.  
    Regular mirrors reflect off the back, or second surface.  This gives ghost reflections, especially with low altitude sights.   Mine was $16.00 plus shipping and 6.5 inches square.  Go to a craft source rather than a science one.   The prices are better.   They use these mirrors to make kaleidoscopes and they are flat and crystal clear.   They tend to collect very fine scratches easily but this doesn't seem to effect image clarity.

    3) Get a big mirror 6 inches or more across.  
    A larger surface allows you to get sights at lower angles and keep dim stars in sight with less effort.   Mine actually hangs over the edge of the plate at the corners.   The bigger mirror also gives more surface area to shoot around the adjusting screws, when doing very low angle sights.

    4) Set the a.h on a small waist-high table or stool so you can stand doing sights.  
    Sitting down had been suggested but I found that being able to move from side to side affects star brightness making dim sights easier.   And with the whole sky open to you you will have to move all the way around the table to shoot various bodies and stand back as far as 15 or more feet to get lower angles.   You can't do this from a chair.

    5) Get an accurate spirit level, small enough to fit between the adjusting screws.  
    I started with a digital level which, though it gave good results, it was slower to use and the results were inconsistent, 'black box' results.   Being large it was always off center from the adjusting screws which might have thrown off the levels.

    6) Use wing-nut heads on the adjusting screws.  
    To turn a round or hex-head screw you use a pinch grip and you have to squeeze, which tends to move the base.  The T-handle of wing nuts allows for a very delicate touch of opposite ends of the wing head.   The standard 1/4 to 20 thread is fine enough and available at any good hardware store for $1.25.

    7) Screw pan head nuts onto the 'foot' end of the adjusting screws.  
    This creates a center pivot point that reduces 'walking' when the screws are turned.    Put the 1 1/2 inch long screws through the base then add the cap nuts.

    8) Use three screws rather than 4.  
    This was suggested by a list member.   Three screws set 120 degrees apart form a "T" that allows you to do leveling in only 2 directions.    Do the vertical of the T first, then the cross, turning the level the same direction each time.  Two rotations like this give the reversing of the level suggested by a list member and slowly reduces the distance the bubble moves when correcting, cutting the bubble "settling" time with each cycle around the T.   It takes  5 minutes to set up and level out.

    9) Attach the mirror to the base with foam double stick tape.  
    This means drilling fewer holes.   It does not affect level because the leveling takes place at the surface of the mirror, not at the plate.

    Some (mirror) artificial horizon advantages:

    You can do sights on windy days.

    You can do sights at night even of the dimmer stars.

    You can do sights from any angle without re-orienting the unit.

    There are no liquids to bother with or store.

    I can take the unit with me anywhere.

    If you use the natural horizon there is always a time pressure due to twilight limits.  

    If you use the natural horizon you can only shoot half the sky. (unless you're at sea of course).  




    Randall F Morrow
    Kern County - Bakersfield, CA


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