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    Re: Two mirror a.h.
    From: Gary LaPook
    Date: 2011 Nov 2, 17:42 -0700
     I have been using mercury for more than thirty years but  I like your idea of a mirror artificial horizon because my mercury only makes a puddle about 2 - 1/2 inches in diameter severely  restricting my field of view. I have have to be a contortionist some times to place my eye close enough so as to be able to keep the star in sight and then I have to keep my head from moving even a little bit or I lose the image.  I am not going to buy several pounds of mercury to make the puddle bigger.

    So I decided to make my own mirror horizon and, as  usual, I wanted to do it down and dirty, simple and cheap. I have a patio table that is quite solid but it is a 18 mils, one degree, out of level. But, even though it is not level, any flat surface has one azimuth that is level. Say the table tilts down one degree towards the north then the table must tilt up one degree towards the south. Some azimuth between north and south must be level. Going with this idea I figured that I could level the mirror in one axis simply by finding the level azimuth on my table. I needed something flat and smooth to use as the base. I went to the craft store and bought a 12 by 12 inch mirror and then a second mirror to be the base, about seven bucks each. I scraped off the little pads on the backs so that I had a smooth flat surface.

    I placed the base mirror on the table and set my gunner's quadrant to zero, placed in on the base mirror aligned with one edge. I then rotated the base mirror slowly until the bubble indicated level and I switched the quadrant end-for-end to recheck the level, it was.

    Next I placed the second mirror aligned on top of the base mirror, offset on one edge by a quarter of an inch, and placed the quadrant on it aligned with the edge at right angles from the first orientation. Then, by trial and error, using the leaves from a common feeler gauge, I found a combination that I slid under the edge of the mirror to level it on the other axis, the edge of the mirror acting as the pivot.

    I took five shots of the sun resulting in intercepts computed from my position of 0.7 A; 1.3 A; 0.6 T; 1.7 A; and 1.5 A. Average is 0.9 and the sigma is also 0.9. Not too bad. So I know it is level in that direction and I will shoot some stars tonight to test for levelness in the other directions.

    It is nice having such a large reflecting surface since I had no problem keeping the reflection of the sun in sight.

    See attached photos.


    --- On Wed, 10/26/11, Randall.F.Morrow@kp.org <Randall.F.Morrow@kp.org> wrote:

    From: Randall.F.Morrow@kp.org <Randall.F.Morrow@kp.org>
    Subject: [NavList] Two mirror a.h.
    To: NavList@fer3.com
    Date: Wednesday, October 26, 2011, 7:59 AM

    My a.h..   The first is made from a plastic cutting board and a second surface mirror.  Look at the back edge in the photo to see the second surface reflection.
    The second is 3/8 inch aluminum plate and if you again look at the back edge you can see the 'first-surface' difference.   The lines drawn on the mirror were suggested on Nav-list, to get the level placed and reversed consistently

    Randall F Morrow PT
    Ergonomics Safety Consultant
    Chronic Pain Program
    Kaiser Permanente - Kern County - Bakersfield

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