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    Re: Mirror Artificial Horizon -- Revisited Once Again
    From: Paul Dolkas
    Date: 2012 Sep 20, 20:19 -0700

    Bruce-

     

    The real crux of trying to float a mirror is to make sure it’s not “listing” to one side – the error is doubled due to reflection, and we are talking a few % of a degree. It’s not at all easy to do.

     

    Here’ a thought though on how you can reduce the effect of wind: Have your mirror float on the boundary between a heavy fluid (say really thick motor oil) and a lighter one (say water). The wind will create ripples on the surface of the top fluid, but the reflection will be very faint compared with the mirror’s image. Small wind ripples will probably not disturb the mirror appreciably. Still doesn’t get you around the problem of listing.

     

    The classic way optical engineers would create a self leveling mirror was to have a dish of mercury with a top layer of some thick, but clear liquid – say glycerin. Works really well, but that much Mercury is super hard to come by these days, and you really don’t want to spill any of it if you are indoors (the vapors are toxic). If you can somehow get about a hundred ml of Mercury, however this is the way to go.

     

    Paul Dolkas

     

    From: navlist-bounce@fer3.com [mailto:navlist-bounce@fer3.com] On Behalf Of Bruce J. Pennino
    Sent: Thursday, September 20, 2012 6:32 PM
    To: NavList@fer3.com
    Subject: [NavList] Mirror Artificial Horizon -- Revisited Once Again

     

    Hello All:

    After reading most of the very helpful postings, particularly those last November,I've decided to try to make a large AH because I really don't want to use a commercial small one. I'm hoping to use it on stars.

    My first effort was "quick and dirty": I used some small planed boards to make a small raft. The raft was symmetrical and I located the center of gravity CG (probably not to the precision required), and placed a low cost 6 inch diameter mirror on the CG.The mirror was in a frame that could not be removed. The results of my sun sights on a windy day were extremely poor. But the raft and mirror simulated a rocking boat and made it possible to do sights. My first question, can a first surface mirror on a reasonable size raft in a baking dish work?

    I have access to a machine shop, steel plate, aluminum etc. My next option is a 3/8" steel plate bigger by 3 inches all around a front surface mirror. I'll use 3, 3/8" leveling screws. I've found at a local mirror shop where 10"x16"x1/8" front surface mirror can be purchased for $25. Seems reasonable to me. I can also buy 6"x8"x1/4" front surface mirror at sciplus.com for $27. Recommendations?

    I could not find the recommended place to buy spirit level vials. There are many sources. Suggestions for source?

    I would use 2 vials. One on the vertical centerline and one on a left-to-right centerline. OK? I understand for reasonable accuracy of a sight (a few minutes) I need a vial with 0.75mm/m (2.4 minute of arc) precision (or accuracy or both). Do I need 0.5 mm/m (1.8 minute of arc? For overall ease, should the spirit vials be at least 4-6 inches in size?

    Thanks for the help.

    Bruce
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