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    Re: Hybrid Artificial Horizon
    From: George Huxtable
    Date: 2008 Sep 12, 08:18 +0100

    Interesting and inventive notions, there, from Ken Muldrew.
    
    He ended-
    
    "What would really be nice would be some kind of simple calibration 
    procedure that could be used to measure the error every time you set it up 
    (like measuring index error) but I can�t think of anything off hand."
    
    If he has something in view with an unchanging altitude, such as a distant 
    street-light or a culminating star, why not measure its altitude, then 
    rotate the whole gear, raft and tray, by 180�, and remeasure?
    
    If a trick like that works, then he can correct for small errors of tilt, as 
    long as he always keeps the orientation of the raft aligned (at least 
    approximately) with the direction of the object being observed. Perhaps, by 
    that technique,  he could find and mark the direction on the raft in which 
    the tilt was zero, and always align that.
    
    But the important aim would be to ensure stability of the flotation plane of 
    the raft, whatever happens. Ken has mentioned the problems that will ensue 
    if water drops on the top upset its balance. Any contact with the sides of 
    the tray would have to be avoided, or even a near-miss, because of 
    surface-tension meniscus effects. To that end, it might be helpful to gently 
    tether the raft by gluing light silk threads to hold it loosely in place, to 
    keep it roughly central. They would have to be long enough to let the raft 
    settle to the bottom when the water was emptied.
    
    But the big problem, as I see it, is to avoid the trapping of air bubbles at 
    the under surface, and for that I have no solution.
    
    George.
    
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
    
    
    
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    ----- Original Message ----- 
    From: "Ken Muldrew" 
    To: 
    Sent: Thursday, September 11, 2008 9:07 PM
    Subject: [NavList 6250] Hybrid Artificial Horizon
    
    
    For those of us who navigate from our back yard, for fun, practice, or
    just as an excuse to watch the night sky, an artificial horizon is a
    necessary part of our kit. Now that mercury is hard to come by, and a bit
    of a nuisance anyway, most of us use a tray of water to reflect the object
    that we are trying to get an altitude for. This works well for the sun and
    moon, and is OK for bright planets, but it can be a trying experience to
    bring down stars. As an alternative, some people of a metrological bent
    will level a mirror or piece of glass, but this is a real challenge in
    precision, and the trying leveling process has to be repeated every time
    the mirror is moved (some back yards have a lot of trees). I think that
    everyone who tries this in the dark begins to wish they had a jar of
    mercury that they could just pour out and start observing.
    
    I decided to try to combine the two approaches by floating a mirror on
    water. I had no wish to try to build a perfectly balanced, hollow mirror,
    so I thought that I could just mark the surface so that I would always use
    it in the same orientation and the error could be calibrated, just as is
    done for the index error with a sextant. So I bought a mirror, cut it in
    half, and used silicone sealant to glue strips of 1/4" plexiglass around
    the sides. The mirror was 1/8" plate glass (it was actually a front
    surface mirror, but polished on the back, so I used it as a rear surface
    mirror to protect the coating). I used 3 pieces of plexi on each side to
    end up with a sealed box that was about 6" x 3.5" x 1". I built a tray to
    float it that was just a bit bigger so that I could stick my fingers in on
    the sides and remove the mirror from the water. The attached pictures show
    the whole setup.
    
    Last night Jupiter culminated just as twilight was ending so I used that
    to get a calibration. Just a note on the ease of use: it set up in
    seconds, and stopped bobbing in about 5 seconds; a light breeze didn�t
    affect it at all, and you can see the whole night sky (piecewise ;-) ) as
    clearly as if you were looking up. The measured maximum altitude was
    31�9�, the index error 8.1�, giving an apparent altitude of 15�30.45�.
    Refraction (10�C, 1000m above sea level) was 3.14� for a meridian altitude
    of 15�27.3�. The declination of Jupiter was 23�9.1� and my latitude was
    51�8.8� so the altitude should have been 15�42.1�. If I add 29.6� to the
    doubled angle for a mirror error, then these numbers match, so I�ll call
    that the mirror error.
    
    To test it out, I took altitudes of Arcturus and Alpheratz, both trivial
    to find in the mirror despite it being just after twilight and all the
    lights of a big city adding to the general lack of darkness (not that it
    should be difficult, but if you have ever hunted for a star in a water
    horizon, you�ll know why I mention it). Here is the data:
    
    Arcturus
    
    9h18m46s 51�7.6'
    9h21m27s 50�15'
    9h22m39s 49�50'
    9h23m24s 49�37'
    -------- ------
    9h21m33s 50�12'24" - 8.1' + 29.6' / 2 = 25�17' - 1.9' refr = 25�15.1'
    
    Almanac gives 25�28.2'for a difference of 13.1'
    
    Alpheratz
    
    9h27m25s 63�19.8'
    9h28m30s 63�40.2'
    9h29m29s 63�57.6'
    9h30m30s 64�15.8'
    9h31m24s 64�32.2'
    -------- ------
    9h29m27s 63�57.12' - 8.1' + 29.6' / 2 = 32�9.3' - 1.4' refr = 32�7.9'
    
    Almanac gives 32�24.8'for a difference of 16.9'
    
    So the results are not very good. Note that I moved the mirror and spilled
    water between each round of sights. Also, when I moved the setup after
    getting Jupiter�s altitude, I noticed that water spilled over the edge
    just as I touched the side of the water box (without actually moving the
    box) so there was a meniscus on one end. I don�t know how much off-level
    the water might have been due to surface forces, but it may have had some
    bearing on the reading. Also, I fear that I slopped some water drops on
    top of the mirror during the moves; that might also have changed the
    balance of the floating box.
    
    I will have to do some more testing to see if better technique can improve
    the results. I hope so, because the ease of use makes this setup really
    attractive. A more careful assembly would probably help (I spent about an
    hour building the whole apparatus). What would really be nice would be
    some kind of simple calibration procedure that could be used to measure
    the error every time you set it up (like measuring index error) but I
    can�t think of anything off hand.
    
    For those interested, the mirror I used was this one:
    http://www.surplusshed.com/pages/item/l3757.html
    everything else was scrap.
    
       \----------------------------+----------------------------+   o_,
     O_/ \    Ken Muldrew, PhD      | Voice: (403) 220-5976      |   <\__/7
     <\__  \  Dept. of Cell Biology | Fax:   (403) 270-0617      |     | /
      "\ L  | University of Calgary | kmuldrew@ucalgary.ca       |   / /
       <    +-----------------------+----------------------------+ / /
                   Morning coffee recapitulate phylogeny          L/
    
    
    
    
    
    
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