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    Re: accuracy of glass artificial horizon figure
    From: George Huxtable
    Date: 2008 Aug 22, 11:39 +0100

    
    Bruce Hamilton asked-
    
    George:
    I clipped the information below from an online tool catalogue.  How little 
    money would I have to spend to buy two levels accurate enough to calibrate a 
    glass sheet (or mirror) horizon?
    
    Horizontal-Mount Levels
    N  5/8" Dia. x 3 3/4" Lg.0.001520 sec./2mm0.228"Brass (Black 
    Finish)2160A5$76.30
    
    N  5/8" Dia. x 3 3/4" Lg.0.0051 min./2mm0.228"Brass (Black 
    Finish)2160A763.04
    
    N  5/8" Dia. x 4 13/16" Lg.0.0072 min./0.1"0.196"Chrome-Plated 
    Brass2160A196.98
    
    P  13/16" Dia. x 6 3/8" Lg.0.00036 sec./2mm0.375"Brass (Black 
    Finish)2160A2171.20
    
    Q 2 15/16" Lg. x  9/16" Wd. x  9/16" Ht.0.03315 
    min./0.050"0.156"Aluminum2160A327.80
    
    Q 2 15/16" Lg. x  9/16" Wd. x  9/16" Ht.0.03315 min./0.1"0.156"Polycarbonate 
    (Black)2160A410.30
    
    Q 2 15/16" Lg. x  9/16" Wd. x  9/16" Ht.0.12245 min./0.1"0.156"Polycarbonate 
    (Black)2160A9*��9.50
    
    Q 2 31/32" Lg. x  9/16" Wd. x  37/64" Ht.0.12245 
    min./0.050"0.156"Polycarbonate (Black)3329A46 10.30
    
    R 2 5/32" Lg. x  29/64" Wd. x  21/64" Ht.0.1851 sec./0.1"0.079"Brass (Black 
    Finish)2160A3611.95
    
    R 2 5/32" Lg. x  29/64" Wd. x  21/64" Ht.0.1851 sec./0.1"0.079"Brass (Chrome 
    Finish)2160A3711.95
    
    R 2 1/2" Lg. x  1/2" Wd. x  5/8" Ht.0.0051 min./2mm0.125"Brass (Black 
    Finish)2160A1177.20
    
    R 3 23/64" Lg. x  5/8" Wd. x  3/4" Ht.0.0051 min./2mm0.125"Brass (Black 
    Finish)2160A672.92
    
    ===============
    
    Although Bruce addressed the question to me, I doubt if I am the best fellow 
    on this list to answer him. To be honest, I've had little experience in 
    using levels and artificial horizons.
    
    And I'm unsure about some of the sensitivity numbers Bruce quoted anyway, as 
    two columns appear to have run together, and there seems to be little 
    correspondence between them. What exactly do those columns represent? I fear 
    that, copied once again here, those tables have suffered further mutilation.
    
    However, let me make some guesses. Clearly, for this job, a highly-sensitive 
    level is called for. The one at the top of Bruce's list, if I follow his 
    numbers right, at 20 sec per 2mm shift, appears to fill that bill, at 
    $76.30. But there are other matters to consider, such as how accurately the 
    tube has been set into the base, and how uniformly it's been ground, for 
    smooth movement of the bubble, and whether the glass plate will be big 
    enough to accomodate the span between its feet..
    
    Bill Morris rightly asked  why  two levels would be called for. What he 
    didn't point out, because it's so obvious, is that you should always try a 
    level both ways round, and take the average.
    
    Bill wrote- "The levelling screws end in ball bearings. One of them sits on 
    a plane , one in a conical depression and one in a vee groove machined in 
    bits of brass let into thesub=frame." Clearly, and this has become 
    inreasingly apparent from his postings, here we have a real expert, with a 
    full understanding of kinematic design, and an asset to this list.
    
    However, I'm a bit uneasy about the way his mirror sits on "a bed of thin 
    felt" . Is he quite certain that when he lifts off the level, after setting 
    the plate horizontal, the felt doesn't spring back, just a touch, when its 
    weight comes off? That might be checked by adding a corresponding bit of 
    extra weight while keeping the level in position, to see what happens..
    
    ================
    
    And Gary LaPook has added-
    
    "I have been taking series of shots of Jupiter in my artificial horizon 
    (since it is favorably placed) and I want to takeseveral more series then I 
    will write up what I found."
    
    Gary has a mercury horizon to use and that's undoubtedly the best, although 
    there are problems. You can alternatively use water, or better, dark oil, or 
    molasses (which here we call treacle), but those are difficult to use with 
    all but very bright stars, and very clear nights. However, Jupiter is 
    presently very bright, and those liquids could well be usable with Jupiter, 
    if anyone wants to do the same as Gary.
    
    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.
    
    
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