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    Re: Aircraft Octant Optimal Bubble Size
    From: Robert Eno
    Date: 2013 Mar 5, 19:36 -0500
    
    I will wade in with a few bits' worth from the cheap seats.
     
    Most of the folks here can run circles around me with respect to their knowledge of astro-nav so I will never presume to be an expert on this subject matter. Nevertheless, I have been using bubble sextants for over 30 years. In my experience, and I am referring to bubble sextant/attachment observations from land (not from the air), having the bubble diameter approximately the same as the sun's diameter yields the best results. Most of my experience is with the RAE Mk IX-A and the old-style C.Plath bubble attachment that has a provision for adjusting the bubble size.
     
    Certainly I have attempted observations with a larger bubble but they were all over the map. No real consistent "towards" or "away".  This is just off the top of my head though. I have never thought to examine the thousands of observations I have taken over the years, and in any case, this would not be possible because I have never bothered to record the size of the bubble in my notes.
     
    Over the years, I have read and heard comments to the effect that the smaller the bubble, the more sluggish it becomes. I cannot challenge this outright, except to say that it may be referring to those sextants that employ relatively viscous liquids in the bubble chamber. The RAE Mk IX-A used hexane (according to an old friend of mine from Aus who worked in the factory in WWII) and one of the first things I did with my C.Plath bubble attachment was to replace the old fluid with hexane. Hexane is a very volatile liquid so any sized bubble that forms in this fluid is very "energetic".  Plus, with a freezing point of minus 98C, hexane is good for observations in extreme cold weather. The bubble formed in hexane is not at all sluggish, even at minus 30, which is about the low limit of my enthusiasm for sextant observations.  
     
    Robert
     
    ----- Original Message -----
    Sent: Tuesday, March 05, 2013 12:23 PM
    Subject: [NavList 22638] Re: Aircraft Octant Optimal Bubble Size


    Greg, your results with 2.5 bubble are like mine with a IX-A and (to my eye) 2-sun bubble.
    
    Hewitt
    
    Sent from my iPad
    
    On Mar 5, 2013, at 8:38 AM, "Greg Rudzinski"  wrote:
    
    > Taken this morning were three sets of ten Sun observations using an A-7 bubble octant  using 1.5, 2.5, and 4.5 solar diameter bubbles.
    > 
    > 1.5 Solar Diameters
    > 
    > Intercepts
    > 4.3' away 
    > 6.8'
    > 3.7'
    > 2.7'
    > 4.4'
    > 3.9'
    > 2.1'
    > 2.9'
    > 3.8'
    > 5.6'
    > 
    > 2.5 Solar Diameters
    > intercepts
    > 2.1' away
    > 3.1'
    > 3.0'
    > 3.8'
    > 0.9'
    > 2.6'
    > 2.0'
    > 3.3'
    > 2.2'
    > 2.4'
    > 
    > 4.5 Solar Diameters
    > intercepts
    > 4.3' away
    > 3.1'
    > 3.6'
    > 2.3'
    > 3.6'
    > 3.6'
    > 7.5'
    > 3.7'
    > 7.6'
    > 5.7'
    > 
    > Looks like there is an away bias of about 3' and the 2.5 Solar Diameter is the most consistent bubble size. Comments ?
    > 
    > Greg Rudzinski
    > 
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