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    Re: Bubble sextant intervention needed.
    From: Greg Rudzinski
    Date: 2012 Oct 19, 10:46 -0700


    Sextants and octants are painted black to prevent stray reflections. Keep sextants and octants out of direct Sun light as a rule. This will protect the scope optic coatings and prevent the instrument frame from becoming warmer than ambient temperature. 5 minutes out of the box should be enough time for the instrument to obtain ambient temperature.

    Greg Rudzinski

    [NavList] Bubble sextant intervention needed.
    From: Paul Dolkas
    Date: 18 Oct 2012 22:04

    I have a stupid question - or perhaps I just need somebody to stop me from doing something stupid. I’ve been playing around with my A-12 bubble sextant for a few months now, and I’ve noticed an interesting phenomena. The bubble cell, unlike some of the more expensive models, has a fixed volume. So when it gets hot outside, the fluid inside the cell expands and the bubble gets smaller. On at least one occasion, it has disappeared entirely. Knowing the pressure that must exist inside the cell to do this, this can’t be good. Luckily, I haven’t had it break the cell window.

    Which brings me to my point. I know some nautical sextants are painted white to avoid getting hot and going out of alignment in the tropics. However, I have yet to see any white aircraft sextants out there. Now, this probably because models like the A-12 were made to be used in open cockpit aircraft, where freezing was more of a concern than baking. But for those of us who use them at sea level, wouldn’t it make sense to paint them white - at least on the outsides?

    So to check this out (and to embrace my inner science geek) I ran an experiment where I took 2 aluminum blocks and painted one white & the other black and left them out in the midday sun (solar elevation: 45o ). Naturally the black one ran hotter, by about 20oF. I did some calculations, and this modest temperature rise is enough to almost make a small (1/16”) bubble disappear.

    So my question is: aside from making what will probably be a truly butt-ugly sextant, is there any reason I shouldn’t paint the outside of the sextant white? (naturally, I would leave the inside black to minimize stray light.) I kinda hate to go against tradition, but there don’t seem to be any real downsides to doing this. Can anybody think of any?



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