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    Re: Corrections for air bubble sextants
    From: Howard G
    Date: 2022 Jul 30, 22:26 +0000
    Hi Juan

    Whilst Art is right about hard mounting the kollsman and a single shot like a marine sextant, you can't do that with the Mk IX.

    The beauty of the averager is that it is there for a reason so use it, you will find you skill level using the bubble increases.

    In an aircraft, the bubble seldom sits nicely in the centre whilst you 'take your shot'. Everything in an aircraft is moving quickly.

    eg in a P3 B Orion at altitude you are travelling at 6 nms a minute.
    At low level monitoring sonobuoys you have turbulence, often, cloud, people walking around in the aircraft.

    The pilot Always deselected autopilot and hand flew and people stopped moving so the aircraft could be flown accurately.

    But despite all this the bubble moves and that is the job of the averager.

    Get use to using it standing still. Gain proficiency, it takes 100s of shots to get comfortable.

    The beauty is you know your location.

    Then just to get a taste of reality and presuming you can pocket the bubble light supply try walking in a straight line dyring the whole shot on an even surface. Difficult, yep.

    Or shooting through the sun roof of a car travelling at say 30 km.

    Just adding some 'fun' to the bubble life.

    That is the beauty of a bubble sextant.

    I believe u boots had gyro stabilised sextants but awfully complex.

    Juan, have fun, but always use averager except when you actually want to emulate a marine shot, which you can't as you inland.

    My bubble USAAF sextant is both a bubble and a real horizon sextant.

    But I also have a plastc Davis marine sextant and a false horizon thingame which is pain to use.

    Howard G



    Sent from my Galaxy



    -------- Original message --------
    From: Art Leung <NoReply_ArtLeung@fer3.com>
    Date: 31/7/22 08:02 (GMT+10:00)
    To: Howard George <HHG@raptorbusinessservices.com.au>
    Subject: [NavList] Re: Corrections for air bubble sextants

    Juanjo - congratulations on picking up your aviation bubble sextants. I run a Kollsman regularly for the same reason that you do and get very good results from mine. 

    Sight reduction corrections for a bubble sextant are actually a tiny bit easier than a nautical one because you do not have to account for Dip correction. If you take sights from an aircraft, altitude and coriolus corrections are necessary but if you are on the ground near sea level and stationary you will not need to use these corrections. You just need to apply the standard refraction correction for the height of the object and use 0 for Dip. 

    One thing you will need to do is establish the index correction for your sextant. There are several methods to do this. I find the easiest is to just take several star or sun sights and do a standard reduction. Ideally you will see your Lines Of Position miss your actual position by a fairly constant amount - that average miss amount is your index error. 

    Another consideration is stabilizing the bubble. I shoot from a tripod using a custom made (3D printed) mount.

    You also have a choice to take a shot and record the time (much like a nautical sextant shot) or use the averager built into the sextant and take timed shots. I tend to use the averager because it's fun. I also precompute my shots to be done at a set time  

    If you need help making the power supply or mount, let me know. 

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