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    Is it a boat? Is it a plane? It's an Astro-Boat
    From: David Pike
    Date: 2015 Feb 13, 12:35 -0800

    In many ways the ideal sextant for inland static situations is a periscopic pendulous reference sextant.  Acceleration errors reduce to zero, and errors inherent in the optical system and from aiming at the middle of things are likely to be no more than with a nautical sextant and external AH arrangement.  The problems are urban clutter, light pollution, somewhere to mount the sextant, and the problems of working outside in the cold and possibly in the dark.  I’ve long thought the best idea would be to install a sextant mounting in an old but roadworthy caravan, and tow it somewhere quiet.  Then you could shoot in clear surroundings and from a nice warm cabin.  Well after looking for nearly a year, I’ve still been unable to discover an old caravan for less than 500GBP, so I decided to modify my river cruiser instead. 
    I made a new hatch with a suitable hole to take the Smiths sextant mounting liberated from Dominie XS714 when she was scrapped.  The new hatch just sits in place of the original, so it can be removed when not required.  The recess would catch rain and cause leaks.  The Mk2C sextant pushes up in the normal way, and the power lead clips onto my coat and is attached to two AA batteries.  The swivel eyepiece allows different heights of operator to use the sextant easily.  Unlike the Kollsman, you have to wind the sextant up to get the reference.  It’s like that so you don’t try to start an unwound sextant.  As you don’t need to use the averager when stationary, you have to remember to run it down when you’re finished to ease the spring. 
    Today was the first try out.  Everything fitted, but someone (I cant think who) had messed up the marking out, and the 10mm chamfer built in to allow for the fore and aft slope of the roof  had found itself applied athwartships instead.  Fortunately, the vessel has a natural list in that direction, so it looks as though it’s there on purpose.  The Smiths mounting still has plenty of wiggle angle left, so I figure if I don’t tell anyone, no one will notice.  Unfortunately, it was 8/8 cloud all day, so I’ll have to wait until next visit to shoot some astro.  Dave

     



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