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    Re: Kollsman periscopic sextant mount
    From: David Pike
    Date: 2016 Nov 28, 10:00 -0800

    That's the exact same dome refracction table as I posted from the 2017 Air Almanac. The dome refraction is caused by the curveture of the dome which caused the inner and the outer surfaces to not be parallel and this is what causes the refraction. Shooting through normal winshields and aircraft windows (as long as you avoid any curved portion common up by the wing roots) then you have no problem with refraction.  gl

    I don’t think it’s just the curvature alone Gary, it’s the difference in curvature.  When you form a dome from a softened sheet of Perspex it ends up thinnest in the centre and thickest near the edges.  There is some argument as to whether it’s best to blow the sheet up into a female mould or pull it over a male mould.  When I worked at Slingsby Sailplanes (for about three weeks in summer 1964, including about one day in the canopy department, so I can speak about this with great authority) they made bubble cockpit covers by covering a male mould all over with about ¼ inch of axle grease.  Then they’d place the Perspex sheet under an array of photoflood lamps to heat it as evenly as possible without the need for a giant oven.  Then they’d quickly place the floppy Perspex over the mould, attach lots of welder style clamps very closely around the edges and pull them down evenly.  The hot Perspex would soften and almost melt the grease so that the Perspex floated on a thin layer of grease above the mould, so that you got a nice smooth finish.  Then they'd leave it overnight to cool. Don’t ask which poor soul had to clean the grease off afterwards, someone did, but it wasn’t me.  https://en.wikipedia.org/wiki/Slingsby_Skylark_4 Clearly, in WW2 they were producing astrodomes by the tens of thousands, but under the stress of wartime production was the thickness close to the expected mean for each part of the dome?  I doubt it, but I bet a standards team tested a few and wrote a report about it.  That’s why I was wondering where it might be.  DaveP

       
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