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    Troughton & Simms box sextant and Davis artificial horizon
    From: Geoffrey Kolbe
    Date: 2015 Oct 16, 20:57 +0100
    After watching ebay offers for some years, I finally took the plunge and acquired a Troughton & Simms box sextant on ebay. It was a clean looking example where the screw slots had obviously not been chewed by someone taking it apart at some stage during the last century. It is somewhat grubby with accumulated dirt, but perfectly usable, so I still have not taken it apart to clean it. I have found it surprisingly accurate.

    I have been using the sextant in conjunction with a Davis artificial horizon. The box sextant has no horizon filters, so I had to ensure that there was sufficient filtering from the covers that form the cloche 'roof' over the water bath for use when taking sun sights. The Davis AI comes with two perspex coloured filter covers, but I measured the intensity reduction to be about equivalent to a Neutral Density Filter 2.5 (a factor of about 320) which (in my opinion) is not enough by a long way. I measured the equivalent NDF reduction of the index mirror filters on the little box sextant to be about 4.5 (an intensity reduction factor of about 32,000) and so to have the sun as reflected from the Davis AI about the same brightness as the filtered sun from the sextant index mirror, I had made two NDF glass filters - one NDF 3.0 and one NDF 1.0 - to form the cloche 'roof' for the AI when using it for sun measurements. (The remaining equivalent NDF 0.5 reduction (or thereabouts) comes from the reflection off the surface of the water in the bath.)

    Adjusting the sextant using the small knurled knob provided is a little finicky, so my method was to set the two sun images slightly apart and watch them come closer together until they just touched - and time this moment.

    I am surprised that the accuracy has generally been better than one MOA! I have just returned from a walking holiday in Tuscany (Italy) and took the sextant, Davis AI, calculator, almanac, clock (deckwatch), stopwatch etc. with me in a backpack on a 15 mile hike through the countryside on the first day. At lunch I took a sight and was 1 MOA away from my position as taken off the map - which I thought quite acceptable in the circumstances. I only did this on the first day however as the weight even for this CN kit in miniature was telling after 15 miles... but the principle of using a sextant to keep oneself found on land when hiking on foot was proven.

    Geoffrey Kolbe
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