From: Michael Bradley
Date: 2014 Aug 25, 15:15 -0700
In the absence of any positive response to the recent Francis Barker posts, I beg leave to report that my sample of the Francis Barber Small Craft Precision Sextant has none of the problems reported by Geoffrey Kolbe. The main scale intervals are 1/2 degrees - 30 intervals in 15 degrees of the main scale. The vernier scale has 31 intervals opposite the 30 intervals of the main scale, thereby indicating increments of 1/30 of the main scale increment that is 1/30 of 1/2 degree, that is 1 minute. Being in effect a 1 3/4" radius instrument, the mechanical precision can reasonably be expected to be at best 1/4 of that achieved by a 7" professional sextant. What would otherwise be the horizon mirror is a solid mirror for the reflected image with the horizon viewed by looking past it's edge, as in the Freiberger yacht sextant. There have been previous posts, I believe, saying the shades cover the horizon. Not true, in my sample at least. The two moveable shades are designed to cover only the mirror half of the field of view. There is an additional darkened eye piece providing extra shading to the whole field of view if required.
In my days of using minimum systems for fun, before smart phones, having done the Astra thing and the 19th century vernier sextant thing, it got me across the Bay of Biscay using Longitude by Chronometer and Mer Pass of the Sun. For a timer I used a £1 battery powered sports timer which wrapped round after 24 hours, rated for a week before departure. For a sun almanac the Henning Umland 9 lines of formula for sun ephemerides ran on a small 400 step programmable scientific calculator bought in a street market for £10. And the two page azimuth nomograph made it possible to plot LOPs from the Long by Chron sights.
What's not to like for such a minimum system?
55 North and about 60 NM South East of Geoffrey Kolbe.....