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    Re: Mark 15 sextants
    From: Michael Bradley
    Date: 2019 Mar 1, 09:47 -0800

    Thanks Greg

    You wrote 'With the Ebbco sextant the index error can change +/- 2' depending on temperature but the average index error seems to hold steady.  Another important correction is arc error. The Ebbco I'm working with has arc errors as high as 6'.  There are a few ways to determine arc error. Use an artificial horizon and work up the arc error to apply to get a zero intercept using the GPS position as the assumed position. Do this through every 10° of arc and keep the table with the sextant.  The plastic sextant arc error cause is from a perminent warping of the sextant arc.'

    My Ebbco copy is quite good enough for star and moon shots i.e. not using shades, at altitudes between 20 and 40 degrees.

    The Ebbco handbook says to expect shade errors of 2 to 3 ' ; presumably of random sign. I note that the shades are two pieces of glass sandwiching a circle of optical film, all free to rotate against each other and within the shade holder. Not much hope of consistency there. I did my sun trial runs as sets of five, within half an hour of each other, similar altitudes, same spot on the beach, horizon, weather, stopwatch, having swapped one heavy shade for the other. So it's crudely fair to assume that arc error problems are not to blame for the large differences between the comparison runs: thanks all the same for the warning and advice.

    Thanks then for your help  with the Ebbco - I shall give up in favour of the Davis Mark 3 for what I have in mind ( see below). I have two of those, second hand from ebay, boxed, and am very happy with the simplicity, directness, reasonable accuracy and repeatability of them both using the 'set and wait' procedure.

    ( I've collected 8 names for a sun sight saturday morning workshop session at my club, 2 bringing their own sextants, plus the two Mark 3s I've collected. The attendees to work in pairs. Timing by my iPhone browser or any other smartphone reading and displaying UTC, reduction by Astron on iPad or laptop, plotting initially on previously set up copies of Frank's UPS. Two attendees (so far) have requested, without prompting, the use of 'trad' methods, Yeeesssssss! They'll get their morning and noonish Astron sights to re-work after lunch using the air almanac and Hav-Doniol.... Thanks to yourself, Hanno, Frank, Peter Hakel, and Vigilance of Brixham ...)

    Good sailing

    Michael Bradley

       
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