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Michael Josefsson

Here are more reasons why an 80-year-old MkIX bubble sextant will never be as accurate as a marine sextant.  Firstly, look at all the levers and cams, all of which must be pivoted.  Secondly, the MkIX only measures over just over nine degrees.  The full range from -10 to 90 degrees is made up by tilting the index mirror through ten different decade stops from D to 80 and the lower mirror by a five degrees using the 5 degree increase lever.  During servicing , the MkIX was calibrated separately for each of the ten decade stops, so if taking a series of shots, you might work your way through more than one decade stop with slightly different calibration.  Thirdly, the mirrors are sprung into the correct decade stop with a slight “ping”, which might lead to wear.  The strength of the “ping” might also affect the amount the index mirror moves.  Fourthly, whilst the MkIX was designed to be “tilt-proof” by ensuring that the radius of the inside of the top glass of the bubble chamber is the same as the focal length of the attendant optical system, the system is rarely perfect although the MkIX seems a lot better than my home filled with white spirits A12.  Therefore, it’s important to keep the bubble as near to the centre of the chamber as possible.  DaveP