NavList:

Many RAF and RCAF bases where MkIX series sextants were used had portable collimators (although they look pretty solid pieces of kit to me).  However, it was stressed that since said collimators couldn’t possibly match the accuracy of the equipment used by the manufacturer (Hughes) or servicing agent (Fenns of Farnborough in the UK), they were only to be used for serviceability checking.  That is, if the average of twelve readings was greater than 2’ of arc, the sextant, after rechecking by supervisor, should be placed unserviceable and returned to the manufacturer/servicing agent.  If the average error was less than 2’ of arc, the sextant was to be judged serviceable, and the manufactures/service agent’s last calibration figure should be used in preference to those just recorded from the portable collimator.

As the fine turning knob of the index mirror of the MkIX series sextants only turned the index mirror just over 5° actual (10° altitude).  The whole mirror assembly was tilted though 5° actual (10° altitude) steps using the coarse knob. Pressing the course knob lifted a pin from one of ten holes 5° actual (10° altitude) apart and allowed a different hole to be selected.  Because of this, the sextant was delivered with a basic index error plus correction figures for each of the 10 stops (9 actually, they never tried to calibrate the Depression Stop). 

To allow for an observation with an Hc between n8° and n12° running out of travel during an observation, the lower mirror could be flicked though 2.5° actual (5° altitude).  E.g.  if Hc was 41° 30’, use 30° on the 10°s knob with the 5°°° increase in.  Suitable metal curtains were arranged to slide into place, so that an Hs around 11° 30’ could be read directly, and you only had to add 30° to 11° 30’ and not 30° to 5° to 6 °30’.

With such a mechanical system, there’s bound to be some backlash after 70 years, and with care in a stable situation you can show it.  However, IMHO it’s probably best to forget about it and hope it will average out in the long term.  In the air, using the averaging mechanism, the most important thing is to follow the star and ignore your feelings irrespective of which way you need to twiddle the knob.  Dickey Richardson gives a particularly good explanation of why this is so in the 1941 edition of AP1234.  On the ground, in the field, you’ll never remember your recent knob twiddling history or the force which you applied.  This is particularly true of stiff old MkIXs. 

Again IMHO, calibration of MkIX series sextants is best done by the amateur enthusiast in this day and age against the bodies they were designed to observe using Frank Reed’s GPS Anti-Spoof app although an average of twelve or more observations should still be used.  My other observation from experience with old aircraft sextants is be prepared to accept large corrections rather than try to take the error out yourself with a screwdriver, tommy bar, or Bristol key.  Touch the adjustment screws without years of experience, and you’ll only make it worse. DaveP