NavList:

Greetings to all of you, passionate Navlist members,

I write today about some calibration issues that I witnesses while using my recently restored Mark-IX-A bubble sextant. To calibrate the beast, I had commission a friend who as a lathe a sextant housing replica made from a bloc of aluminum. The housing replica as a hole drill at the exact center position axes of the index mirror in which I put a riffle laser bore to provide a 0-degree reference alignment. This reference level is use to adequately position a led 15 feet away that will provide a ray of light tin as a needle that looks exactly as a star. The 0-degree led as also another one vertically aligns 5 degrees higher to provide calibration reference for the 5 degrees increment setting on the second mirror.  I won’t detail the entire procedure here on how to calibrate or adjust the calibration station but if some of you are interested by the topic, I can realize a write-up another time.

I will like here to report some interesting observations made while fooling around with my sextant and calibration station. To my great surprise, I noted that the conventional use of the fine setting knob was always offset although calibration was done and perfectly align. For example, when I was turning up the fine setting knob from 0 degree to 5 degrees without exceeding this value, the 5 degrees increment led was always 12’ of arc below expected bubble center. However, if I repeat this operation by first turning the fine setting knob all the way to the upper end rotation and than going down to 5 degrees without exceeding this value on the way down, I was getting a perfect reading each time. I do not know if this is normal or if this is a sign of wearing over time, but since than, I always make sure that each reading maneuvers respect this procedure to increase accuracy. Actually, I always get an average error of less than 2’ of arc which was the OEM standard calibration tolerance required by Hugues industries.

However, here is another strange thing I notices and could hardly explain for the moment. Perhaps some of you Mark-IX user could help me with your expertise. After calibration, I do get nice readings on all ten’s setting knob positions up to 50 degrees (less than 2’). Nevertheless, once the tens setting knob hit the 60 degrees of elevation the error suddenly increases 10 times to reach 20’ steadily. How could this be possible? I check and double check and check again the measurement and always get this kind of stratospheric value. Is there some adjustment to be consider in the Tens setting knob mechanism that could be done to rectified this situation? For the moment, I will test again with high bodies to generate a correction table for these high-altitude situations but will rather like to rectified the mechanism if possible. Hope I will found answers among experts.

Warmest regards from Canada

Jean Villemagne (VE2 CAL)

N45° 44,8’  W073° 38,3