NavList:

Dave,

Thanks for the advices and informations. I will like you to clear for me the meaning of "IMHO" to make sure I do get all that you're saying. Meanwhile, I'm preparing a paper on my home calibration station that I will share with members in the next few days so you can  juge its adequacy (or not). Since I'm on the job and on my lunch time, I do not have the time to answer you properly in details but will do during the week-end. However, most of the information you provided me was already know except the fact that each coarse knob position was having a basic index error plus correction figures. My averager is still in the OEM tolerance limit and I did a chart of it for each degree setting on the fine setting knob. For that, I did 10 averager run for 0 degree, and 10 more for 1 degree and so on and did get a nice correction function that coulb be extrapolated also for arcminutes.

I use this single correction chart for each coarse knob position and it seems to me that till it stays below position 60, it remains the same for both second mirror increment (0 or 5 degrees). I have a steady error of -2' on each reading that is now consider in my calculations. Base on what you told me, I will now establish a correction tables for corse knob position above 60 degrees and try to sea how it is distributed. It still does not make sens to me that the error is constant and than jumps up 10 times it value for high altitude coarse knob setting. As you were depecting, the holes  in the mirror increment dial are distributed proportionnaly and sta constant. Therefore, the error should also stay constant. I simply don't get it and do not understand whats going on for the moment. I will keep my investigation open till I get it for good.

I agree with you, calibration of a Mark-IX sextant (or else) can not be acheive with a screwdriver, tommy bar, or Bristol key. However, it can be done with confidence with the proper tools that as to be built in part to reach OEM calibration standards without having to buy a scientific collimator.

Thanks again and I will keep you inform about the correction table evolution and calibration conclusions. I can way the sky to clear up although the weather will not improve before a few days.

Jean