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Hi Bill

The clear shade is an astigmatiser.  I just tried it on a star without 
worrying about the horizon.  Sure enough, I saw a line.  Reaching around, I 
rotated the clear shade out of the optical path and got the typical star 
point.  Just add that to the feature set.  Thanks for solving that little 
mystery for me, although in retrospect, it was silly of me not to think of 
that myself!  

There is a small clamp that can be attached to the arc.  It is labeled the 
"greatest angle clamp".  I believe that the observer places it on the arc 
prior to meridian crossing, with the clamp pressed against the index arm. The 
clamp is not clamped, rather, it is loose on the arc. The observer takes 
sights as normal, which moves the greatest angle clamp to ever higher angles 
on the arc.  Eventually, the meridian crossing occurs.  The index arm no 
longer moves the clamp.  The observer tightens down the clamping screw and 
can then move the index arm precisely back to the greatest angle. Wonderful 
little feature for LAN.

Thanks very much for the data on the calibration of your Class A sextant from 
NPL.  I do have some questions, if you don't mind.  

Was that bidirectional or unidirectional?  A bi-directional set runs up the 
scale and then back down the scale, stopping in the same locations.   A 
unidirectional data set merely marches up (or down the scale), stopping in 
the same locations from the same direction.  For a micrometer type device, 
then the unidirectional approach would be preferred, due to gear lash and 
lost motion.  For a vernier device, the accuracy and repeatablity is not 
affected by the gear lash in the same way, so I suggest a bidirectional 
approach might be more representative.  That is, the vernier reading is a 
function of the index arm on the arc.  

How many bidirectional or unidirectional runs did your data consist of?  

We need to differentiate between the accuracy of the data and the 
repeatability of the data.   The accuracy would be the numerical average 
while the repeatability at each point would be the statistical  3 sigma 
evaluation of all data for one arc location.  

I suspect that the NPL merely provides us the accuracy figure of merit and not 
the repeatability, which would be affected by many factors, including 
temperature.

Best Regards
Brad 
  


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