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Also wuth PA I take into account that there may be a slight refractuon due to atmosphere which may vary on the wetness of the sky...So even when pole master says it is perfect and you find there is still a slight drift sky refraction may be the reason...These are my ideas and U can cite no authority so I may be talking thru my hat. .. and of course that could apply to everything that I have said really☺.
Alex
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