The separation can be made in the usual way. However, to make the double distance measurement set the fixed wires on the primary with the position angle wire bisecting the primary and the companion. Then move the moveable wire onto the companion. Note the reading on the micrometer screw. Now rotate the micrometer right around so that the position angle wire again bisects the primary and the companion, but the moveable wire is on the opposite side of the primary to the companion. Then move the movable wire back, across the primary, to the companion again. Note the new reading on the micrometer screw. The difference between the two readings gives a measure of the double distance.
Ideally, the companion should be on the position angle wire when the separation measurement is made, but due to field rotation it might have moved away.
The error this would induce would depend on the separation. The error is just p(1 - cos Aq), where p is the separation and Aq is the change in the parallactic angle. If, in the time it took to move the moveable wire on to the companion, the companion had moved two degrees it would induce an error of 0'.'004 in a separation of 10''. As two degrees represents four minutes at a field rotation rate of 30° per hour field rotation would not be a major source of errors in the separation.
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