A commonly used method involves using star transits, but on stars at high declination. With a hand held stopwatch time the transit of a star across the movable wire and note the corresponding value of the micrometer screw. Move the micrometer screw by a fixed amount, say half or one revolution in the direction of the star trail, and time the next transit on the wire. Repeat this for as many revolutions as possible. It will then be possible to calculate a value for one revolution of the screw from all the individual measures. For a star at declination +75 for instance the motion of the star is 15 cos 75'' of time per second so it will take 56.5/15 cos 75 seconds = 14.6 seconds of time to travel the equivalent of one revolution of the micrometer screw in the standard Schmidt-Cassegrain described above. This should be timed to better than 0.5 seconds of time but taking the mean of n revolutions will increase the accuracy of the mean figure by a factor of Vn. The timings should be repeated on other nights to confirm the figure reached. Further checks at regular intervals are also recommended to see if there is any variation of the screw constant with temperature or with time (due to wear and tear).
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