TRUE POLE ALIGNMENT of an
OBSERVATORY CLASS EQUATORIAL

8)

Adjusting Elevation of Hour Axis by Measuring Dec. Drift Rates due to Refraction

i) Centre a star near the meridian and set your filar micrometer to measure dec. drifts.

ii) Follow star for about 2hrs. & measure dec. drift.

iii) Compare the averaged hourly rate to the theoretical rate.

iv) The difference in hourly rate is caused by the elevation of the hour axis deviating from the true pole.

This process can be rather tedious. The determination of the elevation of the hour axis from the true pole may be simplified by preparing two plots beforehand, one of
the theoretical hourly rate in declination when the hour axis is directed at the true pole; the second of the hourly rate in declination due to the departure of the hour axis
from the true pole. This enables the theoretical rate in declination to be read off from the mean HA & dec. of the star. The difference between the observed & theoretical
rate is read off on the second plot,yielding the error in elevation of the hour axis.