The Position Angle

As the zenith is the fixed point on the celestial sphere for an alt-azimuth mounted telescope, position angles measurements made with such a telescope would be referred to the zenith. Let us call position angle measurements made with respect to the zenith, then, the zenithal position angle to distinguish it from the position angle made with respect to the pole.

The direction of the zenith in the field of view can be determined by the same method that would be used to determine the direction of the pole with an equatorially mounted telescope, i.e. a star near the celestial equator is allowed to drift across the field of view, except in this case it must also be close to the meridian.

The position angle can be found by measuring it in the usual way, except, of course, that it is being measured with respect to the zenith. All that needs to be done in addition is to note the time of the observation, so that the parallactic angle can be determined and then subtract the parallactic angle from this zenithal position angle to obtain the position angle with respect to the north pole, i.e.

6= 6z - q where 6 is the position angle, 6z is the zenithal position angle and q is the parallactic angle.

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