In spherical astronomy, the parallactic angle is the angle between the great circle through a celestial object and the zenith, and the hour circle of the object. It is usually denoted q. In the triangle zenith—object—celestial pole, the parallactic angle will be the position angle of the zenith at the celestial object. Despite its name, this angle is unrelated with parallax. The parallactic angle is zero or 180° when the object crosses the meridian.
For ground-based observatories, the Earth atmosphere acts like a prism which disperses light
of different wavelengths such that a star generates a rainbow along the direction that points
to the zenith. So given an astronomical picture with a coordinate system with a known direction
to the Celestial pole, the parallactic angle represents the direction of that prismatic effect relative
to that reference direction. Knowledge of that angle is needed to align Atmospheric Dispersion Correctors with the beam axis of the telescope