The apparent change in the position of an object due to a change in the eye position of the observer is called parallax. In a telescope, parallax occurs when the image formed by the objective does not lie in the plane of the crosshairs. Unless parallax is removed, accurate bisection and objects become difficult to see. Parallax is eliminated by focusing the eyepiece for a clear view of the crosshairs and focusing the objective to bring the image of the object into the plane of the crosshairs as described below.Focusing the eyepiece: To focus the eyepiece for a distinct vision of cross hairs, either hold a white paper in front of the objective or sight the telescope towards the sky. Move the eyepiece in or out till the cross hairs are seen sharp and distinct.
Focusing the objective: After the cross hairs have been properly focused, direct the telescope on a well-defined distinct object and intersect it with a vertical wire. Focus the objective till a sharp image is seen. Moving the eye slowly to one side may check removal of the parallax. If the object still appears intersected, there is no parallax. If on moving the eye laterally, the image of the object appears to move in the same direction as the eye and the observer’s eye and the image of the object are on the opposite sides of the vertical wire, the image of the object and the eye are brought nearer to eliminate the parallax. This parallax is called far-parallax. If, on the other hand, the image appears to move in a reverse direction to the movement of the eye and the observer’s eye and the image of the object are on the same side of the vertical wire, then the parallax is called near-parallax. It may be removed by increasing the distance between the image and the eye.
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