Position of light source

 
If, as it is common, the light source rotates with the collimator, the following method can be used to decide that it is accurately on the axis of rotation of the collimator. The gantry is set to the vertical position and the couch top retracted so that a small field (for example 5 x 5 cm) can be projected onto the floor.
A convex lens with an approximately 400 mm focal length is placed in the beam at about 500 mm from the source. The height of the lens is set so that an image of the filament is projected onto a piece of paper on the floor with a magnification of about 3. The paper is marked at the centre of the filament and the collimator rotated by 180^o. The paper is marked again and the correct centre will be midway between the two marks. The light source can be adjusted if it is necessary. The 3 mm distance between the marks will correspond to a 0.5 mm error in the filament position.