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In order to focus the guiding system, the ends of the bundles must be focused onto the CCD camera. This can be done during daylight and it requires two people speaking to each other; one of them must be in the control room viewing the bundles image on the screen, while the other remains in the dome to perform the focusing.
The bundles are mounted on an interface which connects them to the CCD camera. After illuminating the aperture plate bundles ends, the image of the camera bundles ends can be see on the TV monitor in the control room. In the dome, the micrometric positioning screw can be adjusted until the TV image appears focused. The amount of incoming light onto the bundles must be controlled in order to avoid saturation or fine detail loss.
This focusing method is simple and fairly accurate. However, if better precision is required, a transparency with a fringe pattern can be illuminated behind the coherent bundle, the image of this can be focussed accurately on the TV screen.