TV Focus

  The focus should be checked regularly for all the TV cameras. Inspect the profiles of unsaturated images for any unusual broadening. If a broadening is suspected, the camera focus should be adjusted, which is done manually.

• Focussing the Finder TV camera is a delicate and sometimes very cumbersome operation in which the camera head is positioned by hand. Care should be taken not to displace the field centre. If in the process the finder is moved, it must be returned to a position so that the (prime) CCD chip and guide star overlay can be seen. The most important requirement is that the autoguider reference point (550,900) in guide coordinates is on the screen. Thus, take care that:
  1. For Cassegrain - with TV centred, a star centred on the Cassegrain TV appears near the centre of the Finder TV screen.
  2. For Prime - a star recorded on the centre of the CCD chip appears near the middle of the right-hand side.
• The Cassegrain TV camera should be focussed on the spectrograph slit, which should be done under low illumination. It involves the removal of the cover of the A&G box on the TV side, unclamping the TV camera, screwing the head in and out until the optimum focus is obtained, followed by reclamping the camera and replacing the cover.