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TV Focus

Normal practice is for the telescope operator to focus the telescope on the TV image reflected from the slit jaws. The slit image is of course focussed onto the detector. However for the telescope focus to be correct it is crucial that the TV camera itself is focussed correctly onto the slit. TV focus does not normally change very much unless a change is made to the TV camera itself, and the current value of the TV focus is normally written on the white board in the control room. However if there is any doubt about the value of the TV focus it is necessary to measure this, which can be done during the day:-

  1. Obtain the key to the TV override switch from the technical staff. This is normally kept out of the control room and will have to be signed for.
  2. Override the interlock on the slit viewing TV camera, and only this camera. This will allow the TV and the lights to be on at the same time.
  3. Put on a low level of lighting in the dome; the drum lights are quite sufficient. Ensure by means of a notice than nobody will switch any other lights on in the dome.
  4. Switch on the TV controller for the slit viewing TV and the TV system VME computer. These are in the furthest of the four blue cubicles from the left hand end of the control desk.
  5. Ensure that the slit viewing mirror is in, type AGSLIT at the ICL interface if it is not.
  6. Switch on the slit viewing TV camera only; wait for the delay lights to go out, and turn up the gain on the TV SLOWLY until the slit jaws are visible on the small TV direct monitor. Watch this monitor all the time while turning up the gain.
  7. Adjust the TV focus with the TVFOCUS command until the edges of the slit jaws are sharpest.
  8. Note the new value on the whiteboard.
  9. TURN THE TV OFF AND RETURN THE OVERRIDE SWITCH TO NORMAL STATUS. Failure to do this could result in the destruction of a TV camera.

Note that the correct value of TV focus for direct viewing (i.e. with the ACQCOMP mirror in) is very different from that for slit viewing. The telescope should not normally be focussed on the direct image when using ISIS or FOS. However TV focus will be approximately the same for the two different scales when the slit viewing optics are in. The normal slit viewing mode is at a scale of 4.5 arcsec/mm.



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Fri Jan 7 15:34:48 GMT 1994