Practical Linac Tips (mech and eng)

Replacing the overhead laser (27th Oct 2008)

The lateral overhead laser has become faint. Engineer replaced during down day.

  1. Firstly check that the lasers point to the true isocentre (can check mechanical isoc with pointer, compare to optical, then use the paddle to check lasers co-incident to optical isocentre. Check the line and vertical wall lasers are truly vertical with a plumbob.
  2. We need two points in space to align the laser to once it is replaced. Mark the old laser on the floor (at CAX and off-CAX ie, off the dropping floor) and we will take the front pointer with G=90 as true isocentre.
  3. Remove the old laser assembly, install the new one (check the horizonal/vertical lasers and in the same place as before and not swapped)
  4. Rough-tune the laser to within 2mm of floor and isoc marks using the back plate tilt screws
  5. Fine tune:
    1. adjust the lasers to be straight (vertical). They should pass through both marked centred and also be proved to be truely vertical.
      • use a cap of Mercury to act as a truly flat with gravity mirror to reflect back to the laser source to check the straightness
      • or assuming the couch vert is OK (check), mark laser at max vert and move down and check if the point is followed.
    2. adjust the lasers to match floor markings using the laser shift screws
    3. adjust the lasers to parallel with the floor markings using the laser twist screws
    4. if the laser appears to curve relative to the other markers, then the water vial used to spread the laser may need adjusting (two silver screws on top of the round mounting)
  6. Now verify the overhead laser vs the other lasers at extremities. Re-iterate fine-tuning process until good agreement is found.
  7. Replace the cover, check it has not changed anything.
  8. QA checks:
    1. Turn G=180, see if the laser matches the crosshair (remember there is some sag in the gantry at 180)
    2. Perform the monthly laser QA to check optical isoc vs lasers using the paddle. This creates documentation of the process.
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