Locating the mirror centre
When collimating using a laser collimator it is important to know where the exact centre of the primary mirror is so that the laser can be accurately aligned.

One method is to place a ring binder reinforcement circle neatly on the mirror centre.
  The 10 inch mirror in its cell.

Judging the centre by eye to within a millimetre or two is not easy.
    A steady hand is needed for this - no finger prints or scratches on the mirror!

I used a paper circle from a pack of writeable CDs. It had a neat hole in the centre of just the right size.

The paper disc was lightly lowered onto the mirror and measurements made from the edge of the paper to the edge of the mirror. The paper was gently adjusted by nudging it with a soft artist's paint brush.

When the paper was centred a small metal weight was very carefully lowered to act as a paper weight.

Measurements were repeated in case the paper had moved.
Then the reinforcement circle was held with tweezers and placed in the centre.

A final check showed that the edge of the circle was 12.2 mm from the mirror edge in all directions.


The laser can now be directed to land on the dead centre of the mirror.

Note: It would be easier to cut out a 10 inch diameter paper circle that fitted the mirror and then make a small hole in its centre... but I didn't have a large piece of paper to hand.

The secondary mirror is adjusted to make the laser beam land exactly in the centre of the primary mirror.

Collimation is completed by adjusting the primary mirror to return the laser beam back to the collimator.