While looking through the TELECAT and with its length set to match the
F3.8 of the ASA N12, the secondary projection is first centred evenly
within the sight tube and so that it appears circular. Adjustments
are then made to the primary mirror in an attempt to centre the triangular
centre spot within the cheshire ring (see next picture).

CATSEYE Collimation systems:

http://www.catseyecollimation.com/