Centre mark on a primary mirror

[Avocette]

Is there any particular reason why the centre mark on primary mirrors is a ring?

I ask because I found that the ring on an SW mirror was off-centre by a few mm and was itself not a symmetrical ring (see photo) and yes the mirror really was that dirty!

The third photo shows what I've replaced the original ring with. It's an (approximately) isosceles triangle. A triangle was easy to make since I only have one size of hole punch, and rather than try to make an equilateral triangle, the isosceles shape allows it to point as a reference towards one of the three collimation screws. When I carry out the fine tuning of the primary collimation (ordinary 2x Barlow and Laser) I can see the isosceles triangle image/shadow on the Laser target.   

So far so good, I used Betelgeuse to carry out a star test last night and for the first time with this scope saw circular discs symmetrically either side of the focus point. 

[RT65CB-SWL]

Not owning a 'newtonian', I think it is to aid collimation.

[Glasspusher]

Because hole reinforcement rings for file paper are circular with a hole in the middle.

http://www.ebay.co.uk/sch/i.html?_trksid=p2047675.m570.l1313&_nkw=reinforcement+rings&_sacat=0&_from=R40

John

[Toymaster]

I get the laser collimator to hit the hole in the centre by adjusting the secondary and the adjust the primary.

Think the hole is just there to target the laser beam on.....

Have one newt without the ring and so I just aim for the centre and use a collimation cap to check I'm seeing all the primary.

Ade