I'll just leave this here...

[earth titan]

Centre Spotting a Newtonian Mirror - IceInSpace

Credit to the website owner for this simple and brillantly simple method. Gonna check mine next weekend (I redid my centre dot a while ago) cos this must be the most accurate way I have seen.

[Skills]

Nice link, thank-you.

"just close enough to the centre to be sure that the tip of the pen is marking the primary mirror only in optically inactive areas (in the shadow of the secondary mirror)"

How large is the optically inactive area in say a 8" f5.9 newt?

[earth titan]

Now that i don't know.

[AndyG63]

Nice link, thank-you.

"just close enough to the centre to be sure that the tip of the pen is marking the primary mirror only in optically inactive areas (in the shadow of the secondary mirror)"

How large is the optically inactive area in say a 8" f5.9 newt?

Surely that's just the apparent diameter of the secondary, as cast on the primary from an infinite light source?

Andy

[BlueAstra]

It will be slightly smaller than the shadow from an infinate point source. This is due to the FOV of the scope. If you chose an extreme off-axis point souce, the secondary shadow will be skewed sideways, reducing the unused part of the primary.