[MichaelSW (Michael)]

Doug,

G'day.

I also follow simple method with my 12" collapsable Skywatcher.

As an aide to help me determine if my Secondary mirror was centred within the focuser draw tube, I fitted a piece of graph paper in place to line the inside of the optical tube behind the Secondary mirror. (Had to cut notch in the paper for a spider vane and a screw head that got in the way). I aim to count the same number of 'squares' above and below the secondary to help assess its positioning towards the sides of the optical tube. (Can't count squares towards the outer end of the optical tube because the secondary mirror edge and holder get in the way).

I use a film canister with hole drilled in centre of the base to position my eye.

It seems reasonable to me that once you are satisfied with the positioning of your secondary, and you make any rotation and tilt adjustments of the secondary such that the reflection of the primary mirror is concentric within the secondary mirror, then you have achieved the desired result. With my scope I aim for the reflections of the three primary mirror clips to be equal in size and positioning.

It is useful to paint the inside bottom of the film canister with white paint. When I get to aligning the primary mirror, I hold a red torch carefully inside the optical tube shining it up into the film canister. This shows up the reflection of the hole in the bottom of the canister as a distinct black / dark dot. I then just adjust the primary mirror until this black / dark dot is centred within the reflection of the primary mirror centre-mark circle.

One astronomy friend gave me a tip about looking down the empty focuser draw tube at the reflection of your pupil lined up with the reflection of the centre mark on your primary mirror:- as you move back away from the from the draw tube, if you have done a reasonable job then they stay lined up and eventually fill the view.

Michael.