[mura_gadi (Steve)]

I cover the EP holder all together with a pin hole in the centre to check alignment. The old plastic film canister are custom made for the job, right down to the centre hole being marked.

Look through the pin hole at the secondary, if the pin hole is centred you can check alignment very well by checking for an even gap as per multiweb's comment.

*The pinhole lets you know your looking in along the centre line and gives you a tight frame. The centre line can be a little harder to find if you have the full open focuser in front of you and the edge gap will larger and change easily if you move head position.

PS. You can also just insert a carboard tube down the focuser till its just about resting on the secondary and align from that.