[Moon (James)]

I wouldn't touch the rear of the scope and only make adjustments to the secondary. However having said that, the problem As Trevor states could well be with your focuser.

I don't see how buying a new focuser is going to help collimate the scope if the primary mirror is not collimated properly!
Anyhow step one ( as has already been pointed out) is to check the secondary collimation. A centred defocused star should be concentric.
Step 2 is to focus properly and take a 30 sec exposure and check the stars in the corners. Make sure the camera is attached properly; you are polar aligned accurately and tracking smoothly. If all is not well then I'm afraid you need to collimate the primary. It's not hard, no need to be scared.
James

Edit
And as Paul correctly has pointed out, you will be left with some residual field curvature - but it should be symmetrical, which it's not in that image.