Now you need to zoom in again, twice

Use your eraser tool at the following settings:

Round shape, size 5, hardness 100, opacity 100, step 25, density 100.  Don't build up.

Take off the background a bit closer to image, again being careful not to go over edges.  You really need to start taking your time over this now 'cos we are getting so close to the edge of your graphic.

Save your work.

 

This next bit is where accuracy is the keyword - make sure you are able to fully concentrate on what you are doing at this point.

Zoom in as many times as you need to for it to be comfortable to your eyes... you're going to be spending a long time looking at the same area of the image.  This is how close I've gone with mine:

Use your eraser tool at the following settings:

Round shape, size 1, hardness 100, opacity 100, step 25, density 100.  Don't build up.

Take off the rest of the background 1 pixel at a time being very careful not to go over edges of graphic - use the overview window to see how you're getting on and how the tube is progressing but don't take it as read... use it as a guideline only.  Make sure you take regular breaks away from the screen so that you avoid eyestrain and save regularly as you go.

Make sure that the image is saved and take another break from the screen when you've got to this point... we're nearly there now.