You want it all over the track, and it's hard to do that just through the shaft.
Double check there isn't a grub screw holding it on. If there is, it could only be on the underside of the large rear overhang. Otherwise it must be a push-on type.
The knob should come off with a bit of gentle persuasion. Using a spoon either side of the knob as a lever should do it.

If the gap is too small to get the lip of the spoon under, I often use a flat blade screwdriver. The widest blade you have that will fit in the gap (for leverage). Again, some masking tape will help protect the surface.
Slip the blade of the screwdriver under the knob and turn the blade so it pushes the lip of the knob up slightly. Rotate the knob a bit and repeat. Keep turning the knob back and forth so you gradually lift the knob up. Don't try and do too much at once, maybe 1/2mm at a time.