For an analogue scope, it is almost impossible to capture a trace with a cellphone or cheap camera.
You will just see the point where the dot is, and a bit of the previous phospher trail..
You need a camera that you can set to have a very long shutter period,
then you need to stabilise it (on a tripod, or some other way),
and then use the timer, so that you are not touching the camera when it takes the shot.