Hi,
I have a 50W Raycus fiber marking and i noticed that when the red laser is showing the engraving area it goes out of focus in relation to or focus of the fiber laser. For example: if I use a 110x110mm f-theta, the red laser focus is around 12mm above the fiber laser focus, which makes the red line thicker. I don’t know how to solve because unlike the 30W, the Raycus 50W comes with the red laser inside the sourcer.
After changing the field lens, you need to adjust the focus position, and also adjust the position of the red dot pointer
Thanks for reply but i do not meam red dot auxiliar focus, but red laser diode inside source. In my machine the red line focus is about 10mm above fiber focus, it give me a thick line 
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Because you have changed the lens with different working planes, your focus position has also changed. Suppose your 200200 lens focus position is 20, then if you change to a 9090 lens, your lens will become smaller at 10 , so be sure to re-adjust the focus, otherwise the situation you said will appear