Laser beam appearence without focus lens - tube broken?

Hardware
#1

Hi there,
i am still struggeling with the quality of my cutting/engraving and can not find a perfect focus.
I bought the Laser cutter second hand and have not changed the tube yet. The Tube is 160 Watt - 1850mm long.
When i place a white painted sheet between the tube and the first mirror and fire once, the result looks like this:

2021-04-07 12.33.46-1
2021-04-07 12.33.46-1474×548 27.2 KB

The diameter is about 8mm.
I was expecting an even burnt spot - not an artwork like this.

Did anyone try that yet? How is the spot supposed to look like?

Thanks in advance!

#2

I’ve never seen a 160Watt laser beam, but as a theoretical starting point and my experiences with much smaller tubes, I do not think it looks wrong. If it does not develop in 4 seperate points, then it should be good enough. How well can you focus after the 3rd mirror?

#3

Typically we measure at ~12-15cm from the outlet of the tube and using as low a power as you can get it to fire at.