Nova series machines, with 100 watt and 130 watt SPT tubes only, may or may not come with a dust prevention sleeve (dust cover) that slides into the output coupler of the tube. This help prevent the collection of dust on the output lens of the tube. SPT started putting these in with certain tubes to correct fouling of the output optic. 

Thunder Laser USA has zero reports of fouling of an output coupler as a critical failure. I suspect this is in response to some lesser quality machines with inferior exhaust systems, improper design, loose fit and finish, etc...

They used to be shipped fitted to the tube but they would fall out in transit sometimes and end up in rolling around in the machine somewhere. If it is not in the bag, you were not intended to have one. if you feel that you must have one anyway, you can email [email protected] and we can assist.

The dust cover as shipped	Closeup of the dust cover
Where to fit the dust cover	Correct dust cover placement