Firstly, please be aware that even a large about of dust will not affect image quality, because it’s at the pupil of the optical system and cannot be in focus. The “piston” effect of the focuser drawtube moving back and forth does inevitably draw outside air into the optical tube, and dust will always enter at some point. On most refractors you can remove the dew-shield from the optical tube, by unscrewing the black bezel relative to the dew-shield to separate the dew-shield and the dew-shield bezel. Once you have loosened the dews-shield, pull it off evenly from the front forwards off the scope, and you can then unscrew the front lens cell from the optical tube, clean the back of the lens, and screw it back on, then reverse the procedure. It’s important not to over-tighten the front lens cell when putting it back onto the tube else you can put strain on the whole assembly. Some dust is inevitable and cannot affect performance seeing it’s at the pupil of the system. Sometimes we get dust in-between the elements of doublet or triplet lenses, especially those with higher performance. This is due to the continual adjustments needed to centre the elements relative to each other. In some cases the lens cell needs to be reassembled to get the highest optical performance. In this case it can’t be removed and if not excessive re recommend just leaving it as-is.