A defect in the optics is only caused by an accident. Nothing should be done here by someone without experience in adjusting optical instruments and who does not know what he or she is about.

  Attempts to clean the grating cause a great deal of vexation and cost to inexperienced people. The only permitted cleaning method is blowing with a rubber bulb (do not breathe on it or use compressed air!). Cleaning inside the optics also has risks attached eg. refractors or mirrors may be broken-off. Loose dust on refractors and mirrors should be removed by blowing or by rinsing-off with distilled water. The next stage in ascending order of roughness is alcohol in a cotton-wool swab. If the refrachors/mirrors are not fixed free of distortion and if their removal is undesirable only cotton wool with alcohol remain for cleaning. Other solvents should not be used under any circumstance.