Much less often than instinct suggests. A surprising amount of dust costs very little contrast, and every cleaning risks the coating. Clean when performance is visibly affected, not when the mirror looks dusty under a torch.
More mirrors are damaged by cleaning than by dust, which is why the advice runs the way it does.
What dust actually costs
Scattered light, and not much of it. A mirror that looks alarming under a torch may be costing a fraction of a percent of contrast, which is invisible beside a night of mediocre seeing.
What cleaning risks
The coating. An aluminium coating with an overcoat is thin and can be scratched by a particle dragged across it, and a scratch is permanent where dust is not.
When it is genuinely warranted
When something is on the surface that scatters badly — a film from dew that dried, tree sap, a fingerprint — or when the accumulation is heavy enough to be affecting the view rather than the owner’s peace of mind.
Where to find the method
In the manufacturer’s own guidance for that coating. Makers publish it, it differs between coatings, and it is the document to follow rather than a general rule.
The questions that keep arriving
What about the corrector on a folded telescope?
Same principle, and the front element is the one that gets dewed and handled most. Makers publish their own cleaning guidance for their coatings, and that is what to follow.
Does dust show in the image?
Scattered light rather than spots. Dust out of focus does not appear as marks; it slightly reduces contrast, and the reduction is smaller than most owners assume.
Last reviewed 17 September 2026