Recently, a customer sent us his 2 year old secondary mirror.
It is a good example of why we tell everyone never to put a sock or a "Crown Royal" bag over the secondary mirror.
What happens is that tiny bits of grit fall or get blown into the cloth. The grit becomes imbedded in the weave and over time creates small scratches in the coatings. The scratches tend to form in a similar pattern, usually in one local area.
The scratches can be hard to see when the mirror is in the holder, but here on our lightbox, they are quite evident.
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