John, I was referring to resistance to scratching after three months rather than ultimate longevity of plain aluminium . It depends very much on how clean the mirror has been kept and where the scope is used. I have seen plain aluminium mirrors that have been kept covered , in remarkeable condition after 10 years. I wouldn't however recommend anybody to get a plain alumium coating mirror these days, no matter how `exotic' a vendor might claim there coating to be.
Quote:
Originally Posted by ausastronomer
It might be pretty strong, but it still doesn't last very long. The majority of front surface mirrors in professional observatories are bare aluminium coated, not overcoated and need to be recoated about once a year. Overcoated aluminium usually lasts between 5 and 25 years depending on use and environment.
CS-John B
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