My scope lives outside full-time, under an awning, granted but is still exposed to the rude weather.
After some experimentation, I found an almost ideal covering: those $20 chequered work jackets from Ray's - the more washed the better. The cotton outer and quilted lining absorbs the moisture while the polyester inner protects your scope's finish. A fair bit of cutting and sewing is required for a big OTA, but, combined with something like a small motorbike cover to repel rain, they make a completely weatherproof cover (also eliminates the need for dew-busters).
I also give mine a quick zap with a persistant insecticide to stop spiders from nesting.
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