I believe the original reasoning for a window was to prevent dust build up on the mirror surfaces. In my experience, if a cover is kept on the tube ends, and the focuser when the scope is not in use then dust is not usually a problem. I had a mirror for ten years and rinsing with deionized water a few times is all that was required until the coating started lifting. A simple recoat, and I 'm ready for another ten years. Also try not to use materials in the tube assembly which generate dust like felt, etc.