This tube of approximately the same diameter as your telescope tube reduces the amount of dew that builds up on the corrector plate. It also serves as a lens shade, preventing stray light from falling upon the corrector plate and greatly improving contrast.
This shade is manufactured of rugged, soft Foamex plastic. It can be stowed away flat after usage. Celestron employs a thicker material that won't crack as easily as other manufacturer's efforts, with sewing instead of inexpensive glue for a longer service life.
Celestron Lens Shade/Dew Cap for the Celestron C11 & Other 11.0" Schmidt-Cassegrain Telescopes