This is a Celestron Model #93621 Skylight/Dust Seal Rear Cell SCT filter. It threads on to the back of the SCT. It does work good, does remove some haze on those humid nights. I'm going to 1.25" filters for everything, so don't need this one. $35 shipped CONUS. International shipping extra. Like new, not used very much. Comes in original box. Thanks.