2 inch fit rotating-mount variable polarising filter unit is perfect for controlling brightness when making Lunar or other bright target observations. (Please note: these filters are not suitable for solar observation without an additional solar observation filter). Two identical polarising filters are mounted one above the other - the front filter is mounted in a smoothly rotating ring. By rotating the first filter the light passing through the stacked pair can be modified anywhere between 1 to 40%. The filter is also a very effective way of controlling the image brightness and getting workable exposures when taking webcam or CCD images of bright targets. Black anodised aluminium mounting.