This is a two-part, 2-inch, neutral density filter. Both parts have M48 threads front and back, and the second part rotates. Additional 2-inch filters can be "stacked" when using either or both parts. The rotation mechanism is smooth and not related to the M48 screw thread - there's a knurled ring to grip when screwing/unscrewing the second part, but the front of the filter simply rotates - it rotates continuously but will sit where you leave it.
Each part has around 40% transmission. Together, by rotating the second part, transmission varies from around 3% to 40%. Useful for viewing the moon in all its phases (vary the transmission according to brightness) or as additional filtering for white-light solar viewing (which is what I mostly used it for - e.g. Baader AstroSolar Safety film can still leave the image a bit bright for comfortable viewing depending on the aperture you use).
Selling for $50 including postage Australia-wide, or pickup in Brissie for $40.
