This 30mm extension ring is often used for professional field flatteners such as the SharpStar 2.5" field flattener.
The extension ring has an M69 x 1 female thread on one side and an M69 x 1 male thread on the other side. The optical thickness is 30mm.
|Connection (side a)
||M69 x 1 male
|Connection (side b)||M69 x 1 female|
|Optical thickness||30 mm (not including male thread)|
|Clear aperture||65 mm|