This 3mm extension ring is useful for achieving the correct back focus required for astrophotography, where a few millimetres are critical.
The extension ring has an M48 x 0.75 male thread on one side and an M48 x 0.75 female thread on the other side. The optical thickness is 3mm.
||M48 x 0.75 (2" filter thread)
|Length of female thread||2.5 mm|
|Optical thickness||3 mm (without male thread)|
|Outer diameter||50.9 mm|