The ZWO T2 11mm Extension has M42*075 female to female threads and is useful to obtain precise distancing when using a focal reducer or corrector that needs a particular chip-reducer spacing to reach the proper focus.
This 11 mm T2 Extender is compatible with the ASI1600mm, ASI1600MC, ASI1600MM-C or ASI1600MC-C camera but will work in any optical chain where a T2 to T2 adapter with 11 mm of back focus is required.