The original 10018 adapter plate (STX 3.5" OD / 3 x 24 threaded) provides a 3" OD dovetail plate and a 3.00 x 24 TPI thread, giving you a very robust connection to the telescope and the largest clear aperture. However, it may require adapters on the telescope side. Aluma AC, STX, and STXL cameras have a standard mounting bolt pattern (four 6-32 threaded holes on a 4" diameter). A high quality focuser should be used to ensure that the weight of the camera and accessories are well supported. It is recommended to use at least three grub screws to ensure the camera is sturdily mounted. This plate is 0.2" thick. When using the STX style 3.5" OD / 3 x 24 TPI thread adapter, backfocus is 0.2" (5.08 mm) when using the dovetail connection. When using the threaded interface, the total backfocus distance added is 0.67" (17.02 mm).