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A Barlow lens lets you increase the focal length of your telescope and hence the magnification. The Barlow lens is placed between the telescope and the eyepiece.
The 2x APO extender has in front (telescope side) a female T2 thread, in rear (camera side) a male T2 thread. In order to connect to telescope, you simply need to add a normal T2 adapter. In order to connect cameras to it, you neew a T2 ring if you use a DSLR camera or you can directly connect a CCD camera if it has in front a T2 female thread.
Note: the requested backfocus distance for this extender is 55mm. For DSLR cameras, this distance is normally achieved by positioning (between the extender and the camera body) a normal T2 ring (for example Canon EOS DSLR sensor is normally placed at 44mm from the front bayonet and the T2 ring is usually 11mm thick). The CCD cameras instead have the sensor placed forward so, if you want to connect them directly to this extender, it will require a T2 extension. For example, since the CCD ATIK 428EX or 460EX have a back focus (distance between the sensor and T2 front thread) of 13mm, you'll need a 42mm T2 extension (55mm - 13mm) to bring the sensor to the proper working distance from the field flattener.
