[raymo]

Barlows vary in construction. Most slide into the focuser tube like an
eyepiece does, and the eyepiece then slides into the Barlow. With many
Barlows you can unscrew the lens and use the Barlow as an extension tube.
A few Barlows screw directly onto the bottom end of an eyepiece.
The easiest way to use a standard Barlow for imaging is to use a fixed or
adjustable camera adaptor for eyepiece projection, unscrew the bottom half of the Barlow and slide it into the adaptor as you would whatever
eyepiece you normally use.[ The top half of a Barlow is usually too large to fit inside the adaptor]. As T-rings are normally snug fits onto the camera, and the Barlow [or eyepiece] is tight in the adaptor, and the adaptor is tight in the focuser tube, the only weak link left is any slop in the focuser tube itself. Fixed EP projection adaptors are cheap on Ebay.
raymo