[LaFey]

I think the majority of the Barlows aren't threaded to accept filters, am I right?

When you have a non-threaded Barlow and still want to use filter how do you do it?

Common sense tells me I should screw the filter to the eyepiece, in the regular way and then insert the eyepiece+filter in the Barlow barrel.

Is this common practice? Does this affect the image quality since the eyepiece isn't in direct contact with the Barlow?