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Hi Daniel, not knowing the type of scope you're using makes it a bit hard to solve your dilemma. How are you attaching your DSLR to the focuser? That is quite severe vignetting, and it makes me wonder if you're picking up some of the inner tubing of the draw tube or adapter. My DSLR goes just behind where the focusing knobs are on the draw tube. Are you using a field flattener or corrector of some kind? You don't need a t adapter, just a t ring and you should be able to achieve focus.
I also have one of these clip in cls ccd filters and from my experience using my 550d, they do seem to " create their own noise ". This is one reason why I seldom use it. You need a lot of subs with this filter to hide the noise it generates and due to their selective wavelength filtration, exposures can be very long, especially at lower iso's.
Flat calibration files will help remove the vignetting.
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