Now here is a stupid question - would it be possible to modify the 50 mm filters so that they do not cover the internal guide chip ? and how would one go about that ... modify or buy specials ? What would be the pro's and cons ? I have an STL11000M & the large format FW8-STL. Or are there other options to get good guide stars with narrow band filters ? Many thanks in advance.
I really don't think that's feasible. The light enters the camera as a cone, so if you were to somehow cut away part of the filter (which would be exceedingly difficult to do without destroying it), you would have a region of the main chip that was getting unfiltered light as well as filtered light. You would need to put an off-axis guider in front of the camera and use a separate guide camera (or the STL remote guide head if you have one).
If you're using a SCT or similar, you will probably need an off axis guider. I've used one (Van Slyke Versaport II) on my M12 but it's challenging. If you have a refractor or a scope with fixed mirrors, a separate guide scope is the way to go. I've found the ST-i + the standard kit works fine for my TOA150. A Lodestar may offer you slightly better performance because the Sony chips have less noise.
I should point out that the ST-I has a built-in shutter for dark frames, so there's far less problems with hot pixels. (Yes the Sony chips are low noise, but especially when uncooled they still have hot pixels.)