Carl,
I think you need to consider what is happening when you change these filters....
1. The original filter in the camera is designed to suppress the UV and IR to give a "realistic" natural colouring with minimal chromatic focus issues. Unfortunately for astro imaging the cut-off wavelength in the IR is close to the Ha emission and the efficiency in this area is pretty low (around 25%)
2. If the filter is replaced with a Baader filter (or similar) then the cut-off wavelengths are much more defined (400nm and 700nm) - this allows a much higher efficiency at Ha (up to 50-60%). Yoy still have the benefit of the UV-IR suppression to reduce star image "bloating".
3. If you remove the filter completely...then all the UV and IR light is allowed through. Great for spectroscopy
![Big Grin](/vbiis/images/smilies/happy04.gif)
but it does mean in refractors that the star images will look bloated due to the excess UV (blue ) or the NIR (red). You also need to use the inbuilt Custom colour balance to use the camera for "normal family shots"
4. For option 4 you can "compensate" by adding a clip in filter which effective operates like option#3 Some filters, besides providing UV-IR cut-off can also act as a light pollution filter (ie CLS etc)
In all the modern Canon cameras (400D onwards) there are two filters - one is a "mild" UV-IR which has the shake rattle an' roll for sensor cleaning and a second filter (much thinner than the original back in the 300/ 350D series) which is the actual "heavy" UV-IR filter - this is the one that is normally replaced/ removed. As it's so thin, it can still be removed and the camera still focus normally with standard lenses ( which was NOT the case with the 300/ 350D)
HTH