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if you're using lenses or refractors, you will generally need some kind of UV/IR cutoff filter otherwise you get very bloated stars (since they are not corrected for those extreme wavelengths).
if using a reflector (even with a correcting lens assembly) you can usually get away without one.
at least that's been my experience. my modded 40D and the 20D (when I had one) both had the clear rear filter and i definitely needed a UV/IR blocking filter for widefield work with camera lenses but pretty sure i've taken shots without one when using the epsilon astrograph.
Phil
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