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If you're using the Astronomik RGB filters then ad additional IR blocking filter is not necessary, the Astronomik filters will also be blocking IR (and UV).
The clear (no filter) should probably be fitted with an Astronomik visual bandpass filter, I think they call it an L filter. This will help block IR/UV.
You want to block these things because they focus at different points to the visual spectrum, and will make the images more fuzzy.
cheers, Bird
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