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Eric. as here for the 350D (the 40D would be similar) the green and blue channels are unaffected by filters, they both pass through (the removal of filters is just to extend red response) red and green colours are presnt in both moded and unmoded versions (so images are still RGB). What makes the colour balance so strange with no filter is the over all extra red sensitivity and big green and smaller blue response spikes at 7000 (and another blue spike at 8200), which causes red, green and blue all to accept IR light as extra data at 7000 and beyond, causeing a washed out effect way out side normal balance. The huge red curve gives the overall red hue with something like 3 times ( more than GB) the light collection over a large frequency range.
The UV/IR blocker cuts out the 7000 and above spikes and returns colour balance with just a slightly extended red response to just before 7000. Green and blue response is then back to normal as in an unmodded cam. This is what you want for astro RGB, normal GB and a bit higher red respose to capture Ha.
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