The Astronomik CLS filter response is here as pdf
. Basically there is a big notch from around 540nm to 640nm to take out Sodium and Mercury emissions. It perks up again in time to pass Ha and friends though. The IDAS LPS manages to squeeze in two narrow passbands in that region but it also means the transmission isn't quite zero where the LP emissions are. Curious tradeoff.
I find Christian Buil's response a little confusing now I think about it some more. His charts show the response dropping to zero at 700nm even for the IR block filter removed case. That is clearly incorrect, and I assume either it is a limitation of the measurement equipment used, or there remains some UV/IR block filter as commonly used in astro modifications. For a chart that appears to match what I see with my modified 450D see here
My reason for doing this testing was to get an indication of what the filter and sensor combinations do. I know the CLS does not block IR, but how bad is it? Now I know. I should have paid a bit more for the CCD version initially which is supposed to have IR blocking. However my experience shows that even with a CLS filter in place, broadband visible light imaging is not viable from my location for anything other than the brightest objects.
The other part was the performance of IR pass filters. Research so far has shown there is a spike from Sodium lighting in the IR region around 818nm, which if I read things correctly elsewhere, is of approximately 12% the intensity compared to that of the visible emissions. I was wondering if broad band imaging was possible in the NIR region to get around light pollution, and I would have to avoid that emission. That's where the 850nm IR pass filter comes in, and I found a dirt cheap one on ebay. But does it do what it is supposed to? Seems to so far. Next step, I'll have to try getting a spectrum of the sodium lights outside when it gets dark, and see 1: if the 818nm spike there and 2: does the 850nm filter block it? In case you're wondering, I got the 950nm while I was at it in case it comes in handy later.