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Frank,
I'd advise against using ICNR and to take your own darks. The less you have the camera doing, and the more you do, the more control you have over your images.
The other thing to note is that depending on the bandwidth of the hydrogen alpha filter, you may need to take up to 20+ minute exposures to allow enough light to get through to the sensor. Remember, the red channel only picks up 1/4 of the signal on the Bayer matrix (1 red, 2 green, 1 blue). With ICNR, you'll be exposing for 20 minutes, and then waiting around for 20 minutes for ICNR. I say, grab an hour of darks before you start imaging, spend your night capturing photons and then take an hour or more of darks once you're finished. Median combining the darks should assist in removing the majority of noise.
The other thing to note is that because you're working purely in greyscale, you don't have to worry about chrominance noise, just luminance. These things can be processed out easily enough in Photoshop if you know what you're doing.
I've never imaged in hydrogen alpha before, so can't give you definitive exposure values. I say, get out there, use ISO-400, get your subject roughly in the field of view, take a 1 minute exposure, 2 minute exposure, 4 minute exposure, 8 minute exposure, 16 minute exposure, 32 minute exposure. See which one gives you the best balance between signal, noise and contrast. You might find your optimum exposure may lie somewhere between the values I've mentioned. Once you're happy, frame the subject correctly, then, stick to that exposure value for the rest of the night.
At least, that's what I'd do. And, it is something I'll think about doing in the very near future.
Regards,
Humayun
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