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Hi Lee,
I think CCD's spectral sensitivity varies too little to really explain why in your case the blue filter needs 300% of the exposure of the red filter (from your original post) to be sky-limited.
What you are saying about sky-limited theory makes sense to me, but it does not explain the numbers we are getting from various methods. In my case, I am supposed to expose for MUCH longer with O3 filter (weaker signal) than with Ha filter (stronger signal).
In case of narrowband imaging exposures and signals from various filters do not need to be balanced as nicely as from RGB filters, where we aim for natural colours, that's why I suggested not worrying about sky-limited exposures for RGB, but rather focus on getting a correct exposure ratios for those filters and not saturating the stars if the goal is accurate colouration of the stars.
I think the sky-limited theory does not take into account well depth. So the length of exposure ultimately depends on your objectives: long exposures for maximum SNR or shorter ones for non-saturated stars.
Since with RGB filters we are after colour, shorter exposures make sense to me.
Last edited by Slawomir; 11-10-2015 at 01:37 PM.
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