From a signal standpoint, I don't imagine there would be any harm from combining LPS and L data. However, from a resolution standpoint, I believe there would be some fairly significant differences, as demonstrated by your post above. Whether the loss in resolution and detail is something you are willing to accept is something only you could decide. Personally, I stopped using my LPS filters some time ago because of the blurring issue. Especially for galaxies, it's already tough enough to get decent resolution on them.
One question I do have for you is...are your 2 minute subs clipped? If they are, by how much? What is the difference between say a 2 minute and a 4 minute L sub? Or even a 5 minute sub? It may be that you could expose your L subs longer, so long as you are not clipping, or so long as any clipping is minor and acceptable to you. My ASI1600 only needs about 10-15 seconds to get read noise-swamping signal, however I usually expose for 60 seconds. I clip maybe 5-10 stars, lightly. I could probably go for 120 second subs, however I am really after as much resolution as I can get, so the 60 second subs strike a nice balance point (my issue is more wind than seeing though). Anyway...it's often a matter of balancing the factors that matter, and figuring out which matter most.
There is also still that key factor to consider...the LPS is blocking about 50% of the visible spectrum. That's certainly better than the 75% or around there that a CLS or V4 would block, but it's still HALF the visible spectrum. Signal grows faster than noise. Your L filter will certainly pick up more LP than the LPS will, however the noise is growing slower and slower while the signal grows at the same rate. If you acquire a 100e- skyfog signal with the LPS, and 25e- object signal, your shot noise would be 11.2e-. However, if you acquired 200e- skyfog signal with the L filter and 50e- object signal (same exposure time, twice the spectrum), your shot noise would be 15.8e-. In terms of SNR (ignoring read noise for the moment), the LPS would be 2.23:1, while the L would be 3.16:1. Lets say your LP is even worse than that, and you end up with 300e- skyfog signal and 50e- object signal with the L filter. Your SNR with the L filter would still be better, at 2.67:1. What about a 400e- LP signal? Your SNR with the L filter would STILL be highe than with the LPS filter, at 2.36:1.
Note here that I have not changed the object signal...it's still 50e-. I'm only changing skyfog. So, this is assuming you expose for the same length of time with both filters. If your LP isn't that bad, then the alternative tradeoff is time. You could likely get away with shorter exposures, and still have the same SNR with the L filter, meaning you wouldn't necessarily need to stack that many more subs with the L filter than you were with the LP filter. I would strongly encourage you to do a proper linear fit of your test data and see how things look, and how things measure. You could very well find that the SNR with your L filter is at least as good, if not better, than the LPS filter.
FTR, Things don't change much if you add in read noise, even 8e-, since a 400e- skyfog signal will totally swamp it. With 400e- skyfog, 50e- object signal and 8e- RMS read noise, your final L filter SNR would be 2.17:1. However the LPS filter would lose some SNR as well.
Edited by Jon Rista, 10 April 2017 - 10:12 PM.