I disagree with this point. The subs all end up sharper and so does the final stack. There will always be a handful of pigs, which you might as well throw out, but it makes very little difference to the stack (at least for my setup and experience).
One of the odd things about this is that as you watch the subs come in, every 10 secs for example, most of them look beautifully sharp, but all distinctly offset from each other. One might think they would be trailed in a 'connect the dots' sort of way, but thats not what i see. Every now and again a rough one shows up which I will usually throw out, unless Im just trying to go very deep and not so bothered about resolution, which is rare.
I wouldn't say that all subs are all guaranteed to end up sharper. That is not my experience. Perhaps in some locales, where seeing is good enough on a consistent enough basis, then that could be true. My point is it is not guaranteed to be true. Consider what you would get if you stacked the three exemplars from my GIF above. Would you get a 1.7" FWHM or a 5" FWHM in the stack? You certainly would not get a 1.7" FWHM, and you are likely to get over a 3" FWHM, probably quite close to a 5" FWHM...despite the fact that it is just one sub.
Now if you have a lot of subs, and use a clipping algorithm, then the softest subs will usually have their halos rejected. Problem then is the profiles of good vs. ok vs. poor subs can vary quite a lot, and this affects the quality of the stars in the final stack.
There aren't any guarantees here. Frank is correct that there are added benefits to registration, I agree with that. But it is more complicated than that, and it depends on the night(s) you image over and just how things may vary through the subs as to whether stacking everything with short subs delivers better results than using long subs.
I think there also needs to be consideration for basic psychology. How many of you are going to be willing to throw away even a single 10 minute or longer sub? Let alone a few, many? But 2 minute subs? How many of those could you throw away before you psychologically felt you were starting to lose something? What about 10 second subs? There is a major psychological factor when it comes to culling bad data. With long subs, there is a strong desire to keep as much as you can, as every time you throw out even one, you are tossing a "large" chunk of data, a meaningful portion of signal (whether this is actually true or not, it is still the psychological sense.) On the other hand, throwing away a bunch of 2 minute subs? Eh, who cares. It is easy to get 2 minute subs if you need more...right?
So to Franks point about throwing away the same amount of data with long subs as with short? I don't really agree that is generally the case...in practice. I think people are far more willing to take the resolution hit to keep their SNR with long subs. On the flip side, I think people are far more willing to take an SNR hit to keep their resolution with shorter subs. I think this applies regardless of whether in the end, you might in fact actually end up with the same SNR (I don't actually believe this is necessarily the case either, at lest...it is not guaranteed.)
In my experience, it is both easier to throw away short subs, and when you DO throw away the worst offenders, it does have the ability to help improve the FWHM of the short-sub stack over keeping them in the short-sub stack. This is often true regardless of whether stacking all subs in a short-sub stack is better than a long-sub stack or not. It is easy to optimize with shorter subs, and you can take the benefits of shorter exposures even farther than just stacking them all if you do cull the bad data, than if you just always stack it all.
Edited by Jon Rista, 24 September 2019 - 12:52 PM.