Well, there is always some limit on a signal to noise ratio where you will not be able to produce a usable image. And, it really doesn’t matter much whether you are talking about a small or large aperture, or a bright or dim object, instead what matters is the final signal to noise ratio.
When the Hubble Space telescope does one of its deep fields it will expose for dozens and dozens of hours. But does that mean that the Hubble is a “small” aperture or a “slow” system? Certainly not in amateur terms, but perhaps yes in terms of the largest professional observatories.
I think the only absolute limit to how long you can expose on a single object would be determined by the saturation point of the sensor, when the noise and signal combine to completely overload the recording capabilities of the photo site (the so-called full-well depth). Up until that point you will continue to build signal, but you may never reach a point where you have enough signal to noise to produce a useful image.
Then there is the technique of image stacking where in theory you could stack an infinite number of images into an infinite accumulator (128 bit, 256 bit, etc.). But practically speaking you’d never do that and in fact most users will probably tire of that process after a few thousand subs.
In terms of stacking, I’m not certain whether anyone has demonstrated any limit to how far you can go. And note when it comes to that limit there is really no reason why a small aperature should be any different than a large aperture. Although a small aperture (or high f-number system) will probably take longer to reach a given signal to noise ratio.
So, in the final analysis can you use a barlow to record greater detail in a “small” object? The answer is most likely yes, but you’ll need good seeing, good guiding, and good optics/focus and you will certainly need a longer total exposure time. In any case, as was previously mentioned, there is a limit on resolving power that is based upon aperture and once you reach that limit you won’t gain any more fine detail with greater focal length (unless you also increase your aperture).
Lastly, remember that an object’s magnitude is usually given in terms of its total integrated brightness, not in terms of its surface brightness. Thus, a small planetary nebula or galaxy that has a low total integrated magnitude (say the OP’s sample of magnitude 16) may actually be no more difficult to record than a larger object that has a higher integrated magnitude (within limits, since small remains small and very few objects have uniform surface brightnesses). In fact, the surface brightness on some small planetary nebulae can be so high as to make it easy to overexpose in just a few seconds, even though its magnitude can be rather unimpressive. This is one reason why it may be perfectly fine to use a barlow on a small, high-surface brightness object like a planetary nebulae.
Edited by james7ca, 15 November 2017 - 01:05 AM.