Peter's on point here.
Something that took me a little while to understand in one of Texereau's book was the difference between the physical size of a diffraction spot (airy disc) and its angular size.
My thanks to Mark S. for his patience in explaining this to me and for giving me a good analogy: newsprint halftone dots. It then made a lot of sense.
The first thing that sounds a little odd and counter-intuitive when you first read about it is that the physical radius of an airy disc for example an f/6 objective at a wavelength of let's say 560nm (green) is ~4.1 microns whether it's a 130mm refractor, a 16" RC or a 39m diameter telescope in Chile (assuming the same focal ratio for the sake of the exercise). That's that little disc of light at the focal plane where all the rays converge. Its size is only a linear function of the focal ratio and the radiation wavelength. Nothing to do with the aperture. So it's a little smaller for blue, bigger for green and even bigger for red, etc...
But the angular size of this disc is inversely proportional to the aperture. So as the scope diameter increases you can pack more of these little discs in the focal plane surface. Keep in mind we're talking about light here coming to focus. There is no CCD chip, no pixel size, no eye pupil, no eyepiece magnification of the image plane.
I did a
quick illustration to visualise how it works. The top is a 4" f/6 objective and the bottom double the aperture 8" f/6. Then r
eplicated for a 16" f/6 objective. This was done in PS. I pixelated a photo then applied a circular pattern with a radius matching the box sizes. In reality all the diffraction discs are much much smaller, overlap each others and are fuzzy with a primary concentric ring and other rings of various luminosity if there is a central obstruction and/or spherical aberrations, then on top of that the seeing will blur all this in one blob. It is not to scale but for the illustration this is fine and I respected the proportions between the different aperture panels which is a factor of 2 each time.
All the diffraction spots have the same physical radius in all cases (4.1 microns). So you see a star at the top that covers about 4 dots in the 4" f/6 focal plane. The dot size is the size of the smallest detail that your objective can resolve. In the next panel you see the same star that covers maybe 25 dots in the 8" f/6 focal plane. So if it was a double star the 8" f/6 would have resolved it because there are enough dots in this area to potentially cover the gap if any in between the two stars. The 4" f/6 would see only one star. Also note the 8" f/6 resolves two little stars at 9 o'clock and 5 o'clock in the insert. The 4" f/6 barely shows a hint of the one at 5 o'clock. So it doesn't matter what camera pixel size you fit to the 4" f/6 objective. You still won't resolve these stars because the light is not there. It's too spread out. Similarly you can see the potential resolution in the 16" f/6 objective vs. the 8" f/6. And so on as aperture doubles while keeping the same focal ratio.
The argument that the image in a larger aperture (let's say a 16" RC) is greatly degraded by seeing is flawed. It is equally degraded in a smaller aperture (let's say a 130mm refractor) but it's not as obvious because you can't easily see it.
Now consider deepsky imaging if we used the same CCD chip on the two rigs. The 16" RC might give you slightly larger stellar profiles but there is something called a PSF that any software doing decon can use. One or many non saturated stars in your field will give the algorithm a path to reverse the effects of the atmosphere and partly restore the details in your image. The 16" RC airy disc angular size is smaller than the 130mm refractor and much finer data will be captured by this objective. With the 130mm refractor similar data was never there in the first place because it's simply beyond the reach of the instrument aperture and no amount of processing can make up for it. The limitation is the wave nature of light. Light is not infinitesimal. This is a physical fact.
As a side note, in light of recent trends, use the telescope you have but be aware of its limitations and when processing, if it looks too good to be true, then it usually is. Use common sense. There are a few basic rules and some method to astrophotography in extracting an image from the data your objective is capable of capturing. No more, no less. It's not wedding photography with a free for all you can eat buffet. But if you choose to do so don't mention science. It has nothing to do with it anymore. It's art, compositing, painting, whatever you want to call it and open to interpretation.