Just to make sure i got it right, does this mean that for a given pixel size the resolution depends solely from the focal ratio?
For example, for my 183 cameras (2.4μm pixels) there is not any benefit if i choose to go for the 11inch rasa instead for the 8inch one...?
I'm sure i'm missing something here...
The f-ratio only determines the size of the Airy disk at the focal plane, while aperture determines the angular or arc-second resolution on the target itself. So, it really depends upon what you consider "resolution." However, I'd argue that in astrophotography the most commonly used definition for resolution would be the capability to produce a greater amount of detail in the target itself and that form of resolution scales directly with aperture (not f-ratio).
As for "critical sampling," I'd refer back to the rule of thumb I presented earlier, that being that you'd want to use an f-ratio that is about five times the size of the camera's pixels (with caveats, see my earlier post).
Here is a link to an online calculator where you can input values of aperture and/or f-ratio to see how those change the size of the Airy disk produced by an optical system:
Aperture determines the angular resolution while f-ratio determines the size of the Airy disk at the plane of focus (at the sensor itself).
In terms of either an 8" or 11" RASA, it's kind of complicated. However, at any given image scale a larger aperture will generally produce a "faster" imaging system. However, in terms of exposure for any given camera (pixel size) what really matters is the f-ratio (at least for extended objects, point sources like stars also respond to aperture).
So, if you use the same camera (pixel size) on each of those systems and for extended objects you will see no difference in the exposure time needed to reach a given level of signal to noise (since the f-ratios are the same, or nearly so). However, the 11" RASA will produce a larger image scale since it has a longer focal length which might mean greater detail if your technique and seeing conditions allow for that possibility.
On the flip side, if you could pair the 11" RASA with a camera that had larger pixels to the effect that both systems delivered the same image scale then the larger aperture scope would likely be the "faster" imaging system (since a larger pixel can gather more light/photons per unit of exposure time). Of course, if you were using the same camera on both the 11" RASA would produce a greater image scale (as discussed earlier) and you could resample the image from the larger scope to match the image scale produced on the smaller scope and gain some slight improvement in signal to noise.
Thus, assuming that you are NOT seeing or technique limited the 11" RASA should allow some image quality benefits. Now, would those benefits justify the differences in the cost and size of the competing systems? Well, that's kind of up to each individual person and pocketbook. That said, I think there are some system differences between the 8" RASA and the larger models, so you'd want to weight those factors too.
Edited by james7ca, 06 December 2019 - 01:58 AM.