First there is mechanical difference you didn't mention between the R8 and R11. That is sensor size compatibility and ability to use a filter wheel. The 8 is limited in maximum sensor area and single filter; the 11 can use larger sensors and a filter wheel especially helpful if shooting Mono.
Per Celestron and the 11... "Large 43.3mm optimized image circle maintains pinpoint stars to the far corners of even the largest astroimaging sensors, while the usable field extends even further to 52mm for larger format sensors"
Per Celestron and the 8... "Flat field free of optical aberrations like field curvature, coma, astigmatism and chromatic aberration across an entire APS-C sensor. Unlike the larger RASAs that work with DSLR and large CCD cameras, the 8” model was designed with color astronomical CMOS cameras, smaller CCD cameras, and mirrorless cameras in mind. The telescope does not work with standard DSLR cameras.
Next concerning aperture effects on imaging the same target, the 11 has a longer focal length meaning the maximum field of view is a bit smaller 620 vs 400mm but has about 2/10 arc second greater resolution which doesn't sound significant, but if you're imaging for detail with significant hours of data that is detail that will sift out as signal. The larger aperture means more signal per unit of time to fill the pixel well given near identical focal ratios.
Looking at images on Astrobin is in someways useful but in other ways useless because the data collected more often has gone through levels of processing rather than being a single unaltered image. That processing has the tendency to blend detail to the aesthetic pleasure of the individual. A more helpful resource would be raw single image data of the same target through the 8 & 11 using comparable imaging system to match pixel scale. This would identify better the reality of the difference between 8 & 11.
If I were to state the reasons for choosing the 11 over the 8 they would be as follows:
- greater signal per unit of time
- versatile options for sensor and filter
If I were to state the reasons for choosing the 8 over the 11 they would be as follows:
- budget friendly
- can be used on wider array of mounts
What sort of filter wheel can you use on the RASA 11? I have looked everywhere online and could not find a filter wheel setup which does not substantially increase CO or causes weird asymmetric diffraction spikes.