[Stefan Buda]

My post did not consider any practical aspects of choosing the right instrument. It is simply my back to front way of going about it, from a theoretical point of view, so that one ends up with the right combination of sensor and scope. I guess the main conclusion of the exercise is that the size of the sensor should determine the focal length, when used for wide field (based on preferred field of view). And the size of the pixel should determine it, when used for deep sky. In both cases aperture should be maximised.
The surprising bit is that the sensor puts a limit on the size of the scope. You can waste money on buying a larger scope (longer focal length) without any tangible gains.
That is why one needs to be careful with aperture fewer. In other words aperture rules, but only if it is matched with the right sensor.

I agree with your comments on various combinations that you mention because the practical world is a bit more complicated than the outlined theory which contains a number of assumptions. For instance theory suggests that the FSQ106 should be best combined with smaller pixels than the 9 micron ones that people use so successfully. This particular instrument has been redesigned more than once and I've seen spot diagrams of an earlier version. I was surprised how large the spots were. It seems that they designed it like a photographic lens. They compromised the centre in order to be able to produce a uniformly corrected field. That may explain why it works so well with large pixels.