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F ratios in imaging got faster and faster in the last 10 years. F8 was common around 12 years or so ago.
Faster F ratios with larger apertures makes for more images given the opponents to making an image of clouds/rain, wind, time, moon, equipment problems. They all conspire to reduce the amount of exposure time.
But too fast and everything gets harder in setting up the gear. Add to that these "new" CMOS cameras with small pixels that show up defects easily and you have the other side of this that makes you want to have a longer focal ratio.
So the answer I think depends more on the quality of the gear. To achieve better than F5 is going to require better gear that is well setup. Below F4 is really demanding. Although I see a lot of excellent images from RASA setups at around F2.
So there is the need for speed and there is the rise of equipment problems slowing it.
I often thought F5 is good focal ratio. The FSQ is F5 and that makes a bright image quickly. F6 is still fine. F7 or slower is getting a bit slow.
So my answer is more of a range - F3.8 to F6 is a good imaging range with better gear needed to hit the faster F ratios otherwise F5/6 is practical and balanced between speed and practicality.
The fastest APO refractors are about F4.5 except perhaps a few exceptions like the FSQ with reducer. Large aperture scopes are generally slower and longer focal length.
Also keep in mind that using reducers often adds another layer of complexity and difficulty as exact spacing and the extra glass can add aberrations.
Greg.
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