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Focal length does indeed come into it as you are talking about magnification.
With poor seeing you are now magnifying a blurred object. It will become more obvious with more magnification and less obvious in a wider field image simply because that blur is taking up less angle of the overall image so to speak. The blur gets hidden in the wideness of view.
As far as image scale with the CCD goes, yes I agree that also factors in but even correctly scaled pixel/seeing with long focal length will not give you sharp images under poor seeing.
I believe all scopes are limited by the seeing conditions available but some less than others.
I have found generally speaking that APOs seem less seeing affected but then they tend to be short to medium focal length - 500 to 1300mm focal length.
They also have higher contrast due to no central obstruction and relying on diffraction to work. They are less subsceptible to tube currents and mirrors not being equal in temp to the air around it. They also are not affected by collimation (rarely a problem) and less effect of dew.
Narrowband reduces the signal. So you need low noise and high QE (not always from larger pixels - KAF3200 is 87% QE and its 6.6 micron pixels).
Low noise means high cooling and excellent low noise supporting electronics.
Narrowband in the suburbs needs bigger aperture and faster F ratios for a quicker buildup of signal.
And of course its easier to image the brighter objects.
My experience is large aperture works but it will be more seeing affected more often than wider field imaging.
All a compromise.
Hopefully the above gives you a guideline about what to do with your setup.
In your example you already have a triplet refractor and a smallish sensor. Small sensors of course have the same effect as magnifying the image as now a smaller section of the light being captured becomes the full image.
I would suggest the KAF3200ME chipped cameras, the 4022 you refer is also a good chip, the 3200 is the highest QE around short of super expensive type chips. The 6303 is probably one of the best all round for your type of imaging.
Its your camera more than your scope in my opinion. Your tracking is good.
You could try a reducer to get a lower F ratio going as well to help get the signal levels up with narrowband. If the seeing is poor perhap 2x2 may help.
1.4 isn't too far off the .66 - 1 arc sec/pixel band commonly recommended based on 2-3 arc sec seeing.
Greg.
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