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Always best IMO to flat frame after every time part of the imaging train has been changed, but at least any vignetting should be the same so you should be able to shoot flats per filter and hope dust bunnies doe not really cause issues.
The faster scope will certainly make tilt more of an issue, the faster ratio meaning a small critical focus zone, even if you don't get elongated stars I have seen RASA images where you could see that the sensor was tilted across a diagonal, stars in two corners diagonally opposite were in focus but the other two corners were not.
the 8" RASA is basically intended to be used with an OSC astro cam like my 294, if the pixels were a smidge smaller it could practically be designed for the job. People doing mono with filters have found a way to do it rather than it being the intended configuration.
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