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I can't see how you can successfully use flats taken at a different F ratio. any dust motes on surfaces not specific to the camera will be different and more to the point, any vignetting will be different too. For my SCT in particular, the fall off in light level is starkly different between F10 and F7 so a flat from one configuration would look awful used with lights from the other, you would end up with either bright corners or dark ones depending which way you went.
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