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>Is the spider and mirror mounting centrally placed and the mirror offset applied when gluing the secondary mirror to the flat?<
Yes. See the pic of the front end of Mike Sidonio’s Orion Optics AG12 which clearly shows the arrangement. This is roughly f/4, so an extreme case:
To be honest, at f/6 you won’t notice a lot of difference, assuming the secondary is big enough, especially on such a modest imaging circle. Minor vignetting can be flat-fielded out.
Place your eye at the distance where your CCD sensor will be. Does the secondary show you the whole of the primary with a bit to spare? If so, fine.
For planets, assuming the telescope is collimated and cooled down, seeing conditions are the biggest factor by far. Everything else is pretty trivial!