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Aluminium tube wouldn't be the end of the world but carbon fibre would be so much superior. One of the attractions of CF is stable focus.
My RCOS 12.5 inch with carbon fibre tube would retain the same focus point night after night. Amazing really.
However with colour "parfocal" filters being not really totally parfocal best practice would still be to refocus for each filter when imaging.
But if you use CCDsoft and you program in an imaging run of LRGB for 6 hours throughout the night its nice to know the scope will hold focus despite temperature shifts and perhaps you don't mind the slight difference of focus on RGB (its often very close but not perfect - ie Astrodons and Baaders, Astronomik filters).
Greg.
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