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David's quite correct. If you only need 15-20mm more travel in the focuser, then your primary mirror collimation screws need only be 15-20 mm longer.
Cutting the tube and moving the whole primary mirror and the mirror cell (the holder) is a last resort.
I did this on an 8" tube once. I had the choice of a low profile focuer of using longer screws. The longer screws were cheaper.
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