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Very rough ballpark:
16" f5 = 2.032m focal length,
Assume the focuser is at the bottom of the 18" (457mm) secondary cage, so the pole length is going to be (roughly) 2032mm - 457/2 = about 1.8m per pole.
You should probably allow for at least f5.2 however which gives about 1.9m per pole.
You will therefore get three poles from a 6m length, (order direct from Capral Aluminium @ about $35 per 6m length of 1.25" thickwall Al tube).
You will need 3x 6m lengths for 8 poles.
Edit: Oops forgot about the mirror box height (subtract from pole lenght), still it means each pole is definitely going to be less than 2m.
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