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Hi Andy,
You are correct, tube length is a function of
focal ratio and primary diameter.
A 6" f/8 would have a focal length of
48".
Your tube length for this would be around
48". You need to take into consideration that the primary will be a few
inches inside the tube, and your secondary will be inside the tube 6-8-10 inches
depending on how much light baffling you allow.
I would spend my money on a 5 footer and trim it
down later just to be safe.
Don't know where you got the 8'?
Jump on the ATM Webring and surf some amateur
sites there are some really good ones out there.
Best of luck and Clear skies,
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