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Nick- the 6" 15 you are remebering was probably Wayne Stuarts 6" F11 - it had 3mm ply square tube and aperture stops all the way down to strengthen the tube . It was actually parabolised ( I made the mirror using a collimated light source ) . Unfortunately ( at least in Summer when I used it ) it was next to useless on the planets . Heat always emanating from all the wood and tube currents jumping over the aperture stops into the light path and ociurse trapped in the corners between the formers. I learnt from that that you shouild always leave a Newt tube as smooth and obstruction free as possible to allow the warm air to escape from the tube as uninterupted as possible .
Ralph- again I'd respectfully suggest leaving your tube around 60 max 70 " . That was the longest tube you could put on the Samson mount . By all means figure your mirror on star at focus with a ronchi grating until the bands are straight . Whether you parabolise a little or not is moot as you would be chasing the needed sphere a little just as you might chase a slight parabola. If you null test on a very distant light or star its all the same . Anyway if it your first experience just treat it as learning curve to a perhaps bigger part of the hobby . You can buy a chinese 6" F8 mirror for half the cost you can get one aluminised over here anyway .
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