10" f/8 Planetary Newt: The tube build:

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The 10" F/8 Telescope Tube design:

In my last 'essay' I tried to establish umpteen commandments for maximising the "humble" Newtonian for optimised planetary observation. So I had better follow the rules. No short cuts and no excuses. No sinking into bling for its own sake. If it does not add to the performance then it deserves no place on my telescope.

By chance I had found a (used) long focus mirror and matching flat online. They duly arrived in perfect condition and very well packed. The physical and optical dimensions of my planetary reflector were now set in stone. (Well glass, actually)

With the mirror diameter and focal length now decided I searched online for the concrete pile casting tubes. More commonly known as Sonotube in America. It turned out that there were two stockists of these spiral wound paper tubes. Both within half an hour's drive but in opposite directions. By far the cheapest option was Biltema. A discount DIY chain with a huge range of (almost) affordable products. They call them støberør in Denmark. (casting tubes or pipes)

They listed 12", 302mm internal diameter, spiral wound paper tube of 4.5mm wall thickness for about £12 or 120DKK. (in early 2013) The problem was the length. They only sell the stuff in 1.2 metre/47" lengths. My mirror required at least a two metre, 6'6"  length. I decided I could cope with this if I joined two lengths. I could choose where to place the joint later. After the entire OTA (optical tube assembly's) balance point had been established. At the time I thought that Biltema tubing had no plastic liner or wax coating, inside or out. So I could even glue the spare length of tubing over the joint. Or make a tube twice as thick from several tubes cut to size.

Why not just buy a suitable length of the 300 mm x 8mm thickness from the other nearby, but far more expensive, stockist? I like a challenge and prefer the thinner, untreated tubing for my initial tube design. If it fails I can always order the more expensive tubing. Possibly even going well oversize to 14". 350mm diameter. This would offer a 2", 50mm clearance between the full aperture and the inside wall. Not that vignetting is a problem even now.

Once in the vast Biltema shop I discovered they had absolutely no respect for the roundness of their stock! The tubes were stored on the very top shelf of a towering 20' high rack. The tubes were housed horizontally in a very tall and very rudimentary wooden frame on a pallet. This meant that all of the tubes were horribly oval from constantly pressing down on each other over time! Even the top tubes were oval so they may warp out of round naturally over time.

Why bother to stock something if your means of storage was so obviously ruining the product for its intended use? Oval concrete piles or pillars, anyone? Are oval pillars even legal under Building Regs? Why not stack the tubes vertically in a suitable wooden or metal cage? It would take up exactly the same amount of storage space. All safety factors taken into account, of course. You don't want these tubes dropping from a great height onto unsuspecting customers! How difficult would it be to run some strapping around the cage to allow vertical storage?

I decided the tubes were cheap enough and of suitable material quality to experiment with further. The spiral wraps were very neat and without any overlap. Making for a much prettier tube once filled and painted to keep it protected from inevitable moisture. They also had no (obvious) waxy or polythene lining. So would be much easier to work with and produce a half decent finish. (more on this later)

I'd probably start off by making some internal disks of plywood to force the tubes back into acceptable roundness again. So I bought the best two lengths in stock. (Believe it or not!) The rest were very much worse! In case you were wondering these tubes are far too stiff to make much impression by hand pressure alone. They hardly change shape at all no matter how hard I try to press them round. This is a good thing when the tubes are round. Far more difficult to deal with when they are this oval!

Given the very poor roundness of the tubing and the serious lack of length I decided to house the tubes in a series of plywood rings. These rings would be joined by several wooden dowels. You can call them stringers or longerons, if you like. Model plane, fuselage style.

A telescope tube must be very stiff overall. Even a large, thin-wall tube can become oval and bend regardless of its material. I have seen very long lengths of thick wall, steel tubing, up to 24" diameter, hanging from a crane strop, bend dramatically! Trust nothing to chance. A flexible telescope tube will place the mirror's optical axis outside the field of view. The telescope will constantly change its collimation depending on its angle of inclination. Any attempt at accurate collimation will be an exercise in frustration!

Perhaps the rings and dowels idea seem like overkill but I was determined to avoid an open tube. Nor introduce any unnecessary metal into the tube structure. I was holding a keen eye on the tube's neutral thermal properties. The intended plywood mirror cell was going to keep the bottom of the tube nicely round.

Now I needed the external rings to avoid introducing inner rings or baffles. I still wanted maximum clearance between the light path and the inside walls of the tube. Baffles would only push thermal currents out into the light path under the influence of the cooling fan. Turbulence would make this almost inevitable. A smooth bore tube would allow any warmer air to exit smoothly.

I had several 15" waste circles already cut out in 3/4" 18mm, high quality plywood from another project. The problem was fitting enough dowels, of suitable diameter, into the annular space available. There was very little clearance between the inner cardboard tube diameter and the outside of the circles. So I chose to make new rings in 1/2", 12 mm plywood. Which I already had in the shed. This would  allow an increase of the dowels to 16 mm in diameter from my original plan of 1/2", 12mm.

I would have eight 16mm dowels running as stringers from end to end of the very tall tube. Using fresh plywood rings would allow dowels and rings strong enough to lift the OTA at any point which fell comfortably to hand. All telescope tubes without handles are a pain! Large tubes without handles are completely impossible to manage! The outer rings would help to force the tube round again. The inner roundness "taming" circles could then be safely removed. Or so I hoped.

Besides, the dowels would make useful handles when I wanted to move the OTA manually on its mounting to point to another position. None of this would add enormously to the overall weight of the cardboard tubes on their own. Nor add any metallic content. The cardboard tubes would become a light excluding core with OTA stiffness provided by the stringers and close fitting rings. (Provided, of course, that the cardboard can be made properly round in the first place)

So far I only have the tubes sanded smooth at the roughly cut-off ends. I also have the dowels. All beautifully straight and standing on end in the shed to stay that way. The problem is that the shed is still well below freezing. Working in two duvet jackets is still no fun if one's nose drips constantly onto the floor! I need to be able to rout new rings out-of-doors to avoid the usual "dust everywhere" problem. Routers are awful things for producing lots of dust. They are best used outdoors in the absence of a proper (industrial quality) extraction system. Dust on tools and metal surfaces quickly accelerates their tendency to rust. Since the dust absorbs and holds moisture. It isn't healthy to breathe the dust in either!

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