10" f/8 The 2 metre long, cardboard, telescope tube:

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The view of Jupiter in my 90mm Vixen refractor, the previous evening, spurred me onto finally doing something about my optimised 10" f/8 reflector.  

  

I could not start on the cell or spider until I made a decision on what to do about the main tube. So many options, so very little progress! I had spent ages online trying to source materials:  It seemed I could not obtain flexible plywood in a sensible thickness to allow two layers to be laminated into a roll. 8mm minimum thickness would make a 16mm thick plywood tube. I don't own a fork lift truck!!

Nor could I find any affordable marine ply in a sheet size which would allow a 2 metre tube to be rolled without joints along the length.

The only stockist of longer lengths of cardboard form tube would not deal with private customers. It was expensive enough at wholesale prices without the dealer's mark-up and 25% purchase tax! We are talking cardboard tubing here! Not precious metals!

I had managed to make the original pair of Biltema cardboard tubes round by forcing 12" ply circles inside them. The problem was Biltema's substandard storage. The rest of their stock was far worse than the two I had selected from the very top of the pallet/cage.

I finally made a decision to glue two tubes over two more with staggered joints. The two tubes in the core would be reinforced with 12" ply circles to remain round during gluing. The outer pair would be slit lengthways and clamped firmly on top of the inner pair while the glue dried.

Reluctantly I drove the 20 miles to buy two more Biltema tubes. My fears were confirmed by the sight of foolishly oval tubes. Many of them squashed into a tear-drop shape! They are supposed to be stored vertically to avoid becoming misshapen. Sadly the rudimentary wooden cage would not have allowed this with any degree of safety.

Having driven them 20 miles home again I discovered that they were not even as long as they should have been. Namely 119cm and 119.5cm respectively. With rough, badly sawn ends at that. So another cm or so would be lost before they had square ends. Fortunately I planned to saw them into shorter lengths anyway.

They were so horribly oval that I could not even force my 12" circles inside! I had to work up to  it slowly by squashing 3 of them them as sideways as they would go and then leaving them for a few hours. Much later I was able to beat them square to the tube with a heavy batten. They were then left overnight to try and straighten the ovals into nominal circles. Three plywood circles to a tube.

Next day I wrapped two tubes in lining paper. After bringing the overlapping edges perfectly together the paper was taped in place. This ensured the paper tubes were perfectly square to the cardboard tubes. I could then use the edge of the paper as a guide to saw off two 80cm lengths to go with the two full lengths of 120cm.  Here they are stacked into 2 metre high pairs in the image on the right. [6'6" high in old money] You will see that the joints between the pairs of tubes are staggered by 40cm.

In this picture I have yet to slit along the lengths of one pair of tubes  to allow the outer pair to fit over the core tubes. Though this will leave a gap between the long edges because of the difference in circumference.

By sheer coincidence it is the warmest day so far this year. We may even see 53F, 12C in bright sunshine after months of continuous frosts and snow lying. This will give the PVA glue a chance to dry. Though I will have to bring the tube indoors to cure properly as another frost is forecast for tonight.

I slit the two tubes, which were going on the outside, with a fine toothed hand saw. Then stripped out the very thin plastic lining. It didn't look or feel as if there was anything there but the thin plastic film refused to respond to sandpaper. It would obviously not respond to PVA glue either. So it had to go. Having obtained a clean paper surface I coated the inner tubes generously with the Outdoor PVA wood glue. The two outer tubes slipped over the inner tubes with a batten or two to keep the slit edges apart until the tube sections were in place. Wrapping the sandwich with long lengths of bungee did not produce a perfect job but I'm sure it will do.

If I was doing it again I would definitely use ratchets and webbing straps to clamp the two layers more firmly together. I saw some at the shops yesterday but was too mean to invest that much for only one job. The bungee cord works but it hasn't the brute force to pull the two layers together as well as I would have liked. It is also very hard work turning a big, heavy tube and maintaining tension. The glued surfaces don't slide over each other very well either. One must remember that the tube has a large surface area of over 3' x 6'6". It takes an awful lot of bungee tension to raise the pressure per square inch by very much. Bagging and a vacuum pump would probably be a much better idea but I don't have either.

I was much too impatient to get on with the job because of the unusually warm weather. The very oval tubing hadn't had long enough to stabilise into remotely perfect roundness. This made it harder to make them perfectly concentric. The gap between the long edges of the slit was surprisingly wide at about 1.5". I shall use offcuts of spare tube to close the gap. Then fill the spiral groove and any gaps before sanding. Finally coating the finished tube in epoxy resin to seal it.

After allowing an hour for the glue to dry out of doors it was late afternoon. So I brought the tube indoors to cure in the warm overnight. The tube seems to weigh a ton but some of that is due to the 6 circles of temporary plywood reinforcing. Adding the primary mirror and its cell will increase the overall weight even more! A 3 layer x 2mm, 6mm thickness, teak veneer, marine plywood tube seems (almost) desirable at £450 + glue and ratchet straps, at Danish retail prices! NOT!

If the tube is to be carried far then I shall definitely have to add the shallow, U-shaped drawer handles which I bought for the job. The tube is much too big, awkward and heavy to carry by simply bear hugging and lifting. Fortunately it takes up very little floor space when standing upright. At least I have a workable tube to play with now. I shall leave the bungee cords in place for a couple of days to ensure the PVA gets the best chance to cure properly. Hopefully leaving me with a stiff and strong perfectly round tube.
The bungee won't get in the way while I work on the cell and spider.

Friday: Following a day of curing I removed the bungee cord and reinforcing circles of 3/4"/18mm plywood. The circles came out fairly easily suggesting the laminated tube had relaxed to a better state of roundness. I weighed the bare tube at about 22lbs or 10.5kg. Being so awkward to lift it feels twice that weight!

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