7" f/12 iStar folded refractor 12: Porsa System OTA?

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The area of plywood sheet needed to make my OTA box exceeds the baffle area of Jim Chung's skeleton design. Until I read the article and discovered the finished weight of his OTA I was quite tempted to copy his basic style. Now I really have to completely rethink my own design to make it easily transportable.

I checked the weight of the Vixen 2" focuser at 2.5 lbs. Add 10 lbs for the objective and then more for the two, folding flat mirrors and I now need sheet Unobtainium to build a manageable OTA! Moreover, I have no idea where to buy aluminium sheet other than ridiculously overpriced, cosmetically challenged, roof flashing materials. I could have used four 1" square tubes for stiffening and pop-riveted lightweight sheet metal onto the sides, top and bottom. Without access to affordable aluminium sheet that idea is a non-starter.  

There is a  Danish company called Porsa which sells 1" square tubing for a frame furniture building system. A large variety of tough plastic joints with aluminium reinforcement are offered. The basic idea being to use a soft rubber/plastic hammer to force the joints into place with an interference fit. The tubing is internally ribbed to achieve even greater resistance to coming apart. The system can support large or multiple aquaria without further reinforcement so must be remarkably strong and stiff. The tubing is available as plain or with a variety of "fins" along its length for adding shelves, baffles, or OTA end plates.etc. The images give a basic idea of what is on offer. Search for Porsa  for much more information and the extensive range of tubing and joints. English is available on their website with a click on the flag.

My OTA construction is obvious: Build a simple, rectangular, box frame and fix the optics to that. It should require no stressed sheet covering to add extra weight. The problem is discovering the weight of the finished framework. No weights are listed online against the various components.

I once thought I might make an OTA for my 10" f/8 mirror using this Porsa stuff. But back then I was thinking solely of using an equatorial mounting. The square format did not readily lend itself to tube rotation. Though it made perfect sense for a Dobsonian the cost of building the square tube was rather high when all the components were added up. There is a quarter round [quadrant section] option which might be marginally lighter and cosmetically more attractive at the slight loss of some stiffness.

Now I need an even lighter assembly to avoid mobility problems with the 7" folded refractor. It would still need some flat panels to support the optics and some more to act as baffles. Though the latter need be no more than the thinnest aluminium sheet. Tempting as it might be, to break the basic 'box' unit up into sub-assemblies, the extra joints required would each add their own weight. The minimum of eight corner joints become twelve or sixteen and each has its own weight penalty. Twelve times anything solid is going to quickly add up!

On request I have just been sent a list of weights for the components of the Porsa system. I should mention that the tubing is available in a large range of lengths from the manufacturer. With careful choice of length this should achieve the highest standard of cosmetic finish at the joints. Not to mention improved structural integrity over any likely variations of hand sawn lengths.  

A basic, rectangular, Porsa framework of 90cm x 30cm x 25cm = 5.8m of tube.
[i.e. Four long edges and two rectangular end frames at each end.]
1 meter of plain square section tube weighs 373grams.
Adding one edge flange adds about 75g per meter.
Each, reinforced, square corner joint weighs 140grams.
Tube material = 5.8  x .373 = 2.2kg = 5lbs.
8 joints = ~1kg. or about 2lbs. Tube frame total = 3.3 kg or  ~7.25 lbs.

Partitioning the overall length into two, adds four more joints and another four sided frame to support the second folding mirror. This would add about 1kg extra or 2.2lbs. The second optical flat would need a firm support board anyway. The flanged framework would allow a much lighter local support for the folding mirrors and objective. Without requiring the usually massive boards or heavy alu. plates for stiffness.

So [say] about 9.5-10 lbs total with the partition framing and four additional joints.
Rough estimate of the total price = 1000dkk, £100 or $150US.
Quite pricey, perhaps, but it offers a very stiff, attractive and convenient framework on which to hang the optics. 
  

The 5" and 4" optical flats weigh 2lbs and 1lb respectively. = 3lbs.
The mirror shells and 12mm plywood backing disks add another 2lbs. This could be reduced a little with backplate "ventilation" holes.
The 7" iStar objective in its cell weighs 10 lbs.
The 2" Vixen refractor focuser = 2.5lbs.

So, the essential optical/mechanical components weigh 17.5lbs before the supporting tube is even considered. Helium balloons would only catch the wind and probably end up in the trees.

A round tube is likely to need heavy plywood disks to support the objective and folding mirrors. An entire 12mm [1/2"] plywood box would not be light. Carbon fiber is too expensive to even consider. Though carbon fiber disks to close each end would be ideal from a weight point of view. Hard [phenolic] paper tubes will still need optical and mechanical supporting disks of 12" diameter to be practical. That would mean buying more tube rings and they get very pricey with increasing size. Closing disks might be substituted with baking pans/saucepans but the weight difference is really not that great. I can only speak from my own experience on my straight tubed OTA and the 10" f/8 reflector.  I did look longingly at the builder's straight edges but they do not suggest themselves for any folded refractor layout. Perhaps I just lack enough imagination but I'm thinking about the weight of the required baffles and boards.    

Assuming no other weight gains a 9lb Porsa tube framework means a total of 26.5 lbs before the mirror cells, baffles and rear and front optics supporting boards are added. Thin aluminium for baffles and slightly thicker end plates would do here. That's getting close to 3/4 of the present weight of my straight OTA! And, way, way over the remarkable 16lbs of Tom Gideon's superbly lightweight instrument. At least the completed OTA framework will be easily carried at about 3' long + focuser. The dewshield can be attached after mounting but doesn't weigh much anyway. I am assuming an altazimuth Dobsonian mounting which can be reached without difficulty to place the folded refractor OTA in its trunnions.;)

A refold of the light cone produced a new layout with 80cm to the first reflection, then 48cm and finally 58cm to the focuser backplate. I intend to house this arrangement in a rectangular Porsa frame 100cm long x 40 cm high x 30cm wide. The tubes size can easily be trimmed to a smaller size if it seems beneficial. I am trying to avoid too near a coincidence of the reflected light beams and that coming from the objective. See next "chapter" for a brand new idea on saving weight.  

Click on any image for an enlargement. 

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