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The image shows the latest arrangement of optical folding using two flat mirrors. The optical path easily fits inside a box 95cm long x 30cm wide by 40cm high. The image is taken from a photograph so some distortion is inevitable. I have propped up the mirror shells in the correct places to give a sense of scale. The shells will be used full depth and each hinged via a short piece of plywood cut to match their own circumference. The outer white rim suggests the 1" square framework. With a dropped frame for second mirror cell support. Note the wider angles of reflection at the flat mirrors. This is to avoid stray light from the objective coinciding directly with the last reflected parallel 'leg' of the folded light beam. However tempting it might be to make the instrument as compact as possible there are unavoidable downsides if taken too far.
The image shows the latest arrangement of optical folding using two flat mirrors. The optical path easily fits inside a box 95cm long x 30cm wide by 40cm high. The image is taken from a photograph so some distortion is inevitable. I have propped up the mirror shells in the correct places to give a sense of scale. The shells will be used full depth and each hinged via a short piece of plywood cut to match their own circumference. The outer white rim suggests the 1" square framework. With a dropped frame for second mirror cell support. Note the wider angles of reflection at the flat mirrors. This is to avoid stray light from the objective coinciding directly with the last reflected parallel 'leg' of the folded light beam. However tempting it might be to make the instrument as compact as possible there are unavoidable downsides if taken too far. 
My Porsa OTA tubing package has arrived promptly. There seems to be an awful lot of it for such a simple framework. It feels a bit heavier than I had hoped but the quality and care with which is was packed promises success.The metal reinforced joints are really quite a bit heftier than I could ever have imagined despite knowing their weight beforehand. There lies the secret to the Porsa system. The tubing is not some flimsy, thin wall stuff with a high price tag. The joints are easily strong enough to avoid flexure and require no triangulation to remain rigid. The black finish is excellent and unmarked on first inspection. Never having handled the Porsa Building System in advance I am really quite impressed. At a quick glance one might think the tubing was raggedly cut but, in fact, it has a distinctive internal profile and the cuts are perfectly free of any burr.
Porsa really has this product very finely developed. Note the chamfers on the solid plastic joints to aid the beginning of the forced insertion. Then there is a relieved section to reduce friction followed by the full size again. If the legs were continuously full sized there would be much higher leverage forces at the joints. It all bodes well for very stiff and stable joints once any framework is competed. The cast-in, aluminium reinforcing section is just visible in the narrowest part of the joint.
It might be possible to separate the joints again but I can't imagine it is very easy. Doing so without cosmetic damage only adds to the problems. Perhaps blocks of wood and a heavy weight to resist the hammer blows would help if a mistake were made during assembly.
Since I am aiming for a stepped design I must be careful to ensure the opposing tubes are all of the same length. The joints each add an inch to any run of tubing. This must be taken into account including the corner joints of a box frame. The tubing is sold in stepped lengths in cm with nicely cut ends. If one used [say] 50cm lengths of tube then the finished item would be 50cm + 2 x 2.5cm corner joints. Or 55cm in total length. Not the original 50cm length for which one had planned. This joint addition must be kept firmly in mind for any design using Porsa tubing and joints.
Plain tube, as well as a variety of other flange forms and finishes are all available. White, silver and black options are offered in both joint and tubing colours. Porsa may seem quite expensive but offers genuine furniture quality finishes and results. Its most popular use is probably for aquarium support. The visual sense of lightness and cleanliness of the 1" square tubes is a far cry from heavy slotted [and rusting] angle iron held together with ugly bolts.
Hack-sawing the tubes to length freehand seems a little crude unless I use a miter box to try and ensure my cuts are as neat as Porsa's own. It would be a shame to scratch the tubing by carelessness. I have a cheapo miter handsaw frame with swiveling base but it takes deep, 53cm long saw blades rather than the standard 30cm hacksaw blade. This saw is really intended for cutting neat miters in wooden picture frames. So the small, hardened teeth may object to being [ab]used on aluminium. Perhaps I should buy a fine metal blade for my bandsaw? I have just bought some good quality 24tpi hacksaw blades so I'll see how I get on with the hacksaw and miter box first.
By sheer luck I was able to purchase a metal cutting saw blade for my adjustable miter saw. The blade was slightly too long but could drilled and clipped to the proper length. It has very fine teeth with a wavy edge to avoid jamming in the cut. So now I am equipped to cut the tubes neatly at right angles.
After more folding of the full scale, paper, light cone I discovered a slightly different layout. Where one square section is moved along relative to the other. Construction options exist for T-shaped joints in the adjoining rails. Or even two complete rectangular box frames fixed together with bolts or clamps. I think this arrangement has a more attractive or more traditional[?] appearance than a simple 100x40x30 rectangular box.
Allowing one complete leg to slide over the other provides greater flexibility in design to allow the flat, folding mirrors to be placed optimally. If their positions have already been confirmed then a fixed arrangement using four extra Porsa joints is probably lighter. It rather depends on whether cross bars are considered necessary at the junctions of the legs. If the two square 'tubes' were kept separate but overlapping then the overlapped lengths are duplicated. I doubt there is much difference in weight between these two options. Though both forms are heavier than a simple, rectangular box because there is no central bar dividing the sides of the simple box. The 30cm long section in front of the second folding mirror and above the objective is superfluous anyway. So could be "cut away" from the box to better define the objective section. While leaving the back of the OTA flat.
I read the Porsa instructions online and apparently the cut edge must remain sharp to achieve a cutting action as the plastic joint is driven in. The plastic burr is then trimmed with a sharp knife just before final closure of the joint. To get a close fit for a perfect cosmetic result. The inside of the tube must not be filed to aid the joints insertion. In case you need to put some Porsa tubing together without instructions the smallest section protruding from one end of the joint should point along the vertical axis of a floor standing frame. For an OTA one presumes that the "tongue" should point along the long axis to achieve maximum stiffness.
After failing to find suitable aluminium plate to make the optics supporting plates I came cross some cutting boards in a supermarket. Choosing the darkest grey option gave me four panels of up to a maximum of 30cm x 24cm each if I exclude the cut-out handles and spare material beyond it. The 5mm thick material is described as polypropylene which can be considered an engineering plastic similar to ABS. A lucky find and not expensive at the discount price. The density of aluminium is three times as high as polypropylene. So I am still saving weight even with the 5mm plastic @ nearly 1lb per sheet compared with 4mm aluminium. The PP is remarkably stiff in this thickness and should be easily capable of performing 'bayonet' objective mounting duties.
The downside is some doubts over its ability to take a tapped thread. I shall have to do some research into this matter. [Resulting in much reading and only a little learned.] If the PP is gong to be used as a bayonet plate it will need to accept the collimation 'pull' screws of the heavy [10lb] objective. Clear holes could be backed up by T-nuts. Though the points will need to be anchored to avoid accidental rotation. Pre-drilling for the T-nit anchor points might work provided there is no stress on the plastic. The 'push' screws can have a Nyloc nut attached to avoid local loading. Just as I did with my earlier plywood counter-cell.
A standard engineering thread has the wrong thread flank angles [60 degrees] for strength in thermoplastics. Special, self-tapping screws are available for plastics with low flank angles. [48degrees] Dare I trust my precious lens to self-tapping screws alone? At the cost of a little extra work I could use multiple [self tapping] bayonet screws. 5mm PP should be thick enough to get some serious holding power if I follow pre-drilling recommendations. The OTA support plate can be easily fixed to the Porsa flanges with CSK head bolts in CSK holes and Nyloc nuts.
Click on any image for an enlargement.
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