7" f/12 iStar folded refractor 16: Changed construction details.

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After failing to find suitable aluminium plate to make the optics supporting plates I came cross some 'kitchen 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 the spare material beyond the oval hole. [Which will be very handy for practicing cutting, drilling and tapping this material.] The 5mm thick finely textured material is described as Polypropylene on the manufacturer's label. Which can probably be considered as an engineering plastic similar to ABS. A lucky find and not at all expensive at the discount 'offer' 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 [full] cutting sheet compared with 4mm aluminium. The PP is remarkably stiff in this thickness and should be easily capable of performing 'bayonet' objective mounting duties as far as strength and stability is concerned. This plastic should not be exposed long term to UV but I won't be leaving it outside in sunshine for very long anyway.

The downside with PP are my doubts about its ability to take a tapped thread. I shall have to do some research into this matter. [Resulting in much reading.]

If the PP is going to be used as a 'bayonet' plate it will need to accept the collimation 'pull' screws of the heavy [~10lb] objective cell. Clear 'through' holes could be backed up by recessed and modified T-nuts. The reduced anchor points will need to be located somehow to avoid accidental rotation against the relatively hard, plastic material.  Pre-drilling dimples, for the T-nut anchor points, might work provided there is no local stress on the plastic. The screws are always in tension which helps maintain the T-nuts location anyway.

The collimation 'push' screws can have a Nyloc nut attached to avoid local loading on the face of the plastic 'bayonet' plate. Just as I did with my earlier, straight tube, counter-cell. Which worked well to stop the 'push' screws from slowly boring their way through the relatively soft, plywood counter-cell surface.

A standard engineering thread has the wrong thread flank angles [60 degrees] for holding well in thermoplastics. Though a variety of special, self-tapping screws are available for plastics with low flank angles [48 degrees with sharp, raised threads relative to the core diameter.] 5mm PP should achieve a reasonable level of strength using these specialist self-tapping screws. Arranging eight 'bayonet' screws on a square at 45 degree angles will increase the strength considerably. Spreading the load so widely will avoid local stressing of the plastic material. 

The 'keyholed' OTA support plate can be easily fixed at intervals to the Porsa flanges with small CSK head screws in countersunk holes. With washers backed up by Nyloc retaining nuts on the inside of the flanges. The screw heads must not protrude from the OTA plate or they will spoil the two plate's, flat location against each other to maintain collimation between removal and refitting. Access for tightening the nuts is easy with the Porsa framework completely open at the sides. Cutting more numerous keyhole slots neatly in 5mm PP may require some patience.

Can I trust my full-sized, paper, light cone in placing of the objective, folding mirrors and focuser back plate? Or should I build an optical bench with the optics supported in V-blocks before I begin construction? Perhaps the real question is whether I can measure the components on the optical bench as well as the the simple folds on the paper model? Building the "front" of the OTA would fix the objective and second mirror positions. Leaving the rear of the framework untouched until I have established the positions of the 1st folding mirror, in its cell, and the focuser backplate. The framework can be laid on its side while these components are moved about on blocks. The 2nd mirror can be easily packed away from its supporting frame if it should prove necessary. As can the 1st mirror cell.

Only the focuser backplate position is really critical to avoid falling short. So that no eyepiece can be brought to focus. A middle of the focusing range with the 2" star diagonal in place is optimal. 'Straight through' viewing is only a matter of adding extension tubes to the focuser.

I don't believe it! I laid out the folded paper, light cone on the floor and then laid the Porsa tubing on top.  If I cut the PP boards to 24cm wide x 30cm high I can have a simple rectangular framework 100cm long x 30cm high x 24cm wide [inside measurements.] The joints will add 5cm, 2" to each dimension. This moves the 2nd mirror support to 40cm from the objective support board. 91cm to the 1st folding mirror face and 39cm between the two mirror faces. The focuser then sits comfortably on the outside of the tailboard.

The new framework is longer, but narrower and lower than originally intended. I also seem to have a lot of Porsa bits I probably didn't need! Though I can use two of the unwanted 40cm lengths to reach the joint at the 2nd mirror vertical platform. Which will be reduced in height to only 4" instead of 6". The 75cm lengths can be cut down to reach the backplate as originally intended.

Downside is the potentially greater risk of stray light from the objective entering the eyepiece. A length of tubing with a ring baffle inside the focuser position will help here. As will a baffle behind the 2nd mirror. Pushing the 2nd mirror as high as possible will also help the cause. The 25mm height of the square tubing aids mirror cell clearance. The image shows an uncut board on the left with a 20cm diameter lid placed on top for scale. This diameter is close to that of the 198mm protruding rear of the objective cell, though the clear aperture is only 180mm or 7".

I started to cut off the handle sections of the PP boards with a standard fine toothed, wood tenon saw. It seemed to be going rather slowly so I shifted to a coarser, hard toothed example. This went more quickly and I soon had two plain rectangles 24x30cm. I used a small block plain with the blade set low to clean the cuts though there was not much of a burr anyway. A final rub with fine sandpaper took off the sharp edges.

No pictures? It didn't happen. Capturing the true shade of grey is difficult. It is very like photographer's neutral density grey. The shading is purely an artifact of darkening the image to more realistic shades of grey.  Never knew you could have so much fun with two bits of plastic.

So far so good. Now I need to cut large [8"] holes in both boards. Which will reduce their weight considerably. Positioning the overlapping holes on the objective board will need care. There is the matter of the Porsa flanges and the dewshield fitting. The 'bayonet' slots will also need careful placement.

I think I shall try a normal metal cutting blade at the slowest speed in an electric jigsaw first. The problem with cutting plastics is the tendency to weld the blade to the material due to rapidly reaching the very low melting point. Acrylic is notorious for this problem and jams the blade almost before you turn on the power tool. At least PP does not seem to splinter like acrylic and PP feels quite a bit softer and more forgiving. Both make coarse swarf when being sawn but PP does not stink when heated by the cutting action of the hand saw. Blowing [with the mouth] is the quickest way to remove the dust from gloves and clothing.

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