7" f/12 iStar folded refractor 17: A major rethink.

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Having looked at the pros and cons of various folded layouts I decided I was not willing to sacrifice baffling for compactness. I would also have preferred a projecting 'nose' for the objective. Not for any other reason than cosmetic appeal to the more traditional appearance. I have had to order four more, 3-legged, corner joints from Porsa to allow the indent above the objective. I can build the front end of the framework while I wait for the joints to arrive. Leaving the final length of the framework to the backplate to be decided by the 1st reflection. When the mirror and focuser are both mounted on the backplate the OTA's length can be adjusted in a mock-up on the bench.

The new design will [probably] use a 30cm Porsa tube height for the objective area. Though I could reduce this slightly to [say] 28cm to make the nose more square. [Not ignoring the need for ample bayonet slide to ensure safe locking.] The backplate has to support both the larger 1st folding mirror cell but also the focuser. That is why I have chosen a nominal 40cm for the height of this area. Though this too could be reduced slightly to match the reduced 'nose' height. As the objective framework shrinks in height then so does the rear of the main 'box' section to match. Porsa jointing does not readily support angles and is meant to remain perfectly square. Which means opposing frameworks must all be identical. 

I badly wanted to open up the angles of reflection at the flat mirrors without increasing the area of usage unduly. By making the rear of the framework 40cm high I can push the 2nd mirror higher under the new shoulder. This helps to avoid [stray] light passing straight from the objective to the focuser and thence to the eyepiece. Which would seriously reduce contrast unless very well baffled. This was the major compromise with having a simple, rectangular framework only 35cm high. Only by refolding the full sized, paper model of the light cone could I see the potential for problems and how to overcome them. The tendency with all folded refractor builders is to want them as small as the straight-tubed version. This demand for maximum compactness leaves precious little room for the folded mirrors or the folded light beam.

All crossbars will be cut by hand on my miter saw to 24cm to match the polypropylene objective and backplate board's width. There is really no reason to make the OTA any wider and the original 30cm tube length was very generous. Particularly after adding the width of the tubes and joints to make an overall width of 14". The new width will be 24 + 5 = 29cm outside dimensions. The rest of the framework's measurements are shown on the image above. I really do believe I have reduced the compromises to a minimum with this new layout. It seemed to take an eternity to get there but I think I have now achieved the best design I can come up with for my folded refractor.

First cut: The objective board was cut down to 28 high x24cm wide. The Porsa frame was then cut to a generous size to give some clearance. The miter saw was used to cut the tubing neatly to length. Followed by mitering of all the flanges at 45 degrees. The assembly was then hammered together using a wooden board and rubber hammer  to avoid marring the finish. Care must be taken to ensure that joints can be inserted during assembly. It would be very easy to find it impossible if opposite sides weren't assemble first. Then the sides added in parallel.

The flanges needed care to avoid conflicts as the joints are closed. They must also be oriented as required before the joint has a chance to get a grip. I discovered that the joints were removable provided there was room to hammer and the frame had resistance. Best achieved by clamping in the workbench. The raised shoulder needed some thought as there was only one flange and several possible arrangements. The joints needed quite a wallop with the rubber hammer but thanks to the wooden board and timbers on which I rested the frame for resistance they all closed neatly. Even without the rear frame the assembly is quite stiff at the ends of the tubes. Once the frame is closed it should rival any tube for stiffness.

One thing I discovered today was that the PP boards bow very badly in the cold! I have laid them under the woodburning stove to see what happens with gentle warmth. If they remain bowed they will be a waste of time because I need flat, mating surfaces for the bayonet system to work properly. I may need to obtain some aluminium plate instead. I cut the height of the shoulder tubes down to 4" to make room for the 2nd folding mirror cell. This will lower the backplate by 2."
 
Trimming of the plastic burrs just before final closure of the joints kept things tidy. Simply rubbing with my gloved fingers was usually enough. Possibly because it was so cold and the plastic of the joints slightly more brittle than usual. The temperature outside is 30F, -1C and windy from the north. Requiring this ATM wrapped up warm in a large, down jacket. Thin rubber work gloves helped to avoid chilling my hands on cold metal without being bulky.

I'm afraid you'll have to ignore the almost bare soil where the permafrost and my construction activities have temporarily removed the grass. It will soon return in spring. More telescope making materials await their turn for attention.

Click on any image for an enlargement.

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