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Saturday 25th Jan. 44F. Another grey, overcast day with occasional drizzle. An excuse to work on the new 6" in the shelter of the workshop. Though I always prefer to work outside when I can. The light is much better and wood dust just blows away.
By a happy coincidence standard 150mm PVC drainpipe just fitted inside the telescope's, galvanized steel, main tube. Another scrap extraction duct from a local furniture factory for small change. They use straight seamed tube to avoid sawdust buildup. As occurs all too readily with spiral ducting. This offers a much lower risk of fire.
The straight seamed tube is much nicer cosmetically than spiral, and to work with. It is also very stiff. The single seam can easily be lost in the hinge gap of standard telescope tube ring. Though this precludes easy tube rotation. I have cut the tube to 120cm. 30 cm shorter than the focal length. This allows the vital 200mm inside focus for the PST etalon. With a little spare for adjustment or even a focuser if desired. I made the previous 6" f/8 tube 30cm shorter and this worked well.
Just in case you are thinking steel is a heavy metal, this ducting is thin enough to be similar to 3mmm or 1/8" aluminium. Rare dents are easily popped out with a length of 2" bar slapped down inside tube when laid on a flat, smooth surface.
The galvanized coating makes it highly rust resistant even if left outside in the rain for years on end. I like the appearance as being reminiscent of antique instruments. Perhaps I just have a vivid imagination? Tube paint never seems to last long, for somebody like me. I am always removing and working on my telescopes. Even felt lined rings seem to scratch the paint all too readily.
Just in case you are thinking steel is a heavy metal, this ducting is thin enough to be similar to 3mmm or 1/8" aluminium. Rare dents are easily popped out with a length of 2" bar slapped down inside tube when laid on a flat, smooth surface. The galvanized coating makes it highly rust resistant even if left outside in the rain for years on end. I like the appearance as being reminiscent of antique instruments. Perhaps I just have a vivid imagination? Tube paint never seems to last long, for somebody like me. I am always removing and working on my telescopes. Even felt lined rings seem to scratch the paint all too readily.
After gently forcing a short length of PVC pipe into pipe I made it rounder than when relaxed. I was now able to measure the ID at 160mm and the outside at 162mm. Both +/- ~1mm. The small flange is visible in the image.The flange is used with proprietary clamps to hold lengths of ducting together as required. Straight seamed ducting is available from 4" to 16" in diameter. Perhaps larger, but I haven't found any. Spiral ducting comes in much larger sizes. So the straight seamed may also.
The objective cell adapter will be made from three laminated rings of birch multiply. Fortunately this size is manageable on the lathe. So I can saw out three rings of 200mm OD on the bandsaw. Then put them singly in the 4-jaw chuck to bore the insides to closely fit onto the main tube. An 11x3mm notch will be required on each inside diameter to slide over the single seam of the tube.
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