Building the Octagon: Pt.69 More roof mock-ups.

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A forum member has kindly suggested suitable proportions for a bent, octagonal cone. He suggested I make up a template to check the clearance of the telescope. Since I had a few lengths of plastic conduit tube I used those.

 The first image shows the rather bendy conduit propped up in a 5' + 5' arrangement. This provided adequate clearance when the telescope was pointing high above.

The central prop is 2.3m high and is there to provide stability of the mock-up in an occasionally  gusty wind.

The second image is from a lower viewpoint and the struts have been 'straightened' simply by drawing over the image in red. I think you will agree that the upright "walls" are too high at 5'.

In the third image the "walls" have been reduced to 4' high. The roof struts have been extended to simulate slightly raised height to compensate for the loss of headroom. Overall height of the roof peak is now about 2.4m.

It is fascinating to move the telescope around to check for clearance with the 'virtual' dome.

And now with 4" boards to provide the vital, inward slopes which dictate the roof's geometry.  My 'cheating' using upright conduit to simulate the lower roof was changing everything which mattered.

As the inward tilt on the lower panels increases, the length of the upper panels decreases and the overall height increases.

I drilled some boards at 4" intervals to allow quick changes of length. Then  clamped them to the tops of the octagon support posts. In practice they should be outboard of the octagon. So I had better find a way of providing this overhang to get any real idea of the final appearance and clearance from the telescope inside. It isn't as simple as running a long board across the entire width of the octagon. The mounting and pier get in the way.

I can't believe how blind I was to the errors introduced by using upright lower panels. It was simply too easy to drill holes and stick the conduit into the tops of the octagon posts. From that point on it was all downhill. The geometry was completely wrong unless I overlaid the uprights with suitable angled conduit. I didn't and it made a mockery of everything I tried in the way of strut lengths. The 4" boards are rather heavy and really catch today's 20mph+ winds. So 2"x1" or similar battens would be better.

And now the latest iteration with overhang. I now have 5'+5' boards but connected outboard of the octagon by about 6" per side. With a gale blowing up there it doesn't feel very safe stretching high above my head on a stepladder to loosen the G-cramps at the bends.

So I have to lower each side in turn to make adjustments to angles and lengths. Then put them back up before moving on to the other side. There is barely enough clearance at around the 'bend' between upper and lower sections. Loads of clearance overhead. I want to try reducing the angles from the present 58° on the lower boards. This will lower the peak and provide more clearance around the bend.

It finally feels as if I am getting somewhere on the roof geometry. I could even shorten the upper boards now to flatten the top section. The peak is still too tall at over 12' above the obs. floor.

And last, but not least, 70° lower walls with 5' & 5' boards for a flatter 'roof.' My wife hates it!


Wet paint! What the latest shape would look like painted solid white and the building clad in weathered grooved ply.

Click on any image for an enlargement.

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