26.03.2021 Shutters or slit? In which order?

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My fuzzy logic suggests I start on the shutters next. Build the shutters first. Cut the observation slit last. Otherwise the dome is no longer weatherproof. Tarpaulin and guy lines? Not really. 

Clear, heavy polythene? Or, much better, net.reinforced clear poly "tarpaulin" and guy lines! Then I will have plenty of light to see what I am doing to the dome's base ring and still stay dry in wet weather. Otherwise I'd need working lights!

I have plans to use the 360° laser to mark the lines for dome cutting. The laser has a vertical line mode as well as continuous horizontal. Which will avoid making mistakes while working on the nominally spherical surfaces. Though it and the spare segment, will each have to be perfectly levelled first. 

Gaining access to the top of the dome was the next hurdle. One which was surmounted by a 5m roof ladder. With ridge hooks and wheels. I used this to sweep my own chimney for years. 

Some scraps of camping mattress were wrapped and tied around the upper ladder rails to avoid scratching the dome. Or applying too much local pressure.  The foam also provided lots of friction to the glossy GRP. So that the ladder would not slide sideways off the dome!

The sturdy dome showed no signs of distress, nor flexure. As I clambered up and down the gently sloping ladder almost on my hands and knees. A large dome, like this, needs a surprisingly long tangent to allow access to the very top of the curve. I placed a heavy industrial pallet at the base of the ladder to avoid it slipping backwards along the ground. You can never be too careful. The angle was too low to allow the base of the ladder to dig in as normal.

I then ascended to the summit [from both sides in turn] and marked a centre line and more lines at 50cm, one metre apart. Using the central ridge of the roof moulding seemed the obvious thing to do. Otherwise the slit and shutters might always look lopsided.

Then came the 360° laser on the video tripod at full vertical extension. It was still barely enough for the lines to reach the top of the dome. Then, once I was safely back on the ground, I could no longer see my pencil marks up at the top! So a block of wood, with a clear black line, was marked and placed over my pencil lines. The marked block soon proved invaluable to avoid too many pointless climbs. 

I then had to shuffle the tripod very gently back and forth. While rotating the laser head to align my 1m spaced, pencil marks. Two on the top of the doorway and the other two at the summit. 

A laser level should not be subjected to shocks when active. It has a complex pendulum system to self-level. The Bosch "Universal" 360° laser level I had bought has self-locking when switched off. Which was a real plus. I am very pleased with everything about the instrument so far. Apart from the cost and wishing for brighter lines in daylight. Overcast skies, dusk or even darkness are not difficult to arrange for improved visibility.

It has already saved me countless hours and laid years of nagging doubts to rest. I seriously doubt I would be able to complete this project to a reasonable standard without it. Building accurate base rings from laminated, plywood arcs is an absolute nightmare without such a useful level.

Then the sky darkened. Which made the green, laser lines much more visible. Unfortunately it had darkened for rain! Before I gave up I quickly tried using a steel rule and pencil. To mark out the projected line as high as I could reach from the doorway. Interestingly [?] the projected laser line was slightly curved relative to the rule. The rain steadily increased. So I had to pack up for the night. The ladder stayed where it was.

Tomorrow, if the weather allows, I shall lay masking tape along the laser lines. This should increase contrast and allow me to double check my pencilled cutting lines as I fine adjust the dome's level. I shall also elevate the laser using a tripod clamp on a stepladder.

I intend to use the laser line to confirm that the central, [decorative] moulded ridge is actually perfectly straight and on the vertical. This can then be double checked against the level of the dome skirt. I believe the moulded line is arguably the better reference to use. What reference would they use for cutting the skirt? The edge of the fibreglass mould prior to release? Hopefully, I shall soon find out.

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