3.04.2021 Observation slit cut out.

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Saturday 3rd 33-50F, white, overnight frost. All day sunshine promised. In my dreams, last night, I added more bolts, lower down, to hold the building's frames more firmly together. I shall buy some more next time I am at the builders' merchants.

This morning, I plan to use the profile gauge. To try and draw a centre line from the doorway to the top of the dome. 

This proved to be possible, well within 3mm, using the gauge. I then stretched a ratchet strap along the marks I had made and drew a continuous line. This is only a guide for the observation slit to avoid looking skewed. I have checked the distances from the centre rear of the dome to the doorway and they agree. 

Having a nominally straight, centre line I could then measure 50cm to either side, mark and then draw in continuous lines. This will become the outline for the observation slit. Though I am tempted to cut inside the lines to allow me to correct any mistakes. 

The plywood, slit reinforcing ribs will fit flat against this cut-out. So will tend to emphasise any wandering of the lines. I feel the need to allow a margin for error. There is no real need to stick to exactly 1m wide. [About 40"] I have added 15cm [6"] beyond the top of the dome to the zenith board. This will allow the telescopes to look directly overhead. Subject to them being placed centrally in the dome.

Theoretically I could cut out the slit this afternoon. Am I feeling confident enough to do so? I shall have to arrange proper support inside the dome. The cut-out will not be lightweight! 

As a final measure I used a permanent marker to ink in the pencil lines. Pencil does not show up well against green shiny GRP.

I set up a stepladder with a plank tied on to support the top of the slit cut-out. Then drilled the top corner holes at the zenith. A one piece suit, dust mask and glasses helped to keep the dust at bay. There was lots! Blinding sunshine didn't help.

Starting at the top I sawed out the slit with a fine toothed "Laminate" blade in my jigsaw. It didn't take long even allowing for several ladder changes. I finished off at the front above the arched steel tube with the oscillating multi-cutter. The cut-out piece was lighter than I had expected but still weighty.  

Now I have a skylight rather than a full observation slit. I have to attach plywood ribs to carry the slit down to the  base ring. The arched, reinforcing tube was vital for holding the shape of the dome during the cutting. I shall leave it in place for as long as possible to maintain the correct shape and strength. I have now wrapped bright yellow plastic around the pipe. To avoid [further] collisions with my head!

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