31.03.2021 A huge relief!

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I was not feeling much confidence in my ability to mark out the observation slit from the outside. So I asked on the astro forum for alternative ideas. 

A forum member has just suggested a brilliant, alternative idea. Make a frame to just clear the inside of the dome and a meter wide. Hinge it at skirt level in the centre of the dome so that the corners of the rocking frame trace out the slit. 

The frame would obviously need to be very stiff to avoid distortion, twisting or "lozenging." Easily managed with thin ply cladding to stiffen a batten frame. The hinges/pivots would need to have no play and be accurately levelled and oriented. The exact height and level can be checked with the laser level. While simultaneously checking the skirt is also level. I can drill tiny reference holes for the centre and corners at the top of the slit to double check against the outside mouldings and measurements.

The huge advantage is that I need no longer work from ladders on the outside. I can mark and saw out the slit from the safe height of a beer crate at the zenith. The rest is easily done while standing on the ground. My dust mask and goggles will be needed of course. I'll wear an old, hooded cagoule to keep the GRP dust at bay.

Covering the open slit with the netting reinforced, clear tarpaulin will be easy when required. This will provide plenty of light for working on the dome's base ring. While still safely protected from wet weather. 

After further thought I may use V-grooves for bearings/hinges. Separation of the bearings will reduce play and increase accuracy. The bearings could even be clamped to the dome skirt. A length of 2x4 could act as a baseline. There is only the need for a little over 90° rocking movement. Repeated swings will confirm the lines are not wandering. 

19.30. It is much later now. So I'm off to check the accuracy of the ridge moulding with the laser. It was impossible to see the lines in bright sunshine. I have the action camera clamp fixed to a tall stepladder for extra height. This should aid visibility at the top of the dome. It did. So I marked over the pencilled laser lines with a felt tip pen.

 

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