17.02.2021 Slit and shutter width:

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Here I have drawn the layout with a 1m slit width. A wider slit may be better from a purely cosmetic point of view. The 1m wide slit looks rather mean in proportion to the large dome. Though earlier refractor domes had no need of a wide slit. Observation slits became much wider with the arrival of much larger reflecting telescopes. These soon put an end to ever larger "classical" refractors and the huge and expensive domes they demanded due to their great length.

The controlling dimension, in my case, is the maximum width available which can be cut from the extra [shutter material] panel. The pair of shutters must obviously be slightly wider than the slit itself. To allow room for the thickness of the supporting, plywood ribs on either side. Which are hidden inside them.

The better proportioned 1.2m slit width is shown at right. This would require a pair of shutters at least 64cm x 2 =~128cm total. My calculations suggest 146cm is available minus the thinner [weather proofing] overlaps on the edges of the panel. Plus the width of the necessary saw cuts and any other "wastage." Like wandering saw cuts! 

Selecting precisely where on the length of the extra segment, from which the shutters are cut out, will help.The panel shrinks to a point at the rear. While the doorway sets limits at the front. The shutters must reach from just beyond the zenith right down to the base ring at the front.

The 1.2m slit width also looks better balanced when the shutters are closed. The wider slit demands less GRP material for closure of the arched doorway outboard of the slit. Supported in full [spherical] 3D by the plywood base ring and slit ribs.

Each dome segment has eight, decorative, longitudinal, moulded facets. Which will provide visual interest to the dome and the shutters. Also helping to break up the outline and visual bulk. By providing multiple angles of reflection. From the sky above, the background trees and overall surroundings. These facets will be horizontal on the dome but vertical on the shutters. 

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