Dome build: Making the shutters.

Saturday: I scribed and cut out 6 ribs in 12mm birch ply for the shutters/doors. I discovered I can get 7 half ribs per 5x5 sheet but need at least eight. Two half ribs overlap by about a foot in the middle when I lay them up against the slit framework. [image right] Blowing a gale, with heavy showers, so I had to be very careful about peeling back the tarpaulins.

I think I will internally plywood patch two half  ribs with a slanting butt joint. Rather than double up complete pairs with staggered joints. The weight would rise considerably with 24mm ribs each side of the doors. That would need more than and be even heavier than a single 5x5 sheet! Just for the ribs.

Every millimeter of central shutter rib steals width from the open slit. A narrower slit means the dome must be moved more often to follow an object across the sky. The deeper the shutter ribs the more tunnel-like the view becomes. The more lateral offset of the telescope on its mounting the worse it all becomes.

I wonder whether partial lining of the inside of the shutters with 4mm ply [where possible] would stiffen them more than thicker ribs? A box section would help if it were possible. The inside of the shutters have to clear the dome slit ribs. So any horizontal battens [or ply sheeting] will need to clear too. The inner and outer ribs of the doors must close against the dome's, slit ribs for sealing. An overlap of the door's outer covering 'skins' helps to exclude driven rain when the shutters are closed.

The deeper the shutter's internal crossbars, the greater the shutter projection must be, for sliding clearance. Slotting the slit ribs to clear deeper, internal battens might weaken the weather seal when closed. It would probably weaken the slit ribs as well.

I added 15cm [6"] to the outer radius of the shutter ribs. The image above shows about 3" of outward extension of the ribs. This was because I needed to keep things safe in the very windy conditions. In theory the shutter ribs should just touch the slit ribs to match the radii. Though that wouldn't provide a physical shutter stop nor water seal.

The shutters [doors] can be imagined as pairs of opposing, sliding drawers. When closed, the outer edges of the drawers butt against the slit frame. This is similar to a drawer fascia stopping against its cabinet. Simultaneously, the inner ribs must also butt together.  Which is equivalent to the backs of the drawers meeting in the middle. Unlike most drawers they are in pairs on opposite sides of their their imaginary cabinet. When open the oversized backs of the drawers butt against the outer faces of their cabinet too.

Click on any image for an enlargement.

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