20.07.2021 Of shutter ribs and things.

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Tuesday 20th 60-69F mostly cloudy.

 I have been having another look at the shutter ribs. I cut these to radii to match the slit ribs plus a little extra for clearance. Only later had I added the zenith board. Which had raised the shutters at the top to allow the drawer slides to clear the outer edges of the spherical shutters. They curve downwards from centre to edge.

Logically [?] the shutters should be parallel to and therefore equidistant from the dome throughout their considerable length [and width.] This avoids the complexity of having tapered, shutter ribs. Though in practice there is nothing very difficult about providing them.

Let us assume I shall build the shutters to follow the dome's curvature. That leaves me with a lot of rib arcs which no longer match the shutter's internal radius if they are raised. Nor will the arcs inside radius match their newly raised position. Whoops!

I had hoped to have the outer shutter ribs almost [or actually] resting on the dome. This would provide even support and better weatherproofing when the shutters are closed. 

Now what? Make another lot of arcs? I could move them downwards to rest on the dome as planned. A second set would be made to the correct radii. With a vertical overlap. The problem with that is a step on the lower, very exposed, outer ribs. Rainwater and snow would collect there. Perhaps causing delamination over time. 

A reversed arrangement could have the overlapping rib on the outside.With the inner ribs resting on the dome. I want at least two laminations of 12mm ply anyway.  

I have screwed some 2x4 spacers across the observation slit to fix its width. I then raised ladders around the dome to reach the top from three sides.  Several mock-ups followed to see how the shutter ribs looked with different heights of packing pieces.

The shutters are presently too wide for the slit. This provides the opportunity to have some overhang at the outer edges for better weather protection. Or, I could allow the shutters to be slightly offset to each other. So that one shutter overlaps the other, at the centre, when closed. I would have to add an overlap weather strip anyway. So why not use the full thickness fibreglass for this purpose?

What I do not want is a sticky closure once the dome becomes very inaccessible. Closure has to be effortless and flawless for those sudden rain showers. Though the option exists to have ropes and pulleys to help me close the shutters from the observatory floor. 

I sometimes use a pole with the present shutters. They are beginning to stick sometimes on the zenith board. This is due to the drawer slides fixing board slowly detaching from the outer ribs. Easily fixed from a ladder raised inside the dome. Though I can't see the point. Not this late in the plywood dome's expected life.

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