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I spent most of the morning [and afternoon] arranging the spherical shutters to bring them up to a sensible height and the correct orientation. [Sideways tilt at the top to follow the dome.]
The fibreglass panels are flexible enough to follow any desired radius along their length. So I need the centre ribs' outer curve to be cut to that new radius.
The dome's slit
ribs will be half way in and half way out of the 4.3m
shell. So I'll need to know the inside radius as well. Just so that the
ribs and shutters faithfully follow the dome.
I used 2x4 [50x100] off cuts, on edge, to support the shutters inner edges concentrically with the dome. I would prefer less thickness, for a lower, shutter profile, but the zenith board sets the minimum height. The curvature across the shutters means the outer edges are lower than a normal [flat] bi-parting shutter.. Whether the spherical shutters are open or closed.
The bottom image shows the view from the back of the dome. The white line is drawn in to show the need for a minimum height for the shutters. There must be a close fitting, protective, weather cover for the drawer slides when the shutters are closed. The slides also have vertical depth so must safely clear the top of the dome.
The
long board I used to support the tops of the shutters is actually too
long. So it won't let the shutters droop far enough laterally. So the
shutters are not concentric with the dome laterally. Which makes them
much too high on the outside edges. This also throws the geometry
of the inner shutter edges out of line. They appear twisted or the edges curved. I
can't cut the base of the shutters horizontally until the shutters are
correctly arranged. They appear tilted.
Could
the slides be fitted inside the zenith board for better weather
protection? I doubt it. Not if I need to fit large eye-bolts for the
pulley system. The eye-bolts would have to be fitted where they don't
affect the sliding shutters. I'll have to look into this.
The usual practice is for the top slides to be screwed to the zenith board and top shutter boards. The zenith board also fits half way in and and half way out of the dome shell. I shall scribe a second and third 12mm, plywood board to sandwich the top of the dome. They will beef up the zenith board and spread the loads more evenly though the GRP shell.
The new dome is slightly higher which will help with clearance problems when lifting and lowering, long OTAs. I would never risk lifting the heavy mounting itself using the pulley system.
I
have plans for adding a gently bent and curved, zenith plate. To be
made out of sturdy, 10mm x 150mm [~½" x 6"] scrap aluminium. To closely
match the dome's curvature and facets. This would be securely fixed to
the zenith board, with more brackets, to ensure more than adequate
strength at the top of the dome. The plate could become the support for the pulley lift eye bolts. I would prefer the pulley system to be as near to the slit as possible. To allow OTAs to use the open slit for more vertical clearance.
Further reinforcing plates could be added on the outside top of the dome. To help to spread the loads on the zenith plate when the pulley system is in use.
After another couple of hours fiddling between showers I have decided to make the inner shutter ribs next. The ribs can be attached to the inner edges of the shutters with brackets. Then the outside height and curve of the shutters can be checked knowing that the centre ribs are straight and to the correct radius.
The radius of the inner shutter ribs will probably
end up at least 15cm greater than the dome's 2.15m. I can check the required
radius first. By packing up the tops of the inner shutters with at least 6cm
clearance above the centre top of the dome to make room for the slides. Though attaching the slides to the inner surface of the zenith board is very tempting. If, I can find a way to do it without problems.
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