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Friday 12th -14C/6F at 9am this morning! Bright sunshine all day but remaining well under freezing.
I used a 4m long, telescopic pole to mock up potential Polar Axes for the planned Cross-axis mounting. It was a surprise to discover how long the PA needed to be: 3.6m! Just to allow the present Dec/PA axis crossing height. The lower end of the PA would almost reach the present southern edge of the obs. floor! Raising the southern bearing on a support structure would shorten the PA and help to provide a clear pathway around it.
The North bearing would be 1.5m above the present roller support ring. With the Dec axis at the half way point at 2.3m. This is with a Cross-Axis PA centred on the present, GEM PA but extended, in direct line, [i.e. on axis] above and below it. These dimensions apply to the present, smaller, 3m dome.
The larger 4.3m Ø dome would allow greater lengths and heights. If the axis crossing point is moved up the PA the OTAs need more room in the north side of the dome. Potentially making visual use more difficult due to head clearance problems. The higher the OTAs are mounted on the PA the less room there is for longer dewshields high in the southern half of the dome. Though the 4.3m done should be more than adequate for this. While the present 3m offers anything but adequate clearance for the big refractor's huge dewshield. 10" diameter x 18" long. Nor does it provide headroom enough in the north at low pointing altitudes.
I have ordered six more dome support & rotation rollers/wheels to match the eight originals. 160mm Ø in white Nylon, needle roller bearings. Each wheel with an individual 360kg load capacity. It seemed wise to have a roller above each new upright post to support the larger dome.
It is difficult to judge how much friction is gained or lost with more or less rollers. The present dome has friction issues with the stiff, plastic stand up collar rubbing against the rubber skirt. I'll be glad to see the back of both. The rubber skirt is horribly noisy as it flaps [slaps] almost constantly in the wind. I hope to have much better sealing against the wind with the new dome.
Likely total weight of the new, fibreglass dome will be about 400kg. This is allowing for the shutters, laminated plywood ribs and much thicker, laminated, base ring. The weight of all that birch ply soon climbs once the laminations are added. Whole sheets of ply, in the form of cut arcs, simply vanish into the woodwork.
I cannot remember if I ever worked out the weight of the present, wood and plywood dome. A lot of wood went into its frame. I remember it suddenly being much harder to turn once the 4mm cladding went on. Soon solved with the cranked, friction roller drive. Though I wouldn't want to be without it. Pushing the dome around manually is very hard work!
I cleared the 4" of snow off the observatory veranda in the afternoon. I keep wondering if I need one on the bigger building. It was handy to have a veranda when building and maintaining the smaller dome. Hopefully the new one wont need any more work once it is up there. I can access the top of the dome via the slit using inside ladders. Only the top, shutter, drawer slides will need occasional checking.
That said, a veranda can add to the architectural interest of the building. Otherwise it might look rather austere or even boring. I would probably have to make wooden brackets to support the veranda on the new building.
If I don't have a veranda I would need to fit secure tie off points for ladders. The dome could be rotated to bring the shutters to the front [south.] For easiest access to the lower, shutter, drawer slides.
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