Building the Octagon Pt.42 Flooring the observatory.

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Almost half of the obs. floor is laid but not fixed yet. A poor photo because it is difficult to get far enough away. Both ends of the floorboards have been mitered to a stretched cord line around the perimeter of the octagon's support posts. The red cord is just visible in the foreground.

I don't think the small amount of overhang beyond the nearest joists is serious. Simply because it is impossible to stand so close to the walls. Some of the boards have been exposed for some time to changing weather and sunshine. These have darkened nicely for an aged effect. Other boards are straight from the stack and not yet affected by exposure to sunlight and heavy rain.

I have ordered some of the "Eco" wood treatment, mineral powder and hope it has the desired effect. It would be nice if it had a darkened or weathered look after the water based treatment. It also provided toxin-free protection.

Now I will continue to lay boards on the other side of the obs. floor. Only when they are all laid and straight will I fix them with the specialist, stainless steel, decking screws. These have small, conical, Torx heads to sink just out of sight in the larch boarding. A house floor would be cramped up tight to stop droughts but I'm not worried about a few gaps, within reason. The edges of the boards are slightly beveled so don't show the gaps as much as square-edged boards.

I have some rather fuzzy theory that a "draughty" floor will help to rid the observatory of excess thermal effects. I can leave the door and hatch open to get a chimney effect up through the observation slit. Once the hatch is closed [for safety reasons] there would no longer be an upward [convection] air flow with a tightly sealed obs. floor. A sealed floor will lead to a lack of draught for the "chimney" leading to more prolonged cooling as temperatures drop. Rather than more rapid matching of inner and outer temperatures. A continuous trickle of warmer air out of the slit is then likely. Giving the typical "dome effect" so deleterious to seeing.

The [ROR] roll-off-roof observatory design is wide open and quickly disperses any warm air. Since the dome slit is much smaller in area it cannot rid the space of slab heat in so efficient a manner. The covering roof or dome with its narrow slit also denies the interior, or any large masses like mountings, the ability to radiate directly and efficiently to the cold night sky.

It has occurred to me that a ROR dome would offer the best of both worlds. The interior of the building housing the telescope can be rapidly cooled and then the dome returned to its normal place. Giving the observer the desired shelter from breezes, reduced dewing of the optics and local light pollution. It would obviously be beneficial to avoid massive heat sinks, like concrete slabs and brick or block constructions. Wood construction not only avoids solar gain but has very low heat capacity compared with metal cladding and solid or heavy wall constructions.

Roofing felt is a very poor choice for covering observatory roofs or domes. It gets very hot in sunshine and has enough mass and heat capacity to cause thermal issues for hours after dusk. I cured one shed, afflicted by black roofing felt on a concrete slab and subject to soaring daytime temperatures, simply by covering it in recycled aluminium sheet. The effect was truly remarkable.

Click on any image for an enlargement.

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