The photograph below shows the main rib-work for the observatory dome being gradually fabricated in-situ.
The base of the dome is constructed from two U-shaped girders, manually bent into a semi-circular shape using a large hydraulic trolley-jack, and then welded together to form a circle (with the U-form facing inwards).
The two vertical steel ribs seen in the photograph below were
then drilled, bolted, and finally welded to the circular base.
These two ribs form the edges of the opening aperture, and at
this point in the construction are anchored together by a series
of lateral ribs.