The upper, main window hatch of the dome observatory was clad with galvanized steel sheeting and the lower window hatch frame was fashioned to cover the opening beneath the main hatch.
A front lower window that hinges forwards from under the main shutter was built as a curved square frame, itself to be clad with galvanized sheeting. This frame was pivoted on both sides from two roofing bolts that transfixed the sides of the two main arches at their lower ends. The pivot itself was shaped in the form of a 110° angled flat bar on either side. This was found by experiment to allow the best closure of the frame over the window opening. It was not projected for the window to open fully to maximum aperture because my location did not allow a clear sky view down to the horizon and it was argued that this would have allowed far too much light pollution to reach the scope for the added view to be useful anyway.
The placing of the hinge was designed to allow the front window upper edge to reach just above the lower edge of the main window shutter. This would allow the bottom edge of the main hatch to slip over the upper ridge of the front hatch thus sealing the opening completely and closing off the dome. On closure, the upper edge of the steel sheet covering the front window frame would then be sandwiched between the lowermost limits of the aluminium rail, below the braking block, and the overhang from the lower edge of the main shutter.
Retaining telescopic restraints (not visible in the above) were used to prevent opening of the window beyond the required amount.
These were later scrapped when manual opening was replaced by an electrical servo.