Dome building: Finalising the slit framework.


Thursday 3rd May: I have been working on the dome all morning. First I added more screws to bring the tally up to 6 per pair of joined ribs. This was followed by pilot drilling and screwing the base ring to the bottom ring of horizontal struts with 6x80mm Torx screws.

Then I added a 2x4 to the underside of the base ring to bisect the dome and provide an accurate  reference for dome level and height. It also provides a check for squareness of the observation slit frame relative to the dome gores.

I carefully marked the middle of the 2x4 beam using both a tape measure and checking with a laser rangefinder. I also checked the distance from the center of the top beam down to the inside of the gores at the base ring. The two measurements were within 5mm of each other after several trials.  The height of the dome was then confirmed at 1.5m [5'] inside.

I then turned my efforts to the slit frame. First I cut away its props to allow the top of the frame to sink relative to the dome. Then the opposing ribs had to be cut back as they were overlapping the slit frame, top bar by about 6". 

The plumbline showed I had now increased the sky clearance beyond the zenith to 6". This is not remotely critical IMO. How often does one want to stare at the exact zenith through a telescope? Only half the aperture matters anyway for a full aperture view. The other half is already seeing the sky though the open slit.

The side ribs ought to align with the underside of the slit arcs and now they do from both sides. The [slit opposing] gore ribs were chopped off after dropping verticals from the slit frame's top beam. [See image above.]

Next I have to add short plywood arcs to complete the ribs so that they join the slit frame as a continuation of the dome's external hemisphere.

All the ribs need sanding to even out the lumpiness from roughly jigsawing the flats for the covering panels. I keep wondering if glue is necessary if I can climb on the structure without so much as a creak [from the woodwork.]

Suitably clad in an industrial dust mask, hearing defenders, goggles and a hat I sanded the ribs smooth on their edges. I used an angle grinder and 80 grit disk. By the time I had finished, the dust on my forearms was 3/4" of an inch thick! The racket was drowned out by a neighbour cutting up pallets for firewood and another resurfacing his drive with a JCB. 

Click on any image for an enlargement

No comments: