Building the Octagon Pt.71. Segmented domes.

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Searching for segmented domes threw up a small number of skylight manufacturer's wares and projects.
 

The first is an octagonal roof lantern with only four vertical facets. Still a most attractive structure with jewel-like qualities in special  glass. The crispness of the superbly narrow joints will have to be my target if I build anything similar.

 AutoSpec | Skybright Skylights glass domes and octagons | Browse

The next example is a much closer approximation to a perfect hemisphere thanks to multiple gores [24?] and five [or six] vertical segments. A most attractive form in blue glass. The original is 42' across and is visible in Street View if you find the client's address in the link below.

http://sky-techglazing.com/Half_Dome_Skylights.htm


The desired segmented observatory dome  now needs to be defined as to the number of gores and vertical segments. An octagonal dome is possible but would limit the free radius inside as the straight lines cut across the arc. Only increasing the radius will help here. Adding more gores produces a much more realistic [smoother] approximation to a hemisphere. Adding more segments also smooths the curve. Complexity rapidly increases as does the number of panels to be cut and fitted.

Here I have arranged a hoop of plastic conduit to ensure the telescope would swing freely within its limiting, minimum arc.  I then superimposed circles on the image to conform to the required height [equator] and diameter. Note that the white hoop is skewed relative to the camera so appears oval/elliptical.

By enlarging the dome to the red circle it projects safely out over the octagon below. A suitable dome skirt will ensure weatherproofing. The supporting base track can be reinforced with timber brackets attached outboard of the octagon's upright posts.

The hemisphere's equator needs to be raised to match the mounting's axis. Alternatively, the pier could be lowered but at the expense of a very low eyepiece height when observing overhead. The present height of 70cm, 27" is considered adequate with a star diagonal in place. A low seat helps increase comfort levels on those rare occasions when overhead observation appeals.

Modification of pier height would cause considerable extra work because the heavy mounting would have to be hoisted clear. Using larger [4"] industrial wheels, with their usual, heavyweight, steel frames, will automatically raise the dome. As will using two plywood rings with blocks in between. It is arguably the easier option to physically lowering the pier. Albeit at the cost of a larger object.

I marked each quarter arc of the conduit with four and five pieces respectively to see how large the segments would appear in practice. Five segments seemed rather small. While four segments high seems to work best in practice. Increasing the number of ribs to form multiple gores will increase the dome's weight. 3.2m x Pi = 10m = domes circumference. C/16= height of segments.= 62cm.[Or about 2' high.]  16 segments = 62cm wide.

So the base ring would have equal segment heights and widths. The segment height will remain constant but the width will shrink with increasing height towards the dome's pole. The visual height of each ring of panels will appear to shrink due to perspective. Probably helping to maintain a nicely balanced panel appearance overall. Particularly when seen from a low viewpoint as will occur with my own observatory.

My hope is that the visually diminishing segments near the dome's pole will make it more difficult to determine the exact size and shape of the dome when seen from a distance. Each segment or facet should take on a different shade depending on its orientation to the light. Further confusing the eye and softening the shape by simple, geometric stealth.

Painting the dome [dirty] sage green should help to minimize its impact when contrasted against the mixed background trees. Each panel could even be painted a random variation on a theme of Sage. Perhaps  by adding a few drops of black or white to the paint pot at intervals. Perfectionists, longing for a snow white dome for thermal reasons, don't have to live with it! 

Not much physical progress on the build. Forum discussion surrounding roof design and construction, in particular, continues.

Click on any image for an enlargement.

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