*
Saturday 9th Jan: I have been measuring up the existing observatory
building
for this slightly smaller dome. [Only 10cm smaller than the H&L]
The Agritech Spheribox has some
advantages as a donor for an
observatory dome over the Holm & Laue igloo. There would be no continuity
problems in trying to avoid raised seams with shutter ribs.
This image [Right] of a stack of [white] Spheribox segments seems to show a form of halving joint to left and right. Where the thinned edges overlap the lower segments. This provides a much smoother transition between panels. Rather than having a thick step at the lower edges where the three panels meet.
I just wish I understood the orientation of the stack of panels. There seems to be drill holes along the nearest edge. Which should [in theory] be the lower edge where it rests on the ground. I found another image online of another stack but of inverted panels. The panels look identical to those shown here but this still makes no sense.
The Spheribox's large, semicircular doorway will be closed off. It weakens the
hemispherical form and is usually reinforced with a curved band of
galvanized steel on the edge of the open doorway. Which I obviously can't employ
because it would block the view of the sky.
Fortunately the doorway's open area will be heavily reinforced by the structure around the observation slit and shutters. Only two, curved [3D] triangles will require closing off on either side of the slit. I might mould these area in GRP on a part of the dome. To achieve a neat, three dimensional, closing patch for either side.
These triangular patches are important for carrying the strength and
stiffness of the dome form into the base ring below. There is spherical
curvature in all directions. So flat plywood patches might jar the eye.
The "waste" material from the observation slit cut-out, from the central segment, would provide suitable [spherical] material. This could be bonded to provide neat triangles in the structure. Or I could laminate thin plywood to make curved, triangular patches. The image [left] shows a very rough drawing of a patch. Though not remotely to scale or even in the correct position.
Only the central area
of the wide, arched doorway would become the lower part of the observation slit. With two, curved plywood ribs
extending downwards to the sturdy base ring. Remember that the dome base will
be resting on rollers. Which are, in turn, mounted on 1.5m [5'] high observatory walls. So the existing "doorway" would be
completely unusable anyway.
I
am [wildly] guessing that the lower edge of the GRP dome is thickened to
resist wear from contact with the ground. If that is the case then the
base ring can be raised slightly, within the dome skirt, to provide a
decent, rainwater drip. I was worrying over this detail with the Holm & Laue dome and its base flange. Rainwater might have run between the GRP and the vulnerable, plywood base ring.
I
have been checking for the availability of galvanized, 90° angle brackets
[with pressed inbuilt stiffening ribs.] These would support the base ring mechanically on the inside of the dome skirt. I would pack up the dome on stands using the laser level on the skirt. This should ensure the base ring is perfectly level too.Though this can [and will] be double checked with the laser level, of course.
Fibreglass matt could be added on top of
the ring. To provide a more solid bond between the base ring and the
hemispherical GRP structure. Thereby adding considerable extra stiffness
to the structure. Much like a bulkhead. This would also help to shed
any interior condensation before it soaks into the plywood.
The Agritech Spheribox employs two, lifting brackets. On top, but widely spaced to lie over the segment seams. These seams must provide increased local strength. So I must reinforce the weakened area of the structure where I cut out the wide, observation slit. Width yet to be decided. I am not a fan of wide slits. The recommendation of 1/3 the diameter of the dome just looks wrong to my eyes. I did some experimenting with this on the plywood dome and ended up with a narrower slit. Which has not proved a problem with my refractors and manual dome rotation.
The wide separation of the lifting brackets offers the opportunity to fit a strong, metal, spacing bar between the lifting points. The danger is that lateral, crushing forces would be trying to close the open slit area, during a lift. This must be safely resisted where the slit denies the central panel its full, design strength. The bridging bar could also become a handy, anchor point for internal hoists. Which I use for exchanging long and heavy OTAs on the mounting in my present dome.
*
No comments:
Post a Comment