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> The permanent plaster dome, A hard dome
Ray Worthy
post Feb 24 2010, 08:27 PM
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Chicken wire and plaster. I personally specialised in inflatable domes, but this method is different. If you are lucky enough to have a place which you can dedicate to the construction of a dome, a place where you will not have to pull the dome down when something else comes along demanding the space, then this is for you. Not only that , but I am pretty sure that this could be the dome which costs least of all. Interested ? I have to assume that you are reasonably handy with plans and tools. If you were not then you would not be reading this. First , survey the space and decide upon the radius of the dome. Do not be over ambitious. You have to consider the power of your chosen projector. If there is any doubt about this, then it would be wise to construct a flat area of the plaster surface and fix it to a portable frame. Find a light tight room somewhere and try out the projector and screen to see the quality of the image at various distances. Do not leave this to guesswork and chance. Once the plaster is set then it is too late to change. You need to be handy with saw and hammer etc. , but that goes without saying. There are those among us who would want to use the finest timbers with beautiful finish, but I counsel you to consider that most of the wood will be hidden, covered with plaster or black curtain, so the cheapest soft wood might be chosen. If I start at the wrong end as it were and deal with the essential part , then the method will become clearer. Exactly in the centre of the dome , the geometrical centre of your screen , you must erect a post. Though it will be only temporary, this must be sturdy and fixed in position. Measure this post so that its top wilt be accurately located at the centre of the intended hemispherical screen. Take a piece of plywood about a metre long , and with a piece of string , mark out the intended curve. Drill holes in the wooden curve and attach two lengths of non stretching cord, one at each end. Using your ingenuity, ( you can get a bag of it at the hardware store ) prepare some sort of swivel at the top of the centre post. The cords must be adjusted so that no matter in which direction the board is taken, the arc is always the required radius from the centre. It may seem strange to say this , and it will be an anathema to all careful purists , but the construction of this robust pillar with its swinging arc, will give you all the precision that you will need. The plywood curve will act as the scraper of the final plaster surface , so prepare it well. What you will have to do is to provide the supporting dome. It may even be made of chicken wire netting arranged in curved arches. Whatever you construct will be there only to support the final plaster dome surface. If you are stuck with ideas for producing the support arcs of the dome. take a look at the catalogue of a sail-maker. Some sails require battens to hold their shape. Sail-makers use fibre glass battens for this purpose and sell it in lengths which coil up into rolls with a diameter of about one and a half metres. Take care with the glass slivers which might enter your skin. If the battens sag under their own weight, they can be suspended for a while from above until the dome is set and hardened. This is just a pointer to what is possible. The fun is solving the problems yourself. Ray Worthy 21st. February 2010

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albert
post Feb 25 2010, 06:10 AM
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I also think along similar lines, but I would like to prefabricate the dome with pre bent plaster boards. That is lightweight, can be taken down at any time and gives a first rate projection surface once the panels are sanded and painted white.
Struts and metallic tiers can be made of metal profiles specially intended for lightweight interior walls.
The plaster boards can be bent over a jig that has the necessary curves . On larger diameter domes- and smaller segments-I don't think that the boards will have to be pre-bent at all if the superstructure is solid enough.

I will try to find images...Yves Lhoumeau has sent me a link to a very good video

http://www.knauf-batiment.fr/engine/videoP...&height=315


This shows the basic process and the KNAUF and LAFARGE plaster companies based here in Europe offer "kits" or any other kind of help if one wants to undertake such a construction. Perhaps something similar exists in the U.S.I'll definitely try and get my own dome done this way.
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mrgare5050
post Oct 10 2012, 07:15 PM
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Wonder whatever happened to these permanent domes. After working on ultra lightweights, Im feeling the urge for something solid. real solid ........
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moonmagic
post Oct 11 2012, 10:54 AM
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Since I got interested in this "science" back in the 60's, I've seen a number of different types of domes. Of course, if you manage to acquire a used old professional dome, then 95% of your work is done. The problem is they rarely become available. After all even when the projection technology changes at an existing venue, the dome remains and just gets repainted to fit the need of the newer projection device. But once in a while some installation moves up to a bigger dome, leaving their old one available. Gertting hold of a dome is SO MUCH rarer than finding used machines! One also has to be willing to make the trip and go the expense to acquire the old dome and then remove it. As with ALL things then, there is a cost.

For the vast majority of us in this just for the sake of a hobby, we usual make our own. We have to be willing to accept the imperfections that come with such an endeavor.

For years I have been contemplating creating a framework out of various sizes of PVC pipe. Several people on this form have mentioned creating gores of fabric that would tie together from behind (perhaps with velcro). For some reason this just seems like something I would like to experiment with SOME DAY. Of course a lot depends on how big you want or need a dome to be. I would think for most home use that 12-15 feet diameter would be good.

As long as what ever your dome material is made of can be made to be as taught as possible, I would think it would work fine. The ONLY time that imperfections will ever show up is if you MOVE your starfield during a show. However if the motion is done SLOWLY as it should be anyway, the audience will likely never notice!

For several years I worked at a small Museum that had a 12' diameter dome. That dome had its base ring, top ring and all supporting vertical struts made of wood! The base ring was 1"x4" and the struts were made of 1"x2" pieces. The top ring was also wood. The dome material was a large section of canvas (like tents used to be made of). The canvas was stappled to the FRONT of the struts and was then painted white with latex paint. It was suspended from the ceiling using aircraft type cable (but chains would have worked just as well). OF COURSE it did not provide a perfectly smooth surface, but was NOT noticable for projection UNLESS as mentioned before that you moved the star field rapidly, in which case one can discuss "variable stars" or "grazing occultations." I'll bet several hundred thousand school children saw their first planetarium show under THAT dome and they were as awe-struck in that small domed environment as they would have been at ANY other planetarium anywhere! Even more important is ones ability to dark adapt the room which contains the dome and yet have reasonable safety maintained.

I have also always thought about using LARGE sheets of foam core board to create a more lightweight dome. SO MANY projects running around in my head...SO LITTLE TIME A N D SO LITTLE MONEY! mm


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mrgare5050
post Oct 11 2012, 11:17 AM
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Well said MM! I was impressed with the Moorehead because the dome was 'imbedded' in the building, you couldnt see it from the outside, which of course is my home method, lacking the ability or home depot materials to build a weathertight outer curved structure (thats why I'm so anxious to see how Ron attaches his roofing). Domes really can be anything as you point out MM.

I feel like something permanently solid somewhere. The word plaster is just a good word - a few of my house rooms still have plaster walls .. they just feel different - you can feel the air differently under and within plaster. somehow. it alters the space.
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moonmagic
post Oct 12 2012, 10:08 AM
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Many years ago the Daytona Beach FL planetarium had a plaster 40' dome! What an echo chamber, even with carpeting on the floor AND walls!. I have experience with both a fiberglass 24' and 30' dome before and although they too had echo issues, not as much as that bigger plaster dome. (Physics, busy at work!) I can only guess that they found it cheaper than buying a 40' perforated aluminum dome at the time.
Don't know if they still have it or not, have not had the reason to be there in many, many, years. (They also started with a Viewlex machine and eventually went with a Minolta MS-10. Don't know what they have there today, likely a digital unit like everybody else I guess. mm
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