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Help needed for flexible planetarium dome build

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#1 PaulEK

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Posted 13 September 2019 - 07:04 PM

Hi folks,

 

Cloudynights doesn't have a planetarium forum, so I'm not quite sure where to post this. Bu this is where the builders gather, so it seems like the best fit.

 

I put up a post over at the Observatory Central website, which has several different forums related to planetariums. But it does not get the traffic and views that CN does, and I haven't gotten any responses. It's been up for less than 24 hours, so I may well get some replies, but I figured it wouldn't hurt to post here, too. Rather than r-write everything I wrote there, I'll just paste in what I posted there. If anyone has any ideas, leads, critiques, or whatever, I'd be grateful for them.

 

"I was given a Spitz A4 projector by someone who has posted here. He had tried to get it up in a public dome but couldn't find a way, so offered it anyone who thought they might be able to. I convinced him that I would give it my best, so he gave it to me, with the understanding that I would give it back if things didn't work out for me, either. In my local school district, we recently got a referendum approved to build a new performing arts center (PAC), and our superintendent loves the idea of making a planetarium a part of it, especially since I'll provide the projector.

 

"Here's the issue: we will (almost certainly) not have a dedicated, permanent space for the dome. The hope is to find a way to make a foldable, 24-foot dome that can be raised up into the 'flies' above the stage. I see it as a fabric hemisphere with a steel skeleton made with -- possibly -- electrical conduit. The fabric material does not need to be completely opaque, since the stage can be made completely dark be simply turning off the theater lights. So the weight of the dome can be kept to a minimum, meaning the steel framing does not need to be super heavy-duty. I see the frame being on the outside, made up of about 18 to 24 ribs, and these ribs will slide through sleeves (just once, when we first assemble the thing) on the outside of the dome. When not in use, the dome should open (with a zipper? Velcro?) and the ribs can then be rolled together to flatten the 'tent' of the dome. It would not need to fold up to the smallest size possible; just enough to be flat enough to raise up on a couple of cables attached somehow to one or more of the ribs. We will have motors that will be able to lift several hundred pounds, so I think this would be easily doable.

 

"When the folded dome is lowered from the flies, I see us opening it up and locking the tightness of the dome by lowering braces that fit between the bottoms of the ribs, and then supporting it on its bottom edge with several (4? 6? 8?) posts about four feet tall, which we will install into some kind of holders in the stage floor. Folks will ether sit on the floor, or we will have chairs that can be set up (we have plenty of those on wheels). We'll also need a way to move the projector on some kind of wheeled base from a storage closet on an edge of the stage. I want to find a way to make this as simple and quick to do as possible, so the planetarium can be set up in a reasonable time, for use by students and the public. I live one block from the school, so they'll have someone ready to help.

 

"I've talked this over with some others who think it can be made to work, and I think I have the skills to do much of it, along with help from the shop teachers at the high school. But the one thing I have no skill set for is the making of the dome. What size and shape to make each of the sections of the dome; how best to sew them together; how to make the ribs come together at the top (I'm imagining a small wheel with all the ribs having rings on the ends; like a giant, round shower curtain.) I'm hoping folks here might be able to point me toward sources/ideas, or even to someone who has, or who could, do it. I've been told (but not gotten a guarantee) that the school district could find some funding for the making of it. I'm on the school board, so I might have some influence on that (but again, cannot guarantee it yet). If I had some solid leads, I could present the idea for consideration.

 

"Again, the projector was originally housed in, and is set up to be used in, a 24-foot dome.

 

"I could write a bunch more right now, such as how I own and use a Starlab 16-foot planetarium. And about talking with John Hare from Ash Enterprises about this project, since he services the A3p projectors at two schools (one high school and one university) nearby. But I'll wait to say much more while I wait to see if anyone here can help. The ground-breaking for the new PAC is going to be some time around Thanksgiving this year, with the construction lasting about twelve months. The planetarium project does not to be done as part of the initial building of the PAC, but I think it would help to keep the idea moving forward if it was at least well under way during that time.

I only post here every few years, and always have gotten great advice. Thanks in advance for any ideas!"



#2 PaulEK

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Posted 13 September 2019 - 07:12 PM

Here's a photo that shows what I got. Not my specific projector, but one just like it. I didn't get a desk, but I did get the control panels shown on the desk. When this thing was new in the early 1970s, it cost the price of a modest house. There are still lots of them in use today, including two one at a college an hour and a half from me. They can be updated with LEDs (cooler, lower-power, and much longer-lasting) instead of halogen lights.

 

Most planetariums that could afford it have updated to digital projectors. While digital has some definite advantages (time-travel, better video possibilities, space travel), I think the stars shown by an analog machine look more realistic, and the background sky is black.


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#3 Myk Rian

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Posted 14 September 2019 - 07:17 AM

I'm trying to imagine a removable planetarium on a stage, and have come up with a blank. Good luck.

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#4 Ed Jones

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Posted 14 September 2019 - 08:36 AM

You need something like this



#5 PaulEK

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Posted 14 September 2019 - 11:06 AM

Thanks for the tip, but I already have something like that, though much older. (That's my actual dome, on a page at my website.) But there are two issues with both of them. First, how to get the projector inside. I suppose they could be used by attaching sleeves and metal rods, as in my idea (though that would be very difficult as they are made out of special opaque material). That would allow them to be lowered over the projector (they do not have complete floors underneath). The projector is too big to fit through the portals meant for entry.

 

But the other issue is that they are too small. They are both about 16 feet across, and the A4 is not designed to use on that small a diameter.

 

Sorry for not mentioning that I'd already considered this!

 

I've looked online for plans or calculators that show how to design the panels needed for a fabric dome but have had no luck. If no one else knows of someone who might be able to make this for us, we might consider making it ourselves if we had the pattern.



#6 Ed Jones

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Posted 14 September 2019 - 11:42 AM

They list a 7 meter dome.



#7 PaulEK

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Posted 14 September 2019 - 12:53 PM

Thanks, Ed! I'll look into that, but still see an issue with attaching it to some kind of framework so that it could be lowered onto the projector. And not only the projector, but the station the planetarian will work from. I will check more carefully into what the dome is made from; maybe they can send me a sample.

 

But I'd sure like to hear other ideas, too.



#8 PaulEK

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Posted 14 September 2019 - 01:07 PM

Looking again at the site Ed Jones pointed me to, I see that the manufacturer states that they'll make custom orders. I'm going to write to them with my ideas and see what they say.

 

It was a bit odd to read that they have 'two years' of experience, and they put that like it is supposed to be a claim to instill confidence. Still, if they can do what I need, then that's fine. I ordered a custom-made (sort of; it was put together from components) lifting column from a small Chinese company a year or so ago, and after lots of cross-language clarifications (no one's fault, just the reality), I got a very satisfactory product for a very reasonable price.

 

But this will be taking a risk with an unknown producer halfway around the world. I'll have to convince others to go along with spending funds. So other ideas would still be really helpful, if only to have some other possibilities to compare this one to. I've done lots of searching myself, but drawn a blank. I had not found Ed's link, though, so either it's fairly recent (it's been a while since I looked), or I just missed some search parameters.



#9 jtsenghas

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Posted 15 September 2019 - 09:33 PM

I love the concept and can see why something without a floor is really required in your (likely) unique situation. I also see that your setup is a bit large as compared to most portable setups.

 

I can see how structurally a collapsing ribbed structure poses some problems in going relatively flat, and also that the projector likely needs a fairly elevated horizon.

 

I'm wondering if perhaps the external semicircular ribs should be arranged more in a clamshell arrangement, attached with pivots to a small hub in each side. (think of the longitude lines of a hemisphere of a globe of the world turned sideways).

 

I'm imagining rolling out sections of wall first, including one that has a door in it and latching them together. Then, the collapsed dome is lowered onto it and fastened to one half of the cylindrical shell of the wall. Finally, the structure is opened like clamshell Observatory closing. As soon as the first rib is raised significantly, possibly with a pusher pole, a rope pulling from the opposite side could pull the rest up and over. This would be a bit like extending the hood over an old fashioned baby buggy.

 

Structurally, this could be very simple. 



#10 PaulEK

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Posted 16 September 2019 - 12:03 AM

jtsenghas: I think I can visualize what you are describing, yet I may be missing something. But I don't think we will need walls. The theater will have the capability to go to complete darkness (other than some distant, dim, exit signs). So as long as the bottom edge of the dome can be made stiff, than it can be supported by several vertical posts.

 

I've been away since I last posted here, and I'll think about what you've written.



#11 jtsenghas

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Posted 16 September 2019 - 05:23 AM

Okay, several discrete posts work well too, and promote air circulation among the viewers. It's just that the hub regions will have to be eirher anchored or set on the floor while the structure is opened. I'm thinking of a few ways to handle the pivots. I'll try to make a sketch later today. My last post was a late night musing and I'm off to work now.

 

Edit - Actually I can think of a way that more elegantly manages this for deployment and storage. Both sides can fold up to the center, towards a meridian that passes through zenith. This means that for north and south hubs, lifting lines attached to a few east and west horizon hoops would automatically collapse this upon rising and would lower it without side loads opening it automatically. You would just need a person at each the north and south end  to help place it as it is slowly lowered.

 

Yes! I can see how to manage the potential spinning too, going up or down. Stay tuned. 


Edited by jtsenghas, 16 September 2019 - 05:41 AM.


#12 PaulEK

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Posted 16 September 2019 - 08:28 PM

Oh, you've got me staying tuned now! Thanks! grin.gif



#13 jtsenghas

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Posted 16 September 2019 - 08:47 PM

I am sorry not to have more for you yet. I just had an extraordinarily long day at work today. It was a Murphy's Law Monday at the factory. 

 

I'll try to make a couple of quick sketches tomorrow of the framework I propose. The most difficult part of the project would be the fabric work, I concede that. I don't see how most possible designs would be much simpler regarding the fabric work than my idea, however. You have several advantages in your situation as compared to most collapsible planetariums. The fabric can be lightweight since it may be translucent, and having a large overhead space to hoist it to means semicircular hoops may remain intact. 



#14 PaulEK

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Posted 17 September 2019 - 12:29 AM

I've seen the kind of work you've done, so I'm really looking forward to what you have in mind. But I'm not in a big hurry. I know what long days are like. Thanks for the thought and the help!


Edited by PaulEK, 17 September 2019 - 12:29 AM.


#15 PaulEK

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Posted 17 September 2019 - 08:25 PM

Well, I just noticed my misspelling of 'needed' in the title of this post. As an English major, I'm a bit chagrined.

 

jtsenghas: I realized earlier today that, if I understand your idea correctly (which is only somewhat likely), your flattened dome will be a semi-circle, whereas the idea I described is a quarter of a circle. Won't this mean that, for a dome 24 feet in diameter, your flattened one will be twelve feet from top to bottom, while mine, tilted sideways, would be significantly less? If the little sketch I just made is right, mine would be about 17 feet long and about four feet 'high' when folded up and tilted sideways.

 

Our theater is being built without 'full flies', meaning there will be a limit to how much height we will have to store things up there. I don't know offhand what that height is, but I can find out.



#16 jtsenghas

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Posted 23 September 2019 - 06:07 PM

J. T. 's Design Musings

 

Okay, I had to work even yesterday and personal time has been hard for me to get this week. My wife had shoulder surgery last Wednesday and playing caregiver has been crimping my time too. Now I'm sitting in a waiting room for her physical therapy appointment and have a chance to sketch a little. These offerings aren't my best work, I'm drawing on borrowed photocopier paper and using a credit card as a straight edge.

 

The basic concept I'm thinking of is an almost hemisphere that unfolds 90 degrees on either side from zenith. The hoops and fabric could in fact be mounted within stiffer lightweight flat arcs, perhaps made hollow core by gluing thin plywood to polystyrene insulation. The arcs that fold down to the horizon could perhaps have a 8-10" difference between inside and outside radii. An overhead arc of the same material could be made wider in the middle for increased stiffness, but would have to have a matching inside radius that also closely matches the hoop radii. That overhead arc would look a bit like a basket handle:

20190923_152650.jpg

 

That semicircular piece at each end would be painted white on the interior side and would serve as a hub for all the hoops. The overhead arc would be rigidly attached to that and a horizon section a couple feet long with a few gussets to maintain rigidity. The side horizon arcs would be shy of 180 degrees by the length of those sections attached to both the semicircular hubs and the overhead arc. They would be attached with hinges. A rough draft of the hub region can be seen in the next post. 


Edited by jtsenghas, 23 September 2019 - 07:17 PM.


#17 jtsenghas

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Posted 23 September 2019 - 06:10 PM

Hub detail:

 

20190923_152809_compress96.jpg

 

Description to follow in my next post. 

 


Edited by jtsenghas, 23 September 2019 - 08:06 PM.


#18 jtsenghas

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Posted 23 September 2019 - 06:25 PM

In this concept semicircular bent tubes, possibly conduit as Paul suggested, would have a pivot at each end. These could be screw eyes embedded in tube inserts or, if the tube walls were robust enough, bolts through clearance holes. The key here is that the hub is made large enough that all have sufficient clearance (including for the fabric sleeves each goes through) to rotate vertically. Note that when they are in the down position, as shown to the right, above, that they are all in line with a single point at the horizon.

 

Just as a Japanese paper fan can't have the paper run all the way to the pivot, neither can this dome have fabric all the way to horizon or too much bunching will occur. Note, however, that the fabric can run almost to the ends of each piece of tubing. Thus, the holes will be obscured from within by a semicircular disk of the same color as the fabric - flat white. The fabric can't be attached to the horizon rings since their pivots at the hinges are a bit offset, but the bottom most tube could be fastened down with ties, snaps, velcro, or even magnets when deployed.


Edited by jtsenghas, 23 September 2019 - 06:26 PM.


#19 jtsenghas

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Posted 23 September 2019 - 06:41 PM

There are a number of ways such a structure could be stored and deployed:

 

20190923_152858_compress38.jpg

 

In this concept the structure can be rolled out on casters. Wings that lock outwards using folding table hardware could add stability for when it is used or moved. A few flat topped pedestals could provide additional support in use, perhaps two or three per side simply slid under the horizon ring.

 

I had thought about Paul's first idea to have it lowered from above and think that would work well for a 16' dome, but for one of this scale a LOT of overhead clearance would be required to get it to any appreciable height.

 


Edited by jtsenghas, 23 September 2019 - 06:42 PM.


#20 jtsenghas

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Posted 23 September 2019 - 06:50 PM

Note that the above concept would merely need a curtain or screen at the rear of the stage to hide this in a cavity just a couple feet deep. If it is secured to a wall for safety against being tipped over, I see no reason those caster wings couldn't be folded. They could be made to fold inwards if space is a consideration, but outwards might be more friendly for use.  Safe deployment may involve folding two caster wings out and locking them, unhooking the structure from the wall, rolling it out just enough to lock outward the wings closer to the wall, and then pushing it to the projector in the center of the stage. 

 

Storage would be the reverse. It would be pushed up to the wall until two of the wings nearly touch it. Those wings would be unlatched and folded and the structure would be pushed against the wall. It could then be hooked to the wall at both ends and the remaining wings folded. Finally, a curtain or screen could be closed or lowered to hide it. 

 

This idea also solves the problem of getting the projector INTO the structure.  It can be rolled out and placed like an arch over the projector and then opened with the horizon lower than the top of the projector unit. 

 

Heck, the undersides of the horizon rings could even be painted with stripes of red, orange, yellow, green, blue, indigo and violet! 


Edited by jtsenghas, 23 September 2019 - 08:03 PM.


#21 jtsenghas

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Posted 23 September 2019 - 07:00 PM

The fabric work, as with any similar solution, is the most difficult portion of the project, in my estimation. I could do some CAD work to generate a pattern. I imagine at largest panel sizes 15 degree segments would be used, but I'm inclined to go a little narrower despite odd angles being used for better sphericity and less sag, particularly over time.  Such are minor details. These panels would tend to stretch straight between hoops, so would actually be slightly narrower than corresponding spherical sections. I could work that out. 

 

A scale model from coat hangers or bamboo strips and tissue paper may be in order. 

 

What do you think of the general concept, though? 


Edited by jtsenghas, 23 September 2019 - 08:04 PM.


#22 PaulEK

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Posted 23 September 2019 - 11:18 PM

I think it's great! I've only just seen it, so will need to let the concept percolate in my slower-moving-with-age brain, but I like it quite a bit. Am I right in thinking that each 'gore' (the sections of cloth) would be the same size and shape?

 

I have others to convince, so who can tell what their thoughts will be, but this would solve several problems, including needing to involve specialized people (i.e: people with permission/clearance) to lower the dome from the flies. We are planning a large scene shop/storage area just behind the stage, and the thin shape of this dome could be kept there. Some might be resistant to that, but I think it's better than taking up room in the flies.

 

I can see making the wings a few feet tall, and then securing the bottom edge of the dome to a few removable posts (say, four or six) that can be secured to the stage floor (or maybe they won't even need to be?). The audience could then just stoop down and walk under the edge. 

 

I wonder if there's a way to make this lightweight enough to use as poles some sort of springy material (graphite?) like what fishing and tent poles are often made of. 24 feet mighyt be too much for that, but if it were possible, it seems to me that it would make the dome more smoothly spherical.



#23 PaulEK

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Posted 23 September 2019 - 11:21 PM

I'll share this idea with some other 'stakeholders' to get their thoughts.

 

Another aspect I like about this idea is that it seems much more like something we could make ourselves. as a DIYer, I always think there's more ownership and dedication to an item when folks who will use it are the same folks who actually made it.


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#24 jtsenghas

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Posted 24 September 2019 - 12:42 PM

Tuesday lunch break thoughts:

 

The ribs will have to be about 37 feet long to reach almost a semicircle on a 24 foot dome. They may have to be stiffer than you expect to maintain the desired shape. Stiffness of a beam varies with the cube of the length. Double the length and it will deflect eight times as much under the same load. PVC conduit might do,  PVC pipe might be better despite weighing almost twice as much.  My hunch is that either would be available in 20 foot lengths at the most and that two or three sections would have to be joined for each rib. If an aluminum tube just a few inches long were riveted inside that PVC at each joint with several small pop rivets I expect the result would be smooth arcs that fit well into "curtain rod" sleeves. 

 

A tight fit would be better than a loose one, even if the PVC had to be warmed to fit around the aluminum with a heat gun. 

 

Yes, each "gore" would be the same pattern. In fact, each half gore would match, so a pattern about 18 feet long would be needed. The actual width and angle of each section may depend as much on available material dimensions. 18 foot sections with one central seam that spans a curve from east to west through zenith might be best. This way pieces can be alternately cut for direction to minimize waste. 

 

None of the seams would be more than a few inches from edges of fabric despite the size. Though a huge assembly would have to be moved along, a standard sewing machine would suffice. 

 

Ideally the fabric would be just a little stretchy both east/west and north/south. Perhaps springs salvaged from a trampoline could be hooked into grommets near the hub to maintain tension along each rib. That area would be hidden by the semicircular disk. 

 

Posts to support the horizon could be stanchions such as are used with chains or straps in queueing lines at banks and airports. They could be homemade if disks of adequate mass are used at the base.  Hold down clamps could be fixed atop these to secure the Planetarium down evenly.. 

 

...Lots of DIY possibilities... 


Edited by jtsenghas, 24 September 2019 - 12:51 PM.


#25 jtsenghas

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Posted 24 September 2019 - 12:48 PM

My hunch is that a check of fabric dimensions may need to be done during assembly. If only half the pieces were first cut, sewn, fitted with poles  and attached to the wooden frame and checked for opening angle, a slight pattern adjustment could be made to ensure the total assembly is not more or less than a hemisphere. Slight variation of gore angles should be unnoticeable. 




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