This is the flow-over from the Homemade Observatory (cheap) thread that, as Netwolf said, was hijacked and diverted from the original discussion.
To continue with this, I have a wild idea. Instead of trying to make the usual observatory-type roof with a suitable slit, what about combining this pentagon concept with the roll-off concept.
Only, instead of rolling off to one side on a couple of rails, why not have the roof (which is 5-sided) pivot from one of its corners on a solid Galvanised steel pole cemented into the ground. To support the weight as you swing the roof off and on again, you would not necessarily need any sort of structure. If your ground was reasonably level, you could have a simple jockey wheel (like you use on garden trailers etc) at 3 of the remaining 4 corners. If you say that corner 1 of the roof is at the 3 o'clock position and number the rest clockwise from that, then the corners 3,4 and 5 could have a jockey wheel. Corner 2 couldn't but it's weight could be supported, once it has cleared the base by a clip-on wheel.
The roof section would have to have a few rollers in it and have a facility to bolt it down to the base for weather and security purposes.
One idea I came up with a while back which could be incorporated into your idea is to make the roof as light as possible and remove it with a Hills Hoist Clothes line.
A frame could be attached to the Hills Hoist, you wind it up a few inches to clear the walls, swing it out of the way, then lower it back down a few inches onto a support post or some such object to take the weight of it, rather than the Hills Hoist trying to hold all the weight for all the time you have the roof open.
Bottom of this page give some interesting details of the various dimensions with different strut lengths. My criterion would be the wall height. These guys are only across the ditch in NZ.
One idea I came up with a while back which could be incorporated into your idea is to make the roof as light as possible and remove it with a Hills Hoist Clothes line.
A frame could be attached to the Hills Hoist, you wind it up a few inches to clear the walls, swing it out of the way, then lower it back down a few inches onto a support post or some such object to take the weight of it, rather than the Hills Hoist trying to hold all the weight for all the time you have the roof open.
I can see this working.
Perhaps something like one of those oversized cantilevered deck umbrellas - swing it aside or back as needed. A simple winch to raise and lower. It could work - provided you could keep the roof weight well down. Not sure whether colorbond is lighter than ply. Other options include the new clear Alsynite-type materials - with perhaps some shade cloth under it to break the direct sunlight into the obs a bit. Cut the roof's support framework down to say inch box-section steel as well.
Peter
Peter
Pine plywoods are typically 500 - 650 Kgs/cubic metre.
Colorbond steel is between 4.2 and 4.5 Kgs/square metre.
Polycarbonate sheeting is lighter but needs more in the way of a framework to support it.
Thanks for that info. Polycarbonate roofing sheets seem to be between 1.2 and 1.5 Kgs/M2 for domestic use and depending on brand and thickness. So it's roughly a third to half the weight of colorbond or 7mm plywood.
If you use 2.4 metre lengths to build the frame for one of these things, then the roof will require roughly 5 full sheets of 1.2 x 2.4 length or roughly 12.5 M2 weighing between 15 and 18 Kgs (without frame). If you work with inch box section, you can easily afford to put a bit more support into the panels.
I haven't weighed the steel framework yet but it does seem to me that a full roof for the kit could be made to weight little more than 50Kgs or so. That does seem to suggest that the cantilevered lift-on/lift-off system could work.
[QUOTE=telemarker;701636My criterion would be the wall height. [/QUOTE]
Currently I have an ED80 on an EQPro 6 mount and may mount an 8" or 10" dob tube at a later date.
what wall height (not specific to this style of obs) would people recommend?
Obviously, if the surrounding field of view is restricted then it's no point building lower than that but what about taking into account wind?
Would you build the obs walls as high (or just higher than) the mount or somewhere between the mount and the height of the tube?
Currently I have an ED80 on an EQPro 6 mount and may mount an 8" or 10" dob tube at a later date.
what wall height (not specific to this style of obs) would people recommend?
Obviously, if the surrounding field of view is restricted then it's no point building lower than that but what about taking into account wind?
Would you build the obs walls as high (or just higher than) the mount or somewhere between the mount and the height of the tube?
thanks for your thoughts.
An ED80 is a nice relatively short tube that offers little wind resistance and would therefore be quite stable on an EQ6 even if there was a bit of a gust. Also the FOV of an ED80 is sufficiently wide to forgive the occasional 'shake'. So there would not be an overwhelming need for higher walls - this would be different if you were using say a 200mm F5 newtonian or something with a bit more wind resistance.
Since you won't be imaging bel9ow about 20 degrees in the east or west, high walls there won't be a problem. That will be different if you want to shoot north though where sometimes you have to live with a bit of parallax, dust and light pollution in order to get at objects you won't otherwise ever reach. So it's all about deciding what you want to see/image and with what equipment.
I favour high walls for wind protection since I am using big scopes but for you it would not be a priority item.
An ED80 is a nice relatively short tube that offers little wind resistance and would therefore be quite stable on an EQ6 even if there was a bit of a gust. Also the FOV of an ED80 is sufficiently wide to forgive the occasional 'shake'. So there would not be an overwhelming need for higher walls - this would be different if you were using say a 200mm F5 newtonian or something with a bit more wind resistance.
Since you won't be imaging bel9ow about 20 degrees in the east or west, high walls there won't be a problem. That will be different if you want to shoot north though where sometimes you have to live with a bit of parallax, dust and light pollution in order to get at objects you won't otherwise ever reach. So it's all about deciding what you want to see/image and with what equipment.
I favour high walls for wind protection since I am using big scopes but for you it would not be a priority item.
Peter
thanks, Peter. after reading your post I had more of a think and will probably look to build high enough to block wind for an 8" dob. I will always live in suburbia so will always have houses around to block the view...
since I have hijacked this thread I might start a new one. sorry to the OP.
Fahim, those star joiners have been brought up seversl times in a few Astro forums, and the same point keeps being raised.
Zenith.
Most people just say set the pier off centre
As nice and simple as they are, they are really impractical for Astro purposes.
Someone will come up with a plan where they can be used but no-one has yet.
Maybe no-one is trying hard enough
Fahim, those star joiners have been brought up seversl times in a few Astro forums, and the same point keeps being raised.
Zenith.
Most people just say set the pier off centre
As nice and simple as they are, they are really impractical for Astro purposes.
Someone will come up with a plan where they can be used but no-one has yet.
Maybe no-one is trying hard enough
Well, this comes close: purely by chance, I was driving through a nearby town this morning and noticed a small general engineering factory that had a number of sizeable collapsible cantilevered umbrellas set up outside - their own design. They were in various sizes from 4 to 6 metres in diameter.
The mechanism for opening and closing the folded wings of the umbrella was a simple ratchet winch - wind it up and it pulls the umbrella open and vice versa. Here's a simplified drawing. Basically, it's based on what looked like 50mm gal pipe and is designed for cyclone loads and can handle weights well in excess of mine (and I'm not small) hanging off it so it can certainly handle the roof of one of these designs - provided you use polycarbonate sheeting and 25mm box-section steel to frame it.
In short, the umbrella lifts the roof a few inches and the whole thing just swings aside and is either placed on the ground or on some other support structure. It should be very simple to use one of those inexpensive 4WD winches that have a forward/reverse switch to make the whole process pretty painless.
I'm guessing that with a bit of pipe bending and a bit of welding, this could be built for a few hundred dollars.
Make a plug from it, maybe even make a mould and plug for Wall pannels and wall pannels with shelves.
And get some firbeglass places to user these plugs to make fir us the final pieces. Or those who know how to fiberglass themselves could diy.
From what I can gather once you have the Mould its not that hard to fiberglass it into the final piece.
The problem is coming up with the Mould's
1. Roof Section mould.
2. Wall pannel mould.
3. Optional Wall pannel mould with Shelf.
Then reuse these to make the observatory kits for all.
Could not a Astronomy club sponsor this activity and provide a place where people could go and make there own sections from the moulds?
A sligt difernt way could be to make Plugs of a Geodesic Pannel, and then make moulds of these and create the final sections out of this. Making it smaller like this, A person could build it up as budget allows.
These are very good ideas.
I wouldn't have a clue on how to make the fibreglass mould but it seems obvious that once that is done you can make as many as required for minimal cost.
So....How does one make a fibreglass mould?
You make or find the piece you want and then make a mold from it - cover it in wax, polish it and then it is ready to make the product.
I made a wing for a car like that many years ago.
If you made a dome from closed cell foam over a few bits of judiciously curved framework then fibreglassed it, you'd do it all in one - I helped a guy make a duplicate boat like that in Bermuda he sowed the foam onto an old hull he liked the shape of then glassed the foam and cut it off the old hull - turned it over and lined it with more glass and he had a very nice solid hull.
From what I can work out you need to first make a "plug" this is a replica of the final piece. The plug in this example is 1/12 section of the semisphere. Using this you make your mould which is a "negative" of your shape. Using this you can make as many sections as you need. The plug is no longer required. But making the first plug is usually the most costly and dificult part. The plug needs to be strong enough to take the force aplied by the mould while its forming. You could even use someones existing dome as the plug, but it may need to be braced for strength.
I emailed Ian Sharp and he told me that the mould has been used by several amatuers to build replica obserbatory's. Imagine doing the same here.
Fibreglassing is not that difficult to do.
The hardest part is making a strong, reusable mould.
A form of of section of the dome is made from MDF ribs with a plywood covering to get the bend correct. You may have to steam the ply to get the correct curves.
This is then used to make the mould similar to the one in netwolf's link. I'd probably reinforce the mould with timber as the stronger it is, the longer it will last - and it makes it easier to get out the finished panel if you have to use a bit of force!
Making the mould is a matter of preparing the wooden form so the gelcoat/fibreglass doesn't stick ( polish with, carnauba wax and apply a mould release). Gelcoat and several layers of fibreglass mat and resin are applied to the form and that's it. The trick is making sure the mould releases from the form which is where the waxing/mould release comes in.
Making the panels follows the same routine.
Provided the mould is kept clean and in good repair, it would last for years even with regular use.
This is all from memory, it's been a few years since I did any of this,
Last edited by snowyskiesau; 28-03-2011 at 08:58 AM.
snowyskies is quite right.
The better you make the plug the longer it lasts.
I used to work for Haines Hunter - their boat plugs lasted for years and often made a boat a day from them.
Swimming pools are made the same way, but you don't need to go to those lengths for a one off.
What's a good diameter for a dome?
