What is the "PortaBowl" telescope? It's a simple to build bowl-based telescope designed by Jay Scheuerle. You can download the complete plans here:
http://home.comcast.net/~jayscheuerle/PortaBowl.pdfJay's goal was to design a simple and cheap telescope that was nevertheless useful. I first came across it in the Sept-Oct 2009 issue of Astronomy Technology Today magazine. (Unfortunately they do not sell individual back issues.)
Today there are more resources, in particular there is a Yahoo Discussion Group:
PortaBowl_TelescopeAmateur Astronomy magazine is also planning to publish an article I wrote about the PortaBowl in issue #69 (back issues are $5 each). But in truth, the discussion group and Jay's PDF should provide more than enough information to succeed in building this telescope.
Why? I picked the PortaBowl because I was looking for a telescope to build that didn't look too challenging as a first project, and yet would still be useful. With the low prices on many mail-order telescopes, it's easy to spend more money on a building project than the result is worth. The beauty of the PortaBowl is that both mirrors can be collimated and it includes a red-dot finder. Only the two mirrors are specialty mail order items, the other parts are all purchased locally and at Ikea.
Shopping is the first step in any building process, and for the PortaBowl this is mostly easy... mostly. First you must make sure you have access to an Ikea store. The bowl you want is an 11" Blanda Blank but they do not sell them by mail order!. I was unable to find a suitable alternative. The spacing is critical and the 4" primary mirror must sit inside the bowl. I wound up buying a bowl from the designer! I believe he has offered to do the same for other people.
After you have the Ikea bowl, the rest is pretty easy. A 10 lb. barbell weight from Wal-Mart, plastic plumbing pipes, a few dowels, some screws, etc. You should be able to stay well within the target $90 price estimate, including the $40 mirror pair.
The bowl forms the basis for the telescope. The photo below shows the components, as purchased. (Notice that this bowl is a non-Ikea bowl that did not work out.). The small dowel will be cut in half to form two supports for the large dowel. The PVC fitting will be trimmed to form a wedge securing the large dowel in the center of the weight.
The location of everything is determined by placement of a screw to secure the end of the big dowel, 3" up from the center of the bottom of the bowl. I suggest you assemble all this first, then unscrew the small dowels and slide the glue-rimmed weight down into it's proper place. You have time to re-attach the small dowels and make sure all is well before the glue sets. The plans call for epoxy, but I used construction adhesive because it takes longer to dry and seems more forgiving.
The Primary mirror and a matching secondary are from Meridian Telescope. The mirror is a 4.5" ƒ8 with a focal length of 36". Part No. PM-12 costs $33 and includes both mirrors. My secondary came with the little angled mounting piece already attached. The primary cell itself is a clever design consisting of a triangle of thin masonite above a triangle of plywood or pine. The top piece, with the mirror attached, flexes to permit collimation. Most of the parts are shown below.
I laboriously followed the pattern provided in the plans in cutting the wood pieces, but correspondence with Jay indicates that all those holes other than those in the corners are probably not needed. They might provide better cooling, but they were a lot of work. Not shown clearly is the little triangle of 1" pine used as a spacer (under the clamps). A 7/8" hole will be drilled through the spacer to mount the primary cell to the pole. The last photo shows the completed cell just prior to peeling the double-sided tape backing and installing the mirror. (That's not an Ikea bowl in the background and this is where I realized my original bowl wouldn't work!)
The Secondary mirror assembly includes the focusing mechanism. Here again common hardware store purchases come together to make a very nice telescope. Below you see two pieces of a sink drain, a ruler from Staples, a piece of wood, and a Daisy "Red Dot" scope from K-Mart ($10).
There are two tricky parts here. First, you'll need a big drill bit to make a hole in the wood for the tailpiece. Second, bending the ruler and drilling holes to support the secondary can be hard to do. The plans provide two possible techniques, I wound up devising a third! Jay's prototype has a one-piece secondary holder bend, cut and drilled to provide a tab to mount the secondary. An addendum to the plans suggests not cutting the tab, but adding a small block of wood attached at right angles. Well, I tried to bend things and made a mess of it, but when I had picked up a new ruler I realized I could salvage part of my earlier work by riveting it where the tab had been before.
Making the focuser out of plumbing parts is easy, once you've identified which pieces to use. In the picture below the pipes are show cut into pieces. Only the two center pieces are used, the ones marked X may be discarded. Double-sided foam tape, with the backing left on one side provides the proper friction for the focuser tube. The two flathead screws form a mount for the finder scope.
Final assembly is easy once the individual components are completed. The large dowel provides support for both the primary and secondary cells. Selection of the proper bucket or whatever to sit the bowl in is up to you. I'm still playing with mine to find the right height. Jay's prototype used a coffee can with the plastic lid used as a bearing.