Building an 8" Portaball wannabe with Ikea Blanda Bowls

[geyes30]

I built a Portaball-like scope over six weeks, starting from mid September this year. I detailed the build on cloudy nights, including many missteps along the way. I am going to repost some of the posts here for brevity. Hope this will be of interest to some of you.

I'll start near the end. This is what the scope looks like:

Portabowl on tripod by Cyrus Beh, on Flickr

The finish is pretty awful, mostly because I got impatient with the epoxy coating at the end. It's simple enough to sand down and re-coat, but since the scope is performing well now, and the blemish is mostly cosmetic (the harsh lighting makes it look worse than it actually is), I am reluctant to tear it down and sanding it. So here it is.

For those who aren't familiar, Portaballs are pretty expensive scopes, and the most distinct feature has to be the spherical base. This allows the scope to be pointed in any direction easily, and the eyepiece placement is unlimited by the axes of a traditional Dob. It does not suffer from the so-called "Dob Hole" problem, since movement in any direction is reasonably smooth. The motivation for this project was to make a fast, 8" scope that remains reasonably portable. The hardest part of the build is getting the sphere. I chanced upon the Ikea Blanda Bamboo Bowls (I bought the steel one previously to see if I could polish it into a mirror. I couldn't!), and though they would make great spheres. The largest size had a diameter of 27cm, which is actually a bit small for an 8" Portaball. Nevertheless, it was the largest available, so I went with it. The scope is dubbed Portabowl in honor of the Ikea bowls.

[Airconvent]

Nice effort and it looks good! Next upgrade would be to grind your own Zambuto mirror?

[geyes30]

You're not far off the mark! Before I bought this parabolic mirror, I bought an 8" spherical one by mistake. I am in the process of ordering some stuff to re-figure the mirror But that's a project for another time. My current project is to overhaul a broken CGEM, changing its servos into steppers.

[Airconvent]

You're not far off the mark! Before I bought this parabolic mirror, I bought an 8" spherical one by mistake. I am in the process of ordering some stuff to re-figure the mirror But that's a project for another time. My current project is to overhaul a broken CGEM, changing its servos into steppers.

Thanks for sharing
Do post and share your repair "adventure" here !

[geyes30]

Guess it's time to describe the design. Firstly, I'm using two Blanda Matt bowls, with part of it cut off from the top bowl. The two parts are epoxied together, and perhaps reinforced with some metal plates (undecided). The top bowl is interfaced to a stainless cake ring (9") by a combination of interference fit, epoxy, and 3 retaining screws. The trusses will be attached to this ring, which is stiffer and stronger than the bamboo that make up the bowl (I worried that repeated mounting and unmounting will compromise the structural integrity of the attachment points).

Portaball by Cyrus Beh, on Flickr

To make the top half, I had intended to cut the bowl horizontally (red line below). However, the hole was too small to fit the cake ring through. Out of laziness, I used a jigsaw to cut some more material out (blue lines). This also gave a slightly larger area onto which to epoxy the cake ring.

IKE160012-BACK by Cyrus Beh, on Flickr

The two bowls joined together, with the cake ring, looks like this.

Two bowls are joined together by Cyrus Beh, on Flickr

The ring was sanded, primed, and spray-painted matte black. Because of the difficulty of repositioning the ring, I didn't pull it back out and paint the different parts first, before reassembling. I figured that the process of hammering the ring back into place will probably not be good for the finish, so I just masked off the parts I wanted to keep unpainted. You can see the very poor quality finish of the top edge of the top bowl, where the jigsaw did its thing. That is partly due to cracking when the cake ring was hammered in.

[geyes30]

After making the necessary cuts on the top bowl, I started work on the bottom bit. The unique part of the scope is that it is quite well-balanced and bottom heavy, such that the entire assemble past mid point of the ball (i.e. the top bowl, trusses, secondary, focuser, etc) will be balanced by the bottom. Usually, one uses a larger sphere to attain the necessary moment arm. However, since I am limited by the smallish bowl, I've had to improvise. I tested my Blanda for balance, and as expected, it's extremely difficult to get it down. I wanted to set the mirror as low as possible, but with an 11 inch bowl, 1 inch thickness, and the size of the mirror cell, I was very limited depth-wise.

What I've tried to do to help the balance along is to load the bottom of the bowl with gravel, held together with epoxy.

Gravel in Bottom Half by Cyrus Beh, on Flickr

Here's the amount of gravel I am able to get under the mirror. It came up to just under 4 pounds, I believe. I used some casting epoxy, mixed in small batches, to fill up the air gaps. After about 200mL of the epoxy, the resin started to pool, indicating that it was largely filled up. The epoxy heats up as the reaction proceeds. I should point out that although the epoxy works best when hot, it's not recommended to work with too much volume, since I've heard tales of it getting so hot as to smoke. I tilted the bowl at about 15 degrees, since the off-center focuser will cause a moment (as in rotation) about the optical axis. This was mostly guess work, limited by amount of space I have. An overnight cure (under the shower head, in case the bowl should catch fire!) was sufficient. The bowl did get pretty hot!

Epoxy-encased gravel by Cyrus Beh, on Flickr

This is after curing. You can see that some of the epoxy seeped out on the front end of the tilt, but overall the gravel was held in nicely. I don't have a scale that is suitable for the bowl, epoxy, and gravel, but it didn't seem like it was hefty enough.

Flat black painted interior by Cyrus Beh, on Flickr

This is after painting the interior of the scope. When I got to this point, I noticed that despite the surface prep, the black paint had a tendency to flake when scratched. That's not good!

Anyway, for the collimation brackets, I used 3 Makerbeam 90 degree brackets. These were too long (two drilled holes on each arm) so I had to cut it down. I mounted the two-holed side onto the bowl, with the screw head outside, and recessed. I bent the bracket to slightly less than 90 degrees, so that when mounted on it will be horizontal. This prevents the collimation screws from catching the bracket.

Mounting screws were recessed by Cyrus Beh, on Flickr

The top and bottom halves are the fused together using Araldite two-part epoxy. Because of the slight filleting on the bowls, there was a groove that formed. This was filled with epoxy putty.

Two bowls are joined together by Cyrus Beh, on Flickr

[geyes30]

Since he mirror cell has been mounted, so I cannot take any pictures now. Here's a schematic that's probably a bit clearer anyway.

Mirror Cell by Cyrus Beh, on Flickr

The mirror cell is made of three Makerbeams, held together with 60 degree brackets. The Makerbeams are basically mini T-slot aluminum beams, that are available in black anodized or clear. I used the black anodized version. The developers of the system also created a range of components, which have been useful for prototyping in general. I used silicone to mount the mirror (sorry! I am limited space-wise when loading the mirror cell, so need a firmly mounted mirror!) onto three round plastic pieces, which are attached to the top of the t-slot using an M3 screw and threaded t-slot adapter. The plastic pieces are those that come with servo motors, and which I never use. Looks like this (https://cdn.sparkfun.com/assets/learn_t ... g?size=100).

Three long M3 screws are slotted in, indicated by the blue arrows. These pass through stainless steel springs and the brackets on the bottom bowl. Adjustment is by means of a nut. A 12V cooling fan blows on the underside of the mirror, and is attached to the cell using foam double-sided tape.

[Airconvent]

Looking good!

[geyes30]

Next, I'll describe the UTA. This is a hexagon constructed from Makerbeams and 60 degree brackets (they don't have 120 degree brackets, so I just let the rod slide out the other way).

Upper tube hexagon by Cyrus Beh, on Flickr

As you can see, there is a fair bit of hardware here. The brackets are made of steel, so that does add to the weight a fair bit. A future iteration may see these replaced by machined aluminum. The secondary spider is just 3 more beams. These are super thick, so they are really not ideal. I am working on some curved vanes.

The secondary mirror that came with the kit is quite large, and I have yet to start on the holder, which will be made of a combination of 45 degree sloped balsa block, a small wooden yo-yo, screws, and the curved vanes I mentioned earlier. So I took my old Celestron Tabletop 76, which was in extremely bad condition, and pulled out the secondary. I epoxied around the edges of the original secondary, then siliconed the new mirror onto the old one. I made sure to silicone onto the glass edges of the old mirror, in case the coating fails!

Secondary by Cyrus Beh, on Flickr

This allows me to use the (admittedly poor) collimation mechanism from the old little TT-76.

Lastly, there is no focuser at this point, since all the focusers I have are way too heavy. So, I drilled a 50mm hole in a particle board, and put a 2-inch eyepiece adapter through the hole. On the other side is the 2 inch connector from one of my spare refractor focusers. The board is mounted using screws onto 3 additional short beam sections, resulting in a fairly rigid and stable frame.

Eyepiece holder by Cyrus Beh, on Flickr

And that's that! The top and bottom bits are assembled together using 60cm long beams, which can slide along the rails to provide small degrees of tilt.

Assembled Bowlscope by Cyrus Beh, on Flickr

The Bowlscope, sitting in a pet feeding bowl. How fitting!

[Airconvent]

Great!...Looks relatively top heavy though...?