Building an 8" Portaball wannabe with Ikea Blanda Bowls

Wanna make a scope? Or better still, grind a mirror yourself. Or, you have some good tips in making a really useful accessory? This is the place to show what your hands can do...
geyes30
 
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Building an 8" Portaball wannabe with Ikea Blanda Bowls

Postby geyes30 » Sat Nov 25, 2017 8:41 am

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:

ImagePortabowl 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. :D

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. :D

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Re: Building an 8" Portaball wannabe with Ikea Blanda Bowls

Postby Airconvent » Sat Nov 25, 2017 11:10 pm

Nice effort and it looks good! Next upgrade would be to grind your own Zambuto mirror? :)
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geyes30
 
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Re: Building an 8" Portaball wannabe with Ikea Blanda Bowls

Postby geyes30 » Sun Nov 26, 2017 8:22 am

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 :D But that's a project for another time. My current project is to overhaul a broken CGEM, changing its servos into steppers.

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Re: Building an 8" Portaball wannabe with Ikea Blanda Bowls

Postby Airconvent » Sun Nov 26, 2017 8:15 pm

geyes30 wrote: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 :D 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 ! :)
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geyes30
 
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Re: Building an 8" Portaball wannabe with Ikea Blanda Bowls

Postby geyes30 » Sun Nov 26, 2017 8:39 pm

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).

ImagePortaball 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.

ImageIKE160012-BACK by Cyrus Beh, on Flickr

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

ImageTwo 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
 
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Re: Building an 8" Portaball wannabe with Ikea Blanda Bowls

Postby geyes30 » Thu Nov 30, 2017 10:50 am

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.

ImageGravel 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!

ImageEpoxy-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.

ImageFlat 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.

ImageMounting 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.

ImageTwo bowls are joined together by Cyrus Beh, on Flickr


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