Plastico Gigantico: The First Skyward 400

Pushing the limits of the big printer: one part took 18 hours and 2.5 kg of ABS.

What's big and yellow and rhymes with snobstonian?

That was the question I asked on our Discord last December, and my friend Matteo's 16 inch Dobsonian has been the answer ever since. We have been calling it Plastico Gigantico, and it is the prototype for a new design, the Skyward 400.

How It Started

Matteo came to me with a request for help. He had a 16 inch mirror and he wanted a telescope to put it in. I told him OK, but I was not going to convert my shop into a woodworking shop. If we were going to build it, it would be built from my standard materials, plastic and aluminum. He accepted, and that is how a 16 inch Dobsonian ended up coming off a 3D printer.

The Mirror

This is not a mirror we bought. Matteo hand ground and polished it himself, and Alan Ward did the mirror coatings. It is 16 inches across with a focal ratio of about f/3.8, which works out to a focal length of roughly 1540mm.

Why does that matter? A 16 inch mirror has more than seven times the light gathering area of the mirror in our Skyward 150. More light means fainter galaxies and nebulae, and more detail in the ones you can already see. The fast focal ratio keeps the tube short for a mirror this size, so you can still reach the eyepiece without a tall ladder.

The mirror sits in a cast metal mirror cell that Matteo picked up used. That cell is a one-off, so I designed the printed mirror cell body specifically to fit it. In May we added a nylon sling to support the edge of the mirror.

The printed mirror cell body with Matteo's cast metal mirror cell mounted inside.

Pushing the Big Printer

All of the big parts were printed in ABS on HevOrange, a printer I built a few years ago. It can do large ABS prints and keep the warping under control, which is exactly what this project needed. Even so, Plastico Gigantico pushed HevOrange to its absolute limits.

Some of the numbers:

• One of the big parts took 18 hours and 2.5 kg of filament.

• The rocker box base was printed in two pieces, 3.3 kg together, which were then welded into one. Before committing that much filament, I printed a test fit with leftover filament. A couple of days later, the fit test nailed it.

• One print came within 6 inches of emptying a 2 kg spool.

The rocker box base: two pieces, 3.3 kg of red ABS.

The rocker box, base, uprights and trunnions all came together over the winter. The mirror cell passed its fit test in early January, and we mounted it at a Scope Building Sunday. When Owen saw it, he said "I can't get over the scale of that thing!" Neither can I.

Scope Building Sunday in January.

Mirrors In

At the end of April the mirrors went in. We tried it out by examining tree buds about a kilometre away, which is not very astronomical, but it was a good first look. We also found out that our centre of mass calculations were perfect. It got an 80mm guidescope too.

Mirrors in, and the centre of mass right where we calculated.

Moving a Monster

A 16 inch Dobsonian is not something you carry out to the backyard under one arm. In May Matteo was working on a way to attach wheelbarrow handles to the base, and we tested a Dob-cart concept for rolling it around. To keep it balanced, he uses ankle weights, which are small fabric bags filled with sand.

The rocker box and base in May.

At the end of May we packed it up and it headed off to its forever home.

First Light

On May 28 Matteo reported that first light went well, and he shared a shot of the Moon taken with a camera held up to the eyepiece. Plastico Gigantico is a visual telescope, not an astrophotography rig, but it is a nice first look.

First light: Plastico Gigantico set up outside, and Matteo's handheld shot of the Moon at the eyepiece. Images: Matteo Statti

What's Next: The Skyward 400

Plastico Gigantico is a one-of-a-kind prototype, but the Skyward 400 will be a design anyone can build. We will release it on GitHub like the Skyward 150 and Skyward 200, and unlike the prototype, it will be designed to be friendly to fabricate with inexpensive off the shelf materials. I have several 15.5 inch and 16 inch mirror blanks hanging around, and we will be grinding, polishing and coating those mirrors ourselves. We are also thinking about trying thin slumped mirrors.

The goal is to have several of them around, for star parties and for loans from the library.

A big thank you to Matteo for trusting me with his hand made mirror. Stay tuned for a future post!

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Follow along with the Skyward 400 in the telescope-design-logs channel on our Discord.

Join the Ten Telescopes Discord: discord.gg/kwv2z3wfGd