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Douglas Smith's FirstLight, a truss-tube style Dobsonian telescope.

(Credit: Patty Nason/Gavaphoto)

Oakland resident Douglas Smith has a degree in architecture from UC Berkeley, and works as a BIM / CAD manager at the San Francisco offices of internationally-renowned skyscraper architects Skidmore Owings & Merrill. In 2006, co-worker and amateur astronomer David Frey persuaded him to take an amateur telescope-making class at The Randall Museum, taught by famous amateur astronomer John Dobson.

Optical diagram of a classic Newtonian telescope. A Dobsonian telescope is a Newtonian telescope built using certain low-cost methods pioneered by John Dobson.

Ray tracing diagram of a classic Newtonian reflector. A Dobsonian telescope is a Newtonian reflector built using low-cost methods pioneered by John Dobson.

In 2010, Smith unveiled FirstLight, a homemade reflecting telescope with a 16.5″ primary mirror and a focal length of 117″. It’s 10′ long, fully assembled, with an aluminum truss-tube design that keeps the weight down to just 160 lbs. The base is a signature Dobsonian altitude-azimuth design, with extra amenities such as LED-illuminated scales, a fan to prevent condensation on the mirror, and detachable barrow-wheels for easier movement. Smith ground the large primary mirror, by hand, from a piece of porthole glass.

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Smith is now secretary of the San Francisco Amateur Astronomers, an enthusiast group founded in 1952. He brings FirstLight out regularly for star viewing parties, public outreach, and other events, and now teaches the same telescope-making class that first inspired him. I got a chance to catch up with him, last week, and chat about the journey from point A to point B.

You’ve brought FirstLight to every Bay Area Maker Faire since 2010.

comingtobayareamakerfaire_2013

Yeah, 2010 was the first time it was at Maker Faire. It was good to set myself a deadline to finish it! I’ve been bringing it out since, and also bringing others involved with telescope making trying to have it less about FirstLight itself, but more on showing that building your own telescope is possible. FirstLight can be a little intimidating! Last year my friend George brought his first telescope which he built at the Randall class the first time I taught it. I think he’s going to bring his new 6″ travel scope out this year. It should be a great contrast. His travel scope nests in a box you can fit under an airline seat!

FirstLight was the first telescope you ever built?

Yes. I decided to build a big one my first time. I don’t recommend it for most students though! It was a big project but I learned a lot. It takes some dedication and time to finish a project that size. I’ve met many people who started their telescope and said they still had it under their bed, or in a drawer. But I’ve also met some who finished—and even one guy with a picture of his in his wallet! That one was a bino-scope and I could understand why!

Four years in the making. That’s quite a commitment.

It took roughly 4 years to finish, but that was with some substantial breaks here and there, and also mostly weekend work. Since the mirror was my first one, I did make and learn from some mistakes, so that took some time. That’s why I recommend starting with a small to medium project, then tackling a big one. I’d do it again myself though!

Hand grinding a telescope mirror

Tell me about the mirror-making process.

The glass which becomes the mirror is ground and polished by hand. Essentially the setup is just two pieces of glass (or the glass and a tool made from porcelain tile) with water and grit in-between. There’s a process to the stroke and also the rotation, and which is on top or bottom.

There are three main stages. The first is rough grinding. John Dobson says this is “caveman’s work.” It’s loud and just a little messy. The abrasive grit used at this stage is 60 or 80 grit. The purpose of the first stage is to reach the desired focal point which relates to the depth you have to grind—the sagitta. When rough grinding you can turn out the lights and actually see sparks!

After you reach the desired focal length, which determines the overall size of the telescope, you move on to fine grinding. You’re keeping the shape you made during rough grinding, but smoothing out the surface, moving to finer and finer grits. After the surface has consistent texture from one grit, you move up to the next finer grit.

Then there’s polishing and figuring. You finish fine grinding with about 5 micron grit, which is almost like baby powder, and then start the polishing. To polish, we use a tool called a pitch-lap. Pitch is heated up to molasses consistency and poured on the tool. You put water and polishing compound on the glass which will become the mirror, and press the pitch-lap into it so the grit gets embedded in the pitch. You put grooves in a waffle pattern into the pitch-lap so the slurry can run in and out. The pitch-lap is effective because it makes full constant contact with the curved glass. It takes on the the same shape as the glass and polishes as it wears away.

The final step of the polishing stage is figuring. Depending on the focal length, this could be quick (relatively) or challenging. The shorter the focal length, the deeper the sagitta and the more difficult to figure. Long focal lengths are a flatter curve and more manageable to figure as the parabola is closer to the shape of a sphere which is the default shape when grinding and polishing.

You said you made some mistakes. Does that mean you have to start over?

Don’t have to start over. Worst case you may need to go to a previous size grit. It’s best to test often so you don’t overshoot.

Freshly aluminized 10" primary mirror, courtesy ZWO Optical.

Freshly aluminized 10″ primary mirror, courtesy ZWO Optical.

And how does the glass become a mirror?

Once figured, the glass is aluminized. I took mine to Bob Fies on the peninsula. He’s built the contraption in his garage and he’s been coating mirrors almost as long as JD has been teaching! It’s essentially a vacuum chamber and then aluminum is vaporized inside. The mirror is held upside down inside the chamber and coated. Protective coatings are also added, typically.

What’s the most difficult part of the process?

Actually probably the figuring. Grinding is some work, but there’s usually more resistance when you’re polishing. But overall, naturally the mirror making was the most challenging part. Mostly because it was a new process to learn. Friends with a CNC machine fabricated the wood and aluminum parts from my CAD drawings (and they had a backlog), but having the parts cut precisely helped a lot with the construction.

Any idea of how many hours you have in the mirror?

I should’ve kept track but didn’t. Because I stopped and started here and there it’s hard to guess. At the Randall class, a friend who helps me out will hand out a log so students can keep track. I plan to do that on my next telescope project.

Tell us about your that, your next project.

I actually have two or three planned! I inherited a 4″ off-axis telescope and plan to redo the mount and tube soon. Also I plan an 8″ telescope with an integrated equatorial base—a base that slowly rotates on a virtual axis to compensate for the sky’s movement—that will track with the stars. This will allow for some cool astrophotography. Finally, I just bought a 17 1/2″ piece of glass, already nearly finished. I just need to do some final touches on the figuring. The 17 1/2″ mirror is ground deep for a 70″ focal length, so it will actually be much shorter than FirstLight. I want to build the 8″ with the equatorial base first to be able to figure out any issues with that, and then build the tracking features into the 17 1/2″ ‘scope.

Randall Museum — Teen & Adult Classes — Dobsonian Telescope Making

Sean Michael Ragan

I am descended from 5,000 generations of tool-using primates. Also, I went to college and stuff. I write for MAKE, serve as Technical Editor for MAKE magazine, and develop original DIY content for Make: Projects.


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